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Stop cloning the AST in the transform worker's main Babel pass (#2034) - #2034

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Stop cloning the AST in the transform worker's main Babel pass (#2034)#2034
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@vzaidman vzaidman commented Oct 7, 2026 •
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Summary:

transformJS runs the main Babel pass with cloneInputAst: true, deep-copying every module's AST although nothing reads the original afterwards. It was kept because false broke production bundles (#641). This diff turns cloning off and fixes the underlying bug, saving about 4% wall time and 5% CPU on average in cold builds of three Meta production apps, with byte-identical bundles. D123839895 puts the same change behind an option; this diff makes it unconditional and removes the option.

Root cause

babel/traverse caches paths and scopes globally, keyed by AST node. When a later pass runs on the same, uncloned nodes, Babel reuses the scope crawled by an earlier pass instead of crawling again. Code inserted after that crawl, such as a helper Babel adds while lowering ESM or a call introduced by a plugin, is missing from the cached bindings. The prod-only constant-folding pass deletes every function declaration whose binding is unreferenced, so it deletes live functions while their calls remain:

  • import * as ns (or duplicate imports from one module) without experimentalImportSupport: the inlined _interopRequireWildcard helper is deleted.
  • In the Meta real app 1 bundle, stringifySafe is deleted from a module that still calls it.

The _interopRequireDefault symptom from #641 no longer reproduces only because that helper now comes from babel/runtime as a var, which constant folding never deletes; the bug itself is still present.

Fix

  • cloneInputAst: false in the main pass.
  • traverse.cache.clearScope() right before the prod-only constant-folding pass, so that pass crawls fresh scopes. Dev builds skip that pass and pay nothing.
  • traverse is imported from babel/core, which re-exports the instance Babel uses internally, so the clear always hits the cache Babel reads, and no new dependency is needed.
  • Removes the transformer.unstable_disableInputAstCloning option from D123839895.
  • Tests in index-test.js:
    • keeps functions referenced by earlier passes in production: a prod transform whose Babel transformer adds a call to stringifySafe after the scope crawl. It fails if the clear is removed.
    • Babel keeps stale scope bindings when reusing an AST: asserts the Babel behavior the clear works around. When Babel stops reusing stale scopes, it fails, and its comment and the comment above the clear say to delete both.
  • The test file now uses the real constant-folding plugin instead of a no-op mock; all existing tests and snapshots are unchanged.

Measurements

Measured with scripts/benchmark-patches/benchmark-patches.js bundling_speed from D122997558: 4 candidate fixes against the unpatched tree, on three Meta production apps, referred to as Meta real apps 1–3 (app1–app3 below): app1 android and app2 ios, and separately app3 android (46,534 modules), each in prod and dev, 5 cold builds each.

Variants (all set cloneInputAst: false in the worker's main Babel pass):

  • clear-before-transform: traverse.cache.clear() before the main pass.
  • clear-before-constant-folding: traverse.cache.clear() before the prod-only constant-folding pass.
  • clearscope-before-transform: traverse.cache.clearScope() before the main pass.
  • clearscope-before-constant-folding: traverse.cache.clearScope() before the prod-only constant-folding pass. This is exactly this diff's index.js change relative to D122997558.
Bundle Variant Wall (s) Wall Δ (t) CPU (s) CPU Δ (t) Size (bytes) Same as unpatched
app1-android-prod unpatched 207.5 ± 4.6 2956.1 ± 74.0 37,357,607 yes
clear-before-constant-folding 204.4 ± 5.0 -1.48% (-0.91) 2854.2 ± 41.9 -3.45% (-2.40) 37,357,607 yes
clear-before-transform 203.5 ± 4.1 -1.91% (-1.29) 2856.4 ± 42.4 -3.37% (-2.34) 37,357,607 yes
clearscope-before-constant-folding 204.4 ± 3.9 -1.48% (-1.02) 2853.2 ± 23.5 -3.48% (-2.65) 37,357,607 yes
clearscope-before-transform 210.0 ± 6.6 +1.20% (+0.61) 2917.8 ± 87.5 -1.30% (-0.67) 37,357,607 yes
app1-android-dev unpatched 183.4 ± 6.4 2454.7 ± 66.2 95,688,124 yes
clear-before-constant-folding 178.2 ± 5.6 -2.83% (-1.21) 2304.4 ± 82.6 -6.12% (-2.84) 95,688,124 yes
clear-before-transform 182.6 ± 4.1 -0.42% (-0.19) 2391.7 ± 66.4 -2.57% (-1.34) 95,688,124 yes
clearscope-before-constant-folding 177.2 ± 3.8 -3.37% (-1.65) 2320.7 ± 59.9 -5.46% (-3.00) 95,688,124 yes
clearscope-before-transform 182.0 ± 4.1 -0.73% (-0.34) 2367.3 ± 70.9 -3.56% (-1.80) 95,688,124 yes
app2-ios-prod unpatched 153.5 ± 2.4 1502.7 ± 34.0 18,768,596 yes
clear-before-constant-folding 148.1 ± 3.2 -3.53% (-2.72) 1456.3 ± 43.6 -3.09% (-1.67) 18,768,596 yes
clear-before-transform 148.5 ± 2.9 -3.23% (-2.61) 1430.8 ± 20.5 -4.79% (-3.62) 18,768,596 yes
clearscope-before-constant-folding 143.8 ± 3.5 -6.33% (-4.56) 1402.6 ± 26.1 -6.66% (-4.67) 18,768,596 yes
clearscope-before-transform 144.9 ± 2.5 -5.57% (-4.88) 1431.0 ± 16.4 -4.77% (-3.80) 18,768,596 yes
app2-ios-dev unpatched 132.5 ± 1.9 1135.5 ± 14.8 48,693,569 yes
clear-before-constant-folding 126.3 ± 2.0 -4.68% (-4.51) 1073.1 ± 10.8 -5.50% (-6.83) 48,693,569 yes
clear-before-transform 127.4 ± 2.8 -3.92% (-3.06) 1084.7 ± 19.8 -4.47% (-4.10) 48,693,569 yes
clearscope-before-constant-folding 124.7 ± 4.3 -5.89% (-3.33) 1057.6 ± 37.0 -6.86% (-3.91) 48,693,569 yes
clearscope-before-transform 131.0 ± 4.9 -1.20% (-0.61) 1114.8 ± 50.1 -1.82% (-0.79) 48,693,569 yes
app3-android-prod unpatched 215.3 ± 4.2 3479.3 ± 35.7 114,719,789 yes
clear-before-constant-folding 210.9 ± 3.2 -2.05% (-1.9) 3371.2 ± 61.2 -3.11% (-3.4) 114,719,789 yes
clear-before-transform 212.1 ± 5.3 -1.48% (-1.1) 3415.7 ± 91.2 -1.83% (-1.5) 114,719,789 yes
clearscope-before-constant-folding 211.2 ± 4.6 -1.91% (-1.5) 3382.7 ± 71.0 -2.78% (-2.7) 114,719,789 yes
clearscope-before-transform 226.0 ± 20.2 +4.99% (+1.2) 3633.0 ± 339.7 +4.42% (+1.0) 114,719,789 yes
app3-android-dev unpatched 192.3 ± 7.0 2953.4 ± 130.4 289,411,642 yes
clear-before-constant-folding 184.4 ± 4.2 -4.12% (-2.2) 2777.0 ± 78.3 -5.97% (-2.6) 289,411,642 yes
clear-before-transform 185.8 ± 6.7 -3.39% (-1.5) 2826.0 ± 102.1 -4.31% (-1.7) 289,411,642 yes
clearscope-before-constant-folding 184.4 ± 8.1 -4.09% (-1.6) 2803.5 ± 113.8 -5.08% (-1.9) 289,411,642 yes
clearscope-before-transform 185.6 ± 5.7 -3.46% (-1.7) 2823.2 ± 92.1 -4.41% (-1.8) 289,411,642 yes

5 measured runs per variant, each a cold build (--reset-cache --no-use-saved-state, --max-workers=16) on a 72-core Linux machine. CPU is user + sys of the build and its workers. Values are mean ± sd; t is Welch's t vs unpatched (at 5 runs, |t| < 2.3 is not significant). "Same as unpatched" compares bundle hashes.

app1 and app2 were measured with an earlier revision of the script that ran variants in a fixed order (unpatched first) and printed population sd; their t values were computed afterwards from the same per-run timings. app3 was measured with the current revision: rotating variant order, sample sd, and byte identity checked on every run.

Analysis

  1. All four candidates produce bundles byte-identical to unpatched in all 6 bundle/mode combinations (for app3, in every run), so each one fixes the stale-scope deletion that raw cloneInputAst: false causes in the app1 prod bundle.
  2. Not cloning saves CPU: every candidate uses less CPU than unpatched everywhere except clearscope-before-transform on app3 prod. The size of the effect is similar across apps: for the two before-constant-folding candidates, 2.8–6.9% CPU and 1.5–6.3% wall. Wall-time deltas are mostly within noise at 5 runs; CPU deltas are mostly significant.
  3. Where the clear goes matters more than which clear is used. Clearing before the main pass discards the scopes the Babel transformer just built, so the main pass re-crawls every module, in dev too. The two "before-constant-folding" variants never clear in dev and are faster there on app1 (177–178s vs 182–183s) and app2 (125–126s vs 127–131s); on app3 dev all four are within noise (184.4–185.8s).
  4. clearScope() vs clear() is within noise. In dev the two before-constant-folding variants run identical code, and they still differ by up to 1.4% wall (app3 dev: 184.4s vs 184.4s, 2777s vs 2804s CPU). In prod, clearScope() is ahead on app2 (143.8s vs 148.1s), level on app1 and slightly behind on app3 (-2.78% vs -3.11% CPU). clearScope() discards less, since it keeps the path cache.
  5. clearscope-before-transform is erratic: two of its five app3 prod builds took 245s and 251s, using about 15% more CPU, while the unpatched builds next to them were normal. It also had the largest spread on app1 prod.
  6. clearscope-before-constant-folding has the best average and a significant CPU saving in 5 of 6 combinations (all except app3 dev, t -1.9); clear-before-constant-folding is close behind, also significant in 5 of 6 (all except app2 prod, t -1.67).

Averaged over app1, app2 and app3, prod and dev (6 combinations):

Candidate Avg wall Δ Avg CPU Δ
clearscope-before-constant-folding (this diff) -3.85% -5.05%
clear-before-constant-folding -3.12% -4.54%
clear-before-transform -2.39% -3.56%
clearscope-before-transform -0.80% -1.91%

This diff implements clearscope-before-constant-folding. On app3, the largest app, it saves about 2% wall and 3% CPU in prod, and 4% wall and 5% CPU in dev.

Caveats: 5 runs per variant; the app1 and app2 run used a fixed variant order (unpatched first in each round), while the app3 run rotated it; most wall-time deltas are within noise.

Changelog: [Performance] metro-transform-worker no longer deep-clones each module's AST before the main Babel pass, reducing cold build CPU time by about 5%.

Differential Revision: D123636983

@meta-cla meta-cla Bot added the CLA Signed This label is managed by the Facebook bot. Authors need to sign the CLA before a PR can be reviewed. label Oct 7, 2026
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@vzaidman has exported this pull request. If you are a Meta employee, you can view the originating Diff in D123636983.

@vzaidman
vzaidman requested a review from huntie October 7, 2026 14:38
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vzaidman commented Oct 7, 2026

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will only land this after experimenting with it internally.

@meta-codesync meta-codesync Bot changed the title Stop cloning the AST in the transform worker's main Babel pass Stop cloning the AST in the transform worker's main Babel pass (#2034) Oct 8, 2026
@meta-codesync
meta-codesync Bot force-pushed the export-D123636983 branch from 0221bf0 to f1a8aa3 Compare October 8, 2026 08:57
meta-codesync Bot pushed a commit that referenced this pull request Oct 8, 2026
@vzaidman @facebook-github-bot
Stop cloning the AST in the transform worker's main Babel pass (#2034)
f1a8aa3 
Summary:

`transformJS` runs the main Babel pass with `cloneInputAst: true`, deep-copying every module's AST although nothing reads the original afterwards. It was kept because `false` broke production bundles (#641). This diff turns cloning off and fixes the underlying bug, saving about 4% wall time and 5% CPU on average in cold builds of three Meta production apps, with byte-identical bundles. D123839895 puts the same change behind an option; this diff makes it unconditional and removes the option.

## Root cause

`babel/traverse` caches paths and scopes globally, keyed by AST node. When a later pass runs on the same, uncloned nodes, Babel reuses the scope crawled by an earlier pass instead of crawling again. Code inserted after that crawl, such as a helper Babel adds while lowering ESM or a call introduced by a plugin, is missing from the cached bindings. The prod-only constant-folding pass deletes every function declaration whose binding is unreferenced, so it deletes live functions while their calls remain:
- `import * as ns` (or duplicate imports from one module) without `experimentalImportSupport`: the inlined `_interopRequireWildcard` helper is deleted.
- In the Meta real app 1 bundle, `stringifySafe` is deleted from a module that still calls it.

The `_interopRequireDefault` symptom from #641 no longer reproduces only because that helper now comes from `babel/runtime` as a `var`, which constant folding never deletes; the bug itself is still present.

## Fix

- `cloneInputAst: false` in the main pass.
- `traverse.cache.clearScope()` right before the prod-only constant-folding pass, so that pass crawls fresh scopes. Dev builds skip that pass and pay nothing.
- `traverse` is imported from `babel/core`, which re-exports the instance Babel uses internally, so the clear always hits the cache Babel reads, and no new dependency is needed.
- Removes the `transformer.unstable_disableInputAstCloning` option from D123839895.
- Tests in `index-test.js`:
  - `keeps functions referenced by earlier passes in production`: a prod transform whose Babel transformer adds a call to `stringifySafe` after the scope crawl. It fails if the clear is removed.
  - `Babel keeps stale scope bindings when reusing an AST`: asserts the Babel behavior the clear works around. When Babel stops reusing stale scopes, it fails, and its comment and the comment above the clear say to delete both.
- The test file now uses the real constant-folding plugin instead of a no-op mock; all existing tests and snapshots are unchanged.

## Measurements

Measured with `scripts/benchmark-patches/benchmark-patches.js bundling_speed` from D122997558: 4 candidate fixes against the unpatched tree, on three Meta production apps, referred to as Meta real apps 1–3 (`app1`–`app3` below): app1 android and app2 ios, and separately app3 android (46,534 modules), each in prod and dev, 5 cold builds each.

Variants (all set `cloneInputAst: false` in the worker's main Babel pass):
- `clear-before-transform`: `traverse.cache.clear()` before the main pass.
- `clear-before-constant-folding`: `traverse.cache.clear()` before the prod-only constant-folding pass.
- `clearscope-before-transform`: `traverse.cache.clearScope()` before the main pass.
- `clearscope-before-constant-folding`: `traverse.cache.clearScope()` before the prod-only constant-folding pass. This is exactly this diff's `index.js` change relative to D122997558.

| Bundle | Variant | Wall (s) | Wall Δ (t) | CPU (s) | CPU Δ (t) | Size (bytes) | Same as unpatched |
|---|---|--:|--:|--:|--:|--:|---|
| app1-android-prod | unpatched | 207.5 ± 4.6 | | 2956.1 ± 74.0 | | 37,357,607 | yes |
| | clear-before-constant-folding | 204.4 ± 5.0 | -1.48% (-0.91) | 2854.2 ± 41.9 | -3.45% (-2.40) | 37,357,607 | yes |
| | clear-before-transform | 203.5 ± 4.1 | -1.91% (-1.29) | 2856.4 ± 42.4 | -3.37% (-2.34) | 37,357,607 | yes |
| | clearscope-before-constant-folding | 204.4 ± 3.9 | -1.48% (-1.02) | 2853.2 ± 23.5 | -3.48% (-2.65) | 37,357,607 | yes |
| | clearscope-before-transform | 210.0 ± 6.6 | +1.20% (+0.61) | 2917.8 ± 87.5 | -1.30% (-0.67) | 37,357,607 | yes |
| app1-android-dev | unpatched | 183.4 ± 6.4 | | 2454.7 ± 66.2 | | 95,688,124 | yes |
| | clear-before-constant-folding | 178.2 ± 5.6 | -2.83% (-1.21) | 2304.4 ± 82.6 | -6.12% (-2.84) | 95,688,124 | yes |
| | clear-before-transform | 182.6 ± 4.1 | -0.42% (-0.19) | 2391.7 ± 66.4 | -2.57% (-1.34) | 95,688,124 | yes |
| | clearscope-before-constant-folding | 177.2 ± 3.8 | -3.37% (-1.65) | 2320.7 ± 59.9 | -5.46% (-3.00) | 95,688,124 | yes |
| | clearscope-before-transform | 182.0 ± 4.1 | -0.73% (-0.34) | 2367.3 ± 70.9 | -3.56% (-1.80) | 95,688,124 | yes |
| app2-ios-prod | unpatched | 153.5 ± 2.4 | | 1502.7 ± 34.0 | | 18,768,596 | yes |
| | clear-before-constant-folding | 148.1 ± 3.2 | -3.53% (-2.72) | 1456.3 ± 43.6 | -3.09% (-1.67) | 18,768,596 | yes |
| | clear-before-transform | 148.5 ± 2.9 | -3.23% (-2.61) | 1430.8 ± 20.5 | -4.79% (-3.62) | 18,768,596 | yes |
| | clearscope-before-constant-folding | 143.8 ± 3.5 | -6.33% (-4.56) | 1402.6 ± 26.1 | -6.66% (-4.67) | 18,768,596 | yes |
| | clearscope-before-transform | 144.9 ± 2.5 | -5.57% (-4.88) | 1431.0 ± 16.4 | -4.77% (-3.80) | 18,768,596 | yes |
| app2-ios-dev | unpatched | 132.5 ± 1.9 | | 1135.5 ± 14.8 | | 48,693,569 | yes |
| | clear-before-constant-folding | 126.3 ± 2.0 | -4.68% (-4.51) | 1073.1 ± 10.8 | -5.50% (-6.83) | 48,693,569 | yes |
| | clear-before-transform | 127.4 ± 2.8 | -3.92% (-3.06) | 1084.7 ± 19.8 | -4.47% (-4.10) | 48,693,569 | yes |
| | clearscope-before-constant-folding | 124.7 ± 4.3 | -5.89% (-3.33) | 1057.6 ± 37.0 | -6.86% (-3.91) | 48,693,569 | yes |
| | clearscope-before-transform | 131.0 ± 4.9 | -1.20% (-0.61) | 1114.8 ± 50.1 | -1.82% (-0.79) | 48,693,569 | yes |
| app3-android-prod | unpatched | 215.3 ± 4.2 | | 3479.3 ± 35.7 | | 114,719,789 | yes |
| | clear-before-constant-folding | 210.9 ± 3.2 | -2.05% (-1.9) | 3371.2 ± 61.2 | -3.11% (-3.4) | 114,719,789 | yes |
| | clear-before-transform | 212.1 ± 5.3 | -1.48% (-1.1) | 3415.7 ± 91.2 | -1.83% (-1.5) | 114,719,789 | yes |
| | clearscope-before-constant-folding | 211.2 ± 4.6 | -1.91% (-1.5) | 3382.7 ± 71.0 | -2.78% (-2.7) | 114,719,789 | yes |
| | clearscope-before-transform | 226.0 ± 20.2 | +4.99% (+1.2) | 3633.0 ± 339.7 | +4.42% (+1.0) | 114,719,789 | yes |
| app3-android-dev | unpatched | 192.3 ± 7.0 | | 2953.4 ± 130.4 | | 289,411,642 | yes |
| | clear-before-constant-folding | 184.4 ± 4.2 | -4.12% (-2.2) | 2777.0 ± 78.3 | -5.97% (-2.6) | 289,411,642 | yes |
| | clear-before-transform | 185.8 ± 6.7 | -3.39% (-1.5) | 2826.0 ± 102.1 | -4.31% (-1.7) | 289,411,642 | yes |
| | clearscope-before-constant-folding | 184.4 ± 8.1 | -4.09% (-1.6) | 2803.5 ± 113.8 | -5.08% (-1.9) | 289,411,642 | yes |
| | clearscope-before-transform | 185.6 ± 5.7 | -3.46% (-1.7) | 2823.2 ± 92.1 | -4.41% (-1.8) | 289,411,642 | yes |

5 measured runs per variant, each a cold build (`--reset-cache --no-use-saved-state`, `--max-workers=16`) on a 72-core Linux machine. CPU is user + sys of the build and its workers. Values are mean ± sd; t is Welch's t vs unpatched (at 5 runs, |t| < 2.3 is not significant). "Same as unpatched" compares bundle hashes.

app1 and app2 were measured with an earlier revision of the script that ran variants in a fixed order (unpatched first) and printed population sd; their t values were computed afterwards from the same per-run timings. app3 was measured with the current revision: rotating variant order, sample sd, and byte identity checked on every run.

## Analysis

1. All four candidates produce bundles byte-identical to unpatched in all 6 bundle/mode combinations (for app3, in every run), so each one fixes the stale-scope deletion that raw `cloneInputAst: false` causes in the app1 prod bundle.
2. Not cloning saves CPU: every candidate uses less CPU than unpatched everywhere except `clearscope-before-transform` on app3 prod. The size of the effect is similar across apps: for the two before-constant-folding candidates, 2.8–6.9% CPU and 1.5–6.3% wall. Wall-time deltas are mostly within noise at 5 runs; CPU deltas are mostly significant.
3. Where the clear goes matters more than which clear is used. Clearing before the main pass discards the scopes the Babel transformer just built, so the main pass re-crawls every module, in dev too. The two "before-constant-folding" variants never clear in dev and are faster there on app1 (177–178s vs 182–183s) and app2 (125–126s vs 127–131s); on app3 dev all four are within noise (184.4–185.8s).
4. `clearScope()` vs `clear()` is within noise. In dev the two before-constant-folding variants run identical code, and they still differ by up to 1.4% wall (app3 dev: 184.4s vs 184.4s, 2777s vs 2804s CPU). In prod, `clearScope()` is ahead on app2 (143.8s vs 148.1s), level on app1 and slightly behind on app3 (-2.78% vs -3.11% CPU). `clearScope()` discards less, since it keeps the path cache.
5. `clearscope-before-transform` is erratic: two of its five app3 prod builds took 245s and 251s, using about 15% more CPU, while the unpatched builds next to them were normal. It also had the largest spread on app1 prod.
6. `clearscope-before-constant-folding` has the best average and a significant CPU saving in 5 of 6 combinations (all except app3 dev, t -1.9); `clear-before-constant-folding` is close behind, also significant in 5 of 6 (all except app2 prod, t -1.67).

Averaged over app1, app2 and app3, prod and dev (6 combinations):

| Candidate | Avg wall Δ | Avg CPU Δ |
|---|--:|--:|
| `clearscope-before-constant-folding` (this diff) | -3.85% | -5.05% |
| `clear-before-constant-folding` | -3.12% | -4.54% |
| `clear-before-transform` | -2.39% | -3.56% |
| `clearscope-before-transform` | -0.80% | -1.91% |

This diff implements `clearscope-before-constant-folding`. On app3, the largest app, it saves about 2% wall and 3% CPU in prod, and 4% wall and 5% CPU in dev.

Caveats: 5 runs per variant; the app1 and app2 run used a fixed variant order (unpatched first in each round), while the app3 run rotated it; most wall-time deltas are within noise.

Changelog: [Performance] `metro-transform-worker` no longer deep-clones each module's AST before the main Babel pass, reducing cold build CPU time by about 5%.

Differential Revision: D123636983
@meta-codesync
meta-codesync Bot force-pushed the export-D123636983 branch from f1a8aa3 to ff77d81 Compare October 8, 2026 08:59
meta-codesync Bot pushed a commit that referenced this pull request Oct 8, 2026
@vzaidman @meta-codesync
Stop cloning the AST in the transform worker's main Babel pass (#2034)
ff77d81 
Summary:
Pull Request resolved: #2034

`transformJS` runs the main Babel pass with `cloneInputAst: true`, deep-copying every module's AST although nothing reads the original afterwards. It was kept because `false` broke production bundles (#641). This diff turns cloning off and fixes the underlying bug, saving about 4% wall time and 5% CPU on average in cold builds of three Meta production apps, with byte-identical bundles. D123839895 puts the same change behind an option; this diff makes it unconditional and removes the option.

## Root cause

`babel/traverse` caches paths and scopes globally, keyed by AST node. When a later pass runs on the same, uncloned nodes, Babel reuses the scope crawled by an earlier pass instead of crawling again. Code inserted after that crawl, such as a helper Babel adds while lowering ESM or a call introduced by a plugin, is missing from the cached bindings. The prod-only constant-folding pass deletes every function declaration whose binding is unreferenced, so it deletes live functions while their calls remain:
- `import * as ns` (or duplicate imports from one module) without `experimentalImportSupport`: the inlined `_interopRequireWildcard` helper is deleted.
- In the Meta real app 1 bundle, `stringifySafe` is deleted from a module that still calls it.

The `_interopRequireDefault` symptom from #641 no longer reproduces only because that helper now comes from `babel/runtime` as a `var`, which constant folding never deletes; the bug itself is still present.

## Fix

- `cloneInputAst: false` in the main pass.
- `traverse.cache.clearScope()` right before the prod-only constant-folding pass, so that pass crawls fresh scopes. Dev builds skip that pass and pay nothing.
- `traverse` is imported from `babel/core`, which re-exports the instance Babel uses internally, so the clear always hits the cache Babel reads, and no new dependency is needed.
- Removes the `transformer.unstable_disableInputAstCloning` option from D123839895.
- Tests in `index-test.js`:
  - `keeps functions referenced by earlier passes in production`: a prod transform whose Babel transformer adds a call to `stringifySafe` after the scope crawl. It fails if the clear is removed.
  - `Babel keeps stale scope bindings when reusing an AST`: asserts the Babel behavior the clear works around. When Babel stops reusing stale scopes, it fails, and its comment and the comment above the clear say to delete both.
- The test file now uses the real constant-folding plugin instead of a no-op mock; all existing tests and snapshots are unchanged.

## Measurements

Measured with `scripts/benchmark-patches/benchmark-patches.js bundling_speed` from D122997558: 4 candidate fixes against the unpatched tree, on three Meta production apps, referred to as Meta real apps 1–3 (`app1`–`app3` below): app1 android and app2 ios, and separately app3 android (46,534 modules), each in prod and dev, 5 cold builds each.

Variants (all set `cloneInputAst: false` in the worker's main Babel pass):
- `clear-before-transform`: `traverse.cache.clear()` before the main pass.
- `clear-before-constant-folding`: `traverse.cache.clear()` before the prod-only constant-folding pass.
- `clearscope-before-transform`: `traverse.cache.clearScope()` before the main pass.
- `clearscope-before-constant-folding`: `traverse.cache.clearScope()` before the prod-only constant-folding pass. This is exactly this diff's `index.js` change relative to D122997558.

| Bundle | Variant | Wall (s) | Wall Δ (t) | CPU (s) | CPU Δ (t) | Size (bytes) | Same as unpatched |
|---|---|--:|--:|--:|--:|--:|---|
| app1-android-prod | unpatched | 207.5 ± 4.6 | | 2956.1 ± 74.0 | | 37,357,607 | yes |
| | clear-before-constant-folding | 204.4 ± 5.0 | -1.48% (-0.91) | 2854.2 ± 41.9 | -3.45% (-2.40) | 37,357,607 | yes |
| | clear-before-transform | 203.5 ± 4.1 | -1.91% (-1.29) | 2856.4 ± 42.4 | -3.37% (-2.34) | 37,357,607 | yes |
| | clearscope-before-constant-folding | 204.4 ± 3.9 | -1.48% (-1.02) | 2853.2 ± 23.5 | -3.48% (-2.65) | 37,357,607 | yes |
| | clearscope-before-transform | 210.0 ± 6.6 | +1.20% (+0.61) | 2917.8 ± 87.5 | -1.30% (-0.67) | 37,357,607 | yes |
| app1-android-dev | unpatched | 183.4 ± 6.4 | | 2454.7 ± 66.2 | | 95,688,124 | yes |
| | clear-before-constant-folding | 178.2 ± 5.6 | -2.83% (-1.21) | 2304.4 ± 82.6 | -6.12% (-2.84) | 95,688,124 | yes |
| | clear-before-transform | 182.6 ± 4.1 | -0.42% (-0.19) | 2391.7 ± 66.4 | -2.57% (-1.34) | 95,688,124 | yes |
| | clearscope-before-constant-folding | 177.2 ± 3.8 | -3.37% (-1.65) | 2320.7 ± 59.9 | -5.46% (-3.00) | 95,688,124 | yes |
| | clearscope-before-transform | 182.0 ± 4.1 | -0.73% (-0.34) | 2367.3 ± 70.9 | -3.56% (-1.80) | 95,688,124 | yes |
| app2-ios-prod | unpatched | 153.5 ± 2.4 | | 1502.7 ± 34.0 | | 18,768,596 | yes |
| | clear-before-constant-folding | 148.1 ± 3.2 | -3.53% (-2.72) | 1456.3 ± 43.6 | -3.09% (-1.67) | 18,768,596 | yes |
| | clear-before-transform | 148.5 ± 2.9 | -3.23% (-2.61) | 1430.8 ± 20.5 | -4.79% (-3.62) | 18,768,596 | yes |
| | clearscope-before-constant-folding | 143.8 ± 3.5 | -6.33% (-4.56) | 1402.6 ± 26.1 | -6.66% (-4.67) | 18,768,596 | yes |
| | clearscope-before-transform | 144.9 ± 2.5 | -5.57% (-4.88) | 1431.0 ± 16.4 | -4.77% (-3.80) | 18,768,596 | yes |
| app2-ios-dev | unpatched | 132.5 ± 1.9 | | 1135.5 ± 14.8 | | 48,693,569 | yes |
| | clear-before-constant-folding | 126.3 ± 2.0 | -4.68% (-4.51) | 1073.1 ± 10.8 | -5.50% (-6.83) | 48,693,569 | yes |
| | clear-before-transform | 127.4 ± 2.8 | -3.92% (-3.06) | 1084.7 ± 19.8 | -4.47% (-4.10) | 48,693,569 | yes |
| | clearscope-before-constant-folding | 124.7 ± 4.3 | -5.89% (-3.33) | 1057.6 ± 37.0 | -6.86% (-3.91) | 48,693,569 | yes |
| | clearscope-before-transform | 131.0 ± 4.9 | -1.20% (-0.61) | 1114.8 ± 50.1 | -1.82% (-0.79) | 48,693,569 | yes |
| app3-android-prod | unpatched | 215.3 ± 4.2 | | 3479.3 ± 35.7 | | 114,719,789 | yes |
| | clear-before-constant-folding | 210.9 ± 3.2 | -2.05% (-1.9) | 3371.2 ± 61.2 | -3.11% (-3.4) | 114,719,789 | yes |
| | clear-before-transform | 212.1 ± 5.3 | -1.48% (-1.1) | 3415.7 ± 91.2 | -1.83% (-1.5) | 114,719,789 | yes |
| | clearscope-before-constant-folding | 211.2 ± 4.6 | -1.91% (-1.5) | 3382.7 ± 71.0 | -2.78% (-2.7) | 114,719,789 | yes |
| | clearscope-before-transform | 226.0 ± 20.2 | +4.99% (+1.2) | 3633.0 ± 339.7 | +4.42% (+1.0) | 114,719,789 | yes |
| app3-android-dev | unpatched | 192.3 ± 7.0 | | 2953.4 ± 130.4 | | 289,411,642 | yes |
| | clear-before-constant-folding | 184.4 ± 4.2 | -4.12% (-2.2) | 2777.0 ± 78.3 | -5.97% (-2.6) | 289,411,642 | yes |
| | clear-before-transform | 185.8 ± 6.7 | -3.39% (-1.5) | 2826.0 ± 102.1 | -4.31% (-1.7) | 289,411,642 | yes |
| | clearscope-before-constant-folding | 184.4 ± 8.1 | -4.09% (-1.6) | 2803.5 ± 113.8 | -5.08% (-1.9) | 289,411,642 | yes |
| | clearscope-before-transform | 185.6 ± 5.7 | -3.46% (-1.7) | 2823.2 ± 92.1 | -4.41% (-1.8) | 289,411,642 | yes |

5 measured runs per variant, each a cold build (`--reset-cache --no-use-saved-state`, `--max-workers=16`) on a 72-core Linux machine. CPU is user + sys of the build and its workers. Values are mean ± sd; t is Welch's t vs unpatched (at 5 runs, |t| < 2.3 is not significant). "Same as unpatched" compares bundle hashes.

app1 and app2 were measured with an earlier revision of the script that ran variants in a fixed order (unpatched first) and printed population sd; their t values were computed afterwards from the same per-run timings. app3 was measured with the current revision: rotating variant order, sample sd, and byte identity checked on every run.

## Analysis

1. All four candidates produce bundles byte-identical to unpatched in all 6 bundle/mode combinations (for app3, in every run), so each one fixes the stale-scope deletion that raw `cloneInputAst: false` causes in the app1 prod bundle.
2. Not cloning saves CPU: every candidate uses less CPU than unpatched everywhere except `clearscope-before-transform` on app3 prod. The size of the effect is similar across apps: for the two before-constant-folding candidates, 2.8–6.9% CPU and 1.5–6.3% wall. Wall-time deltas are mostly within noise at 5 runs; CPU deltas are mostly significant.
3. Where the clear goes matters more than which clear is used. Clearing before the main pass discards the scopes the Babel transformer just built, so the main pass re-crawls every module, in dev too. The two "before-constant-folding" variants never clear in dev and are faster there on app1 (177–178s vs 182–183s) and app2 (125–126s vs 127–131s); on app3 dev all four are within noise (184.4–185.8s).
4. `clearScope()` vs `clear()` is within noise. In dev the two before-constant-folding variants run identical code, and they still differ by up to 1.4% wall (app3 dev: 184.4s vs 184.4s, 2777s vs 2804s CPU). In prod, `clearScope()` is ahead on app2 (143.8s vs 148.1s), level on app1 and slightly behind on app3 (-2.78% vs -3.11% CPU). `clearScope()` discards less, since it keeps the path cache.
5. `clearscope-before-transform` is erratic: two of its five app3 prod builds took 245s and 251s, using about 15% more CPU, while the unpatched builds next to them were normal. It also had the largest spread on app1 prod.
6. `clearscope-before-constant-folding` has the best average and a significant CPU saving in 5 of 6 combinations (all except app3 dev, t -1.9); `clear-before-constant-folding` is close behind, also significant in 5 of 6 (all except app2 prod, t -1.67).

Averaged over app1, app2 and app3, prod and dev (6 combinations):

| Candidate | Avg wall Δ | Avg CPU Δ |
|---|--:|--:|
| `clearscope-before-constant-folding` (this diff) | -3.85% | -5.05% |
| `clear-before-constant-folding` | -3.12% | -4.54% |
| `clear-before-transform` | -2.39% | -3.56% |
| `clearscope-before-transform` | -0.80% | -1.91% |

This diff implements `clearscope-before-constant-folding`. On app3, the largest app, it saves about 2% wall and 3% CPU in prod, and 4% wall and 5% CPU in dev.

Caveats: 5 runs per variant; the app1 and app2 run used a fixed variant order (unpatched first in each round), while the app3 run rotated it; most wall-time deltas are within noise.

Changelog: [Performance] `metro-transform-worker` no longer deep-clones each module's AST before the main Babel pass, reducing cold build CPU time by about 5%.

Differential Revision: D123636983
meta-codesync Bot pushed a commit that referenced this pull request Oct 9, 2026
@vzaidman @facebook-github-bot
Stop cloning the AST in the transform worker's main Babel pass (#2034)
9abde35 
Summary:

`transformJS` runs the main Babel pass with `cloneInputAst: true`, deep-copying every module's AST although nothing reads the original afterwards. It was kept because `false` broke production bundles (#641). This diff turns cloning off and fixes the underlying bug, saving about 4% wall time and 5% CPU on average in cold builds of three Meta production apps, with byte-identical bundles. D123839895 puts the same change behind an option; this diff makes it unconditional and removes the option.

## Root cause

`babel/traverse` caches paths and scopes globally, keyed by AST node. When a later pass runs on the same, uncloned nodes, Babel reuses the scope crawled by an earlier pass instead of crawling again. Code inserted after that crawl, such as a helper Babel adds while lowering ESM or a call introduced by a plugin, is missing from the cached bindings. The prod-only constant-folding pass deletes every function declaration whose binding is unreferenced, so it deletes live functions while their calls remain:
- `import * as ns` (or duplicate imports from one module) without `experimentalImportSupport`: the inlined `_interopRequireWildcard` helper is deleted.
- In the Meta real app 1 bundle, `stringifySafe` is deleted from a module that still calls it.

The `_interopRequireDefault` symptom from #641 no longer reproduces only because that helper now comes from `babel/runtime` as a `var`, which constant folding never deletes; the bug itself is still present.

## Fix

- `cloneInputAst: false` in the main pass.
- `traverse.cache.clearScope()` right before the prod-only constant-folding pass, so that pass crawls fresh scopes. Dev builds skip that pass and pay nothing.
- `traverse` is imported from `babel/core`, which re-exports the instance Babel uses internally, so the clear always hits the cache Babel reads, and no new dependency is needed.
- Removes the `transformer.unstable_disableInputAstCloning` option from D123839895.
- Tests in `index-test.js`:
  - `keeps functions referenced by earlier passes in production`: a prod transform whose Babel transformer adds a call to `stringifySafe` after the scope crawl. It fails if the clear is removed.
  - `Babel keeps stale scope bindings when reusing an AST`: asserts the Babel behavior the clear works around. When Babel stops reusing stale scopes, it fails, and its comment and the comment above the clear say to delete both.
- The test file now uses the real constant-folding plugin instead of a no-op mock; all existing tests and snapshots are unchanged.

## Measurements

Measured with `scripts/benchmark-patches/benchmark-patches.js bundling_speed` from D122997558: 4 candidate fixes against the unpatched tree, on three Meta production apps, referred to as Meta real apps 1–3 (`app1`–`app3` below): app1 android and app2 ios, and separately app3 android (46,534 modules), each in prod and dev, 5 cold builds each.

Variants (all set `cloneInputAst: false` in the worker's main Babel pass):
- `clear-before-transform`: `traverse.cache.clear()` before the main pass.
- `clear-before-constant-folding`: `traverse.cache.clear()` before the prod-only constant-folding pass.
- `clearscope-before-transform`: `traverse.cache.clearScope()` before the main pass.
- `clearscope-before-constant-folding`: `traverse.cache.clearScope()` before the prod-only constant-folding pass. This is exactly this diff's `index.js` change relative to D122997558.

| Bundle | Variant | Wall (s) | Wall Δ (t) | CPU (s) | CPU Δ (t) | Size (bytes) | Same as unpatched |
|---|---|--:|--:|--:|--:|--:|---|
| app1-android-prod | unpatched | 207.5 ± 4.6 | | 2956.1 ± 74.0 | | 37,357,607 | yes |
| | clear-before-constant-folding | 204.4 ± 5.0 | -1.48% (-0.91) | 2854.2 ± 41.9 | -3.45% (-2.40) | 37,357,607 | yes |
| | clear-before-transform | 203.5 ± 4.1 | -1.91% (-1.29) | 2856.4 ± 42.4 | -3.37% (-2.34) | 37,357,607 | yes |
| | clearscope-before-constant-folding | 204.4 ± 3.9 | -1.48% (-1.02) | 2853.2 ± 23.5 | -3.48% (-2.65) | 37,357,607 | yes |
| | clearscope-before-transform | 210.0 ± 6.6 | +1.20% (+0.61) | 2917.8 ± 87.5 | -1.30% (-0.67) | 37,357,607 | yes |
| app1-android-dev | unpatched | 183.4 ± 6.4 | | 2454.7 ± 66.2 | | 95,688,124 | yes |
| | clear-before-constant-folding | 178.2 ± 5.6 | -2.83% (-1.21) | 2304.4 ± 82.6 | -6.12% (-2.84) | 95,688,124 | yes |
| | clear-before-transform | 182.6 ± 4.1 | -0.42% (-0.19) | 2391.7 ± 66.4 | -2.57% (-1.34) | 95,688,124 | yes |
| | clearscope-before-constant-folding | 177.2 ± 3.8 | -3.37% (-1.65) | 2320.7 ± 59.9 | -5.46% (-3.00) | 95,688,124 | yes |
| | clearscope-before-transform | 182.0 ± 4.1 | -0.73% (-0.34) | 2367.3 ± 70.9 | -3.56% (-1.80) | 95,688,124 | yes |
| app2-ios-prod | unpatched | 153.5 ± 2.4 | | 1502.7 ± 34.0 | | 18,768,596 | yes |
| | clear-before-constant-folding | 148.1 ± 3.2 | -3.53% (-2.72) | 1456.3 ± 43.6 | -3.09% (-1.67) | 18,768,596 | yes |
| | clear-before-transform | 148.5 ± 2.9 | -3.23% (-2.61) | 1430.8 ± 20.5 | -4.79% (-3.62) | 18,768,596 | yes |
| | clearscope-before-constant-folding | 143.8 ± 3.5 | -6.33% (-4.56) | 1402.6 ± 26.1 | -6.66% (-4.67) | 18,768,596 | yes |
| | clearscope-before-transform | 144.9 ± 2.5 | -5.57% (-4.88) | 1431.0 ± 16.4 | -4.77% (-3.80) | 18,768,596 | yes |
| app2-ios-dev | unpatched | 132.5 ± 1.9 | | 1135.5 ± 14.8 | | 48,693,569 | yes |
| | clear-before-constant-folding | 126.3 ± 2.0 | -4.68% (-4.51) | 1073.1 ± 10.8 | -5.50% (-6.83) | 48,693,569 | yes |
| | clear-before-transform | 127.4 ± 2.8 | -3.92% (-3.06) | 1084.7 ± 19.8 | -4.47% (-4.10) | 48,693,569 | yes |
| | clearscope-before-constant-folding | 124.7 ± 4.3 | -5.89% (-3.33) | 1057.6 ± 37.0 | -6.86% (-3.91) | 48,693,569 | yes |
| | clearscope-before-transform | 131.0 ± 4.9 | -1.20% (-0.61) | 1114.8 ± 50.1 | -1.82% (-0.79) | 48,693,569 | yes |
| app3-android-prod | unpatched | 215.3 ± 4.2 | | 3479.3 ± 35.7 | | 114,719,789 | yes |
| | clear-before-constant-folding | 210.9 ± 3.2 | -2.05% (-1.9) | 3371.2 ± 61.2 | -3.11% (-3.4) | 114,719,789 | yes |
| | clear-before-transform | 212.1 ± 5.3 | -1.48% (-1.1) | 3415.7 ± 91.2 | -1.83% (-1.5) | 114,719,789 | yes |
| | clearscope-before-constant-folding | 211.2 ± 4.6 | -1.91% (-1.5) | 3382.7 ± 71.0 | -2.78% (-2.7) | 114,719,789 | yes |
| | clearscope-before-transform | 226.0 ± 20.2 | +4.99% (+1.2) | 3633.0 ± 339.7 | +4.42% (+1.0) | 114,719,789 | yes |
| app3-android-dev | unpatched | 192.3 ± 7.0 | | 2953.4 ± 130.4 | | 289,411,642 | yes |
| | clear-before-constant-folding | 184.4 ± 4.2 | -4.12% (-2.2) | 2777.0 ± 78.3 | -5.97% (-2.6) | 289,411,642 | yes |
| | clear-before-transform | 185.8 ± 6.7 | -3.39% (-1.5) | 2826.0 ± 102.1 | -4.31% (-1.7) | 289,411,642 | yes |
| | clearscope-before-constant-folding | 184.4 ± 8.1 | -4.09% (-1.6) | 2803.5 ± 113.8 | -5.08% (-1.9) | 289,411,642 | yes |
| | clearscope-before-transform | 185.6 ± 5.7 | -3.46% (-1.7) | 2823.2 ± 92.1 | -4.41% (-1.8) | 289,411,642 | yes |

5 measured runs per variant, each a cold build (`--reset-cache --no-use-saved-state`, `--max-workers=16`) on a 72-core Linux machine. CPU is user + sys of the build and its workers. Values are mean ± sd; t is Welch's t vs unpatched (at 5 runs, |t| < 2.3 is not significant). "Same as unpatched" compares bundle hashes.

app1 and app2 were measured with an earlier revision of the script that ran variants in a fixed order (unpatched first) and printed population sd; their t values were computed afterwards from the same per-run timings. app3 was measured with the current revision: rotating variant order, sample sd, and byte identity checked on every run.

## Analysis

1. All four candidates produce bundles byte-identical to unpatched in all 6 bundle/mode combinations (for app3, in every run), so each one fixes the stale-scope deletion that raw `cloneInputAst: false` causes in the app1 prod bundle.
2. Not cloning saves CPU: every candidate uses less CPU than unpatched everywhere except `clearscope-before-transform` on app3 prod. The size of the effect is similar across apps: for the two before-constant-folding candidates, 2.8–6.9% CPU and 1.5–6.3% wall. Wall-time deltas are mostly within noise at 5 runs; CPU deltas are mostly significant.
3. Where the clear goes matters more than which clear is used. Clearing before the main pass discards the scopes the Babel transformer just built, so the main pass re-crawls every module, in dev too. The two "before-constant-folding" variants never clear in dev and are faster there on app1 (177–178s vs 182–183s) and app2 (125–126s vs 127–131s); on app3 dev all four are within noise (184.4–185.8s).
4. `clearScope()` vs `clear()` is within noise. In dev the two before-constant-folding variants run identical code, and they still differ by up to 1.4% wall (app3 dev: 184.4s vs 184.4s, 2777s vs 2804s CPU). In prod, `clearScope()` is ahead on app2 (143.8s vs 148.1s), level on app1 and slightly behind on app3 (-2.78% vs -3.11% CPU). `clearScope()` discards less, since it keeps the path cache.
5. `clearscope-before-transform` is erratic: two of its five app3 prod builds took 245s and 251s, using about 15% more CPU, while the unpatched builds next to them were normal. It also had the largest spread on app1 prod.
6. `clearscope-before-constant-folding` has the best average and a significant CPU saving in 5 of 6 combinations (all except app3 dev, t -1.9); `clear-before-constant-folding` is close behind, also significant in 5 of 6 (all except app2 prod, t -1.67).

Averaged over app1, app2 and app3, prod and dev (6 combinations):

| Candidate | Avg wall Δ | Avg CPU Δ |
|---|--:|--:|
| `clearscope-before-constant-folding` (this diff) | -3.85% | -5.05% |
| `clear-before-constant-folding` | -3.12% | -4.54% |
| `clear-before-transform` | -2.39% | -3.56% |
| `clearscope-before-transform` | -0.80% | -1.91% |

This diff implements `clearscope-before-constant-folding`. On app3, the largest app, it saves about 2% wall and 3% CPU in prod, and 4% wall and 5% CPU in dev.

Caveats: 5 runs per variant; the app1 and app2 run used a fixed variant order (unpatched first in each round), while the app3 run rotated it; most wall-time deltas are within noise.

Changelog: [Performance] `metro-transform-worker` no longer deep-clones each module's AST before the main Babel pass, reducing cold build CPU time by about 5%.

Differential Revision: D123636983
@meta-codesync
meta-codesync Bot force-pushed the export-D123636983 branch from ff77d81 to 9abde35 Compare October 9, 2026 07:07
meta-codesync Bot pushed a commit that referenced this pull request Oct 9, 2026
@vzaidman @meta-codesync
Stop cloning the AST in the transform worker's main Babel pass (#2034)
5f4a64a 
Summary:
Pull Request resolved: #2034

`transformJS` runs the main Babel pass with `cloneInputAst: true`, deep-copying every module's AST although nothing reads the original afterwards. It was kept because `false` broke production bundles (#641). This diff turns cloning off and fixes the underlying bug, saving about 4% wall time and 5% CPU on average in cold builds of three Meta production apps, with byte-identical bundles. D123839895 puts the same change behind an option; this diff makes it unconditional and removes the option.

## Root cause

`babel/traverse` caches paths and scopes globally, keyed by AST node. When a later pass runs on the same, uncloned nodes, Babel reuses the scope crawled by an earlier pass instead of crawling again. Code inserted after that crawl, such as a helper Babel adds while lowering ESM or a call introduced by a plugin, is missing from the cached bindings. The prod-only constant-folding pass deletes every function declaration whose binding is unreferenced, so it deletes live functions while their calls remain:
- `import * as ns` (or duplicate imports from one module) without `experimentalImportSupport`: the inlined `_interopRequireWildcard` helper is deleted.
- In the Meta real app 1 bundle, `stringifySafe` is deleted from a module that still calls it.

The `_interopRequireDefault` symptom from #641 no longer reproduces only because that helper now comes from `babel/runtime` as a `var`, which constant folding never deletes; the bug itself is still present.

## Fix

- `cloneInputAst: false` in the main pass.
- `traverse.cache.clearScope()` right before the prod-only constant-folding pass, so that pass crawls fresh scopes. Dev builds skip that pass and pay nothing.
- `traverse` is imported from `babel/core`, which re-exports the instance Babel uses internally, so the clear always hits the cache Babel reads, and no new dependency is needed.
- Removes the `transformer.unstable_disableInputAstCloning` option from D123839895.
- Tests in `index-test.js`:
  - `keeps functions referenced by earlier passes in production`: a prod transform whose Babel transformer adds a call to `stringifySafe` after the scope crawl. It fails if the clear is removed.
  - `Babel keeps stale scope bindings when reusing an AST`: asserts the Babel behavior the clear works around. When Babel stops reusing stale scopes, it fails, and its comment and the comment above the clear say to delete both.
- The test file now uses the real constant-folding plugin instead of a no-op mock; all existing tests and snapshots are unchanged.

## Measurements

Measured with `scripts/benchmark-patches/benchmark-patches.js bundling_speed` from D122997558: 4 candidate fixes against the unpatched tree, on three Meta production apps, referred to as Meta real apps 1–3 (`app1`–`app3` below): app1 android and app2 ios, and separately app3 android (46,534 modules), each in prod and dev, 5 cold builds each.

Variants (all set `cloneInputAst: false` in the worker's main Babel pass):
- `clear-before-transform`: `traverse.cache.clear()` before the main pass.
- `clear-before-constant-folding`: `traverse.cache.clear()` before the prod-only constant-folding pass.
- `clearscope-before-transform`: `traverse.cache.clearScope()` before the main pass.
- `clearscope-before-constant-folding`: `traverse.cache.clearScope()` before the prod-only constant-folding pass. This is exactly this diff's `index.js` change relative to D122997558.

| Bundle | Variant | Wall (s) | Wall Δ (t) | CPU (s) | CPU Δ (t) | Size (bytes) | Same as unpatched |
|---|---|--:|--:|--:|--:|--:|---|
| app1-android-prod | unpatched | 207.5 ± 4.6 | | 2956.1 ± 74.0 | | 37,357,607 | yes |
| | clear-before-constant-folding | 204.4 ± 5.0 | -1.48% (-0.91) | 2854.2 ± 41.9 | -3.45% (-2.40) | 37,357,607 | yes |
| | clear-before-transform | 203.5 ± 4.1 | -1.91% (-1.29) | 2856.4 ± 42.4 | -3.37% (-2.34) | 37,357,607 | yes |
| | clearscope-before-constant-folding | 204.4 ± 3.9 | -1.48% (-1.02) | 2853.2 ± 23.5 | -3.48% (-2.65) | 37,357,607 | yes |
| | clearscope-before-transform | 210.0 ± 6.6 | +1.20% (+0.61) | 2917.8 ± 87.5 | -1.30% (-0.67) | 37,357,607 | yes |
| app1-android-dev | unpatched | 183.4 ± 6.4 | | 2454.7 ± 66.2 | | 95,688,124 | yes |
| | clear-before-constant-folding | 178.2 ± 5.6 | -2.83% (-1.21) | 2304.4 ± 82.6 | -6.12% (-2.84) | 95,688,124 | yes |
| | clear-before-transform | 182.6 ± 4.1 | -0.42% (-0.19) | 2391.7 ± 66.4 | -2.57% (-1.34) | 95,688,124 | yes |
| | clearscope-before-constant-folding | 177.2 ± 3.8 | -3.37% (-1.65) | 2320.7 ± 59.9 | -5.46% (-3.00) | 95,688,124 | yes |
| | clearscope-before-transform | 182.0 ± 4.1 | -0.73% (-0.34) | 2367.3 ± 70.9 | -3.56% (-1.80) | 95,688,124 | yes |
| app2-ios-prod | unpatched | 153.5 ± 2.4 | | 1502.7 ± 34.0 | | 18,768,596 | yes |
| | clear-before-constant-folding | 148.1 ± 3.2 | -3.53% (-2.72) | 1456.3 ± 43.6 | -3.09% (-1.67) | 18,768,596 | yes |
| | clear-before-transform | 148.5 ± 2.9 | -3.23% (-2.61) | 1430.8 ± 20.5 | -4.79% (-3.62) | 18,768,596 | yes |
| | clearscope-before-constant-folding | 143.8 ± 3.5 | -6.33% (-4.56) | 1402.6 ± 26.1 | -6.66% (-4.67) | 18,768,596 | yes |
| | clearscope-before-transform | 144.9 ± 2.5 | -5.57% (-4.88) | 1431.0 ± 16.4 | -4.77% (-3.80) | 18,768,596 | yes |
| app2-ios-dev | unpatched | 132.5 ± 1.9 | | 1135.5 ± 14.8 | | 48,693,569 | yes |
| | clear-before-constant-folding | 126.3 ± 2.0 | -4.68% (-4.51) | 1073.1 ± 10.8 | -5.50% (-6.83) | 48,693,569 | yes |
| | clear-before-transform | 127.4 ± 2.8 | -3.92% (-3.06) | 1084.7 ± 19.8 | -4.47% (-4.10) | 48,693,569 | yes |
| | clearscope-before-constant-folding | 124.7 ± 4.3 | -5.89% (-3.33) | 1057.6 ± 37.0 | -6.86% (-3.91) | 48,693,569 | yes |
| | clearscope-before-transform | 131.0 ± 4.9 | -1.20% (-0.61) | 1114.8 ± 50.1 | -1.82% (-0.79) | 48,693,569 | yes |
| app3-android-prod | unpatched | 215.3 ± 4.2 | | 3479.3 ± 35.7 | | 114,719,789 | yes |
| | clear-before-constant-folding | 210.9 ± 3.2 | -2.05% (-1.9) | 3371.2 ± 61.2 | -3.11% (-3.4) | 114,719,789 | yes |
| | clear-before-transform | 212.1 ± 5.3 | -1.48% (-1.1) | 3415.7 ± 91.2 | -1.83% (-1.5) | 114,719,789 | yes |
| | clearscope-before-constant-folding | 211.2 ± 4.6 | -1.91% (-1.5) | 3382.7 ± 71.0 | -2.78% (-2.7) | 114,719,789 | yes |
| | clearscope-before-transform | 226.0 ± 20.2 | +4.99% (+1.2) | 3633.0 ± 339.7 | +4.42% (+1.0) | 114,719,789 | yes |
| app3-android-dev | unpatched | 192.3 ± 7.0 | | 2953.4 ± 130.4 | | 289,411,642 | yes |
| | clear-before-constant-folding | 184.4 ± 4.2 | -4.12% (-2.2) | 2777.0 ± 78.3 | -5.97% (-2.6) | 289,411,642 | yes |
| | clear-before-transform | 185.8 ± 6.7 | -3.39% (-1.5) | 2826.0 ± 102.1 | -4.31% (-1.7) | 289,411,642 | yes |
| | clearscope-before-constant-folding | 184.4 ± 8.1 | -4.09% (-1.6) | 2803.5 ± 113.8 | -5.08% (-1.9) | 289,411,642 | yes |
| | clearscope-before-transform | 185.6 ± 5.7 | -3.46% (-1.7) | 2823.2 ± 92.1 | -4.41% (-1.8) | 289,411,642 | yes |

5 measured runs per variant, each a cold build (`--reset-cache --no-use-saved-state`, `--max-workers=16`) on a 72-core Linux machine. CPU is user + sys of the build and its workers. Values are mean ± sd; t is Welch's t vs unpatched (at 5 runs, |t| < 2.3 is not significant). "Same as unpatched" compares bundle hashes.

app1 and app2 were measured with an earlier revision of the script that ran variants in a fixed order (unpatched first) and printed population sd; their t values were computed afterwards from the same per-run timings. app3 was measured with the current revision: rotating variant order, sample sd, and byte identity checked on every run.

## Analysis

1. All four candidates produce bundles byte-identical to unpatched in all 6 bundle/mode combinations (for app3, in every run), so each one fixes the stale-scope deletion that raw `cloneInputAst: false` causes in the app1 prod bundle.
2. Not cloning saves CPU: every candidate uses less CPU than unpatched everywhere except `clearscope-before-transform` on app3 prod. The size of the effect is similar across apps: for the two before-constant-folding candidates, 2.8–6.9% CPU and 1.5–6.3% wall. Wall-time deltas are mostly within noise at 5 runs; CPU deltas are mostly significant.
3. Where the clear goes matters more than which clear is used. Clearing before the main pass discards the scopes the Babel transformer just built, so the main pass re-crawls every module, in dev too. The two "before-constant-folding" variants never clear in dev and are faster there on app1 (177–178s vs 182–183s) and app2 (125–126s vs 127–131s); on app3 dev all four are within noise (184.4–185.8s).
4. `clearScope()` vs `clear()` is within noise. In dev the two before-constant-folding variants run identical code, and they still differ by up to 1.4% wall (app3 dev: 184.4s vs 184.4s, 2777s vs 2804s CPU). In prod, `clearScope()` is ahead on app2 (143.8s vs 148.1s), level on app1 and slightly behind on app3 (-2.78% vs -3.11% CPU). `clearScope()` discards less, since it keeps the path cache.
5. `clearscope-before-transform` is erratic: two of its five app3 prod builds took 245s and 251s, using about 15% more CPU, while the unpatched builds next to them were normal. It also had the largest spread on app1 prod.
6. `clearscope-before-constant-folding` has the best average and a significant CPU saving in 5 of 6 combinations (all except app3 dev, t -1.9); `clear-before-constant-folding` is close behind, also significant in 5 of 6 (all except app2 prod, t -1.67).

Averaged over app1, app2 and app3, prod and dev (6 combinations):

| Candidate | Avg wall Δ | Avg CPU Δ |
|---|--:|--:|
| `clearscope-before-constant-folding` (this diff) | -3.85% | -5.05% |
| `clear-before-constant-folding` | -3.12% | -4.54% |
| `clear-before-transform` | -2.39% | -3.56% |
| `clearscope-before-transform` | -0.80% | -1.91% |

This diff implements `clearscope-before-constant-folding`. On app3, the largest app, it saves about 2% wall and 3% CPU in prod, and 4% wall and 5% CPU in dev.

Caveats: 5 runs per variant; the app1 and app2 run used a fixed variant order (unpatched first in each round), while the app3 run rotated it; most wall-time deltas are within noise.

Changelog: [Performance] `metro-transform-worker` no longer deep-clones each module's AST before the main Babel pass, reducing cold build CPU time by about 5%.

Differential Revision: D123636983
@meta-codesync
meta-codesync Bot force-pushed the export-D123636983 branch from 9abde35 to 5f4a64a Compare October 9, 2026 07:10
robhogan
robhogan approved these changes Oct 24, 2026
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LGTM once Flow’s fixed - can we get this in?

meta-codesync Bot pushed a commit that referenced this pull request Oct 27, 2026
@vzaidman @facebook-github-bot
Stop cloning the AST in the transform worker's main Babel pass (#2034)
10d6b2e 
Summary:

`transformJS` runs the main Babel pass with `cloneInputAst: true`, deep-copying every module's AST although nothing reads the original afterwards. It was kept because `false` broke production bundles (#641). This diff turns cloning off and fixes the underlying bug, saving about 4% wall time and 5% CPU on average in cold builds of three Meta production apps, with byte-identical bundles. D123839895 puts the same change behind an option; this diff makes it unconditional and removes the option.

## Root cause

`babel/traverse` caches paths and scopes globally, keyed by AST node. When a later pass runs on the same, uncloned nodes, Babel reuses the scope crawled by an earlier pass instead of crawling again. Code inserted after that crawl, such as a helper Babel adds while lowering ESM or a call introduced by a plugin, is missing from the cached bindings. The prod-only constant-folding pass deletes every function declaration whose binding is unreferenced, so it deletes live functions while their calls remain:
- `import * as ns` (or duplicate imports from one module) without `experimentalImportSupport`: the inlined `_interopRequireWildcard` helper is deleted.
- In the Meta real app 1 bundle, `stringifySafe` is deleted from a module that still calls it.

The `_interopRequireDefault` symptom from #641 no longer reproduces only because that helper now comes from `babel/runtime` as a `var`, which constant folding never deletes; the bug itself is still present.

## Fix

- `cloneInputAst: false` in the main pass.
- `traverse.cache.clearScope()` right before the prod-only constant-folding pass, so that pass crawls fresh scopes. Dev builds skip that pass and pay nothing.
- `traverse` is imported from `babel/core`, which re-exports the instance Babel uses internally, so the clear always hits the cache Babel reads, and no new dependency is needed.
- Removes the `transformer.unstable_disableInputAstCloning` option from D123839895.
- Tests in `index-test.js`:
  - `keeps functions referenced by earlier passes in production`: a prod transform whose Babel transformer adds a call to `stringifySafe` after the scope crawl. It fails if the clear is removed.
  - `Babel keeps stale scope bindings when reusing an AST`: asserts the Babel behavior the clear works around. When Babel stops reusing stale scopes, it fails, and its comment and the comment above the clear say to delete both.
- The test file now uses the real constant-folding plugin instead of a no-op mock; all existing tests and snapshots are unchanged.

## Measurements

Measured with `scripts/benchmark-patches/benchmark-patches.js bundling_speed` from D122997558: 4 candidate fixes against the unpatched tree, on three Meta production apps, referred to as Meta real apps 1–3 (`app1`–`app3` below): app1 android and app2 ios, and separately app3 android (46,534 modules), each in prod and dev, 5 cold builds each.

Variants (all set `cloneInputAst: false` in the worker's main Babel pass):
- `clear-before-transform`: `traverse.cache.clear()` before the main pass.
- `clear-before-constant-folding`: `traverse.cache.clear()` before the prod-only constant-folding pass.
- `clearscope-before-transform`: `traverse.cache.clearScope()` before the main pass.
- `clearscope-before-constant-folding`: `traverse.cache.clearScope()` before the prod-only constant-folding pass. This is exactly this diff's `index.js` change relative to D122997558.

| Bundle | Variant | Wall (s) | Wall Δ (t) | CPU (s) | CPU Δ (t) | Size (bytes) | Same as unpatched |
|---|---|--:|--:|--:|--:|--:|---|
| app1-android-prod | unpatched | 207.5 ± 4.6 | | 2956.1 ± 74.0 | | 37,357,607 | yes |
| | clear-before-constant-folding | 204.4 ± 5.0 | -1.48% (-0.91) | 2854.2 ± 41.9 | -3.45% (-2.40) | 37,357,607 | yes |
| | clear-before-transform | 203.5 ± 4.1 | -1.91% (-1.29) | 2856.4 ± 42.4 | -3.37% (-2.34) | 37,357,607 | yes |
| | clearscope-before-constant-folding | 204.4 ± 3.9 | -1.48% (-1.02) | 2853.2 ± 23.5 | -3.48% (-2.65) | 37,357,607 | yes |
| | clearscope-before-transform | 210.0 ± 6.6 | +1.20% (+0.61) | 2917.8 ± 87.5 | -1.30% (-0.67) | 37,357,607 | yes |
| app1-android-dev | unpatched | 183.4 ± 6.4 | | 2454.7 ± 66.2 | | 95,688,124 | yes |
| | clear-before-constant-folding | 178.2 ± 5.6 | -2.83% (-1.21) | 2304.4 ± 82.6 | -6.12% (-2.84) | 95,688,124 | yes |
| | clear-before-transform | 182.6 ± 4.1 | -0.42% (-0.19) | 2391.7 ± 66.4 | -2.57% (-1.34) | 95,688,124 | yes |
| | clearscope-before-constant-folding | 177.2 ± 3.8 | -3.37% (-1.65) | 2320.7 ± 59.9 | -5.46% (-3.00) | 95,688,124 | yes |
| | clearscope-before-transform | 182.0 ± 4.1 | -0.73% (-0.34) | 2367.3 ± 70.9 | -3.56% (-1.80) | 95,688,124 | yes |
| app2-ios-prod | unpatched | 153.5 ± 2.4 | | 1502.7 ± 34.0 | | 18,768,596 | yes |
| | clear-before-constant-folding | 148.1 ± 3.2 | -3.53% (-2.72) | 1456.3 ± 43.6 | -3.09% (-1.67) | 18,768,596 | yes |
| | clear-before-transform | 148.5 ± 2.9 | -3.23% (-2.61) | 1430.8 ± 20.5 | -4.79% (-3.62) | 18,768,596 | yes |
| | clearscope-before-constant-folding | 143.8 ± 3.5 | -6.33% (-4.56) | 1402.6 ± 26.1 | -6.66% (-4.67) | 18,768,596 | yes |
| | clearscope-before-transform | 144.9 ± 2.5 | -5.57% (-4.88) | 1431.0 ± 16.4 | -4.77% (-3.80) | 18,768,596 | yes |
| app2-ios-dev | unpatched | 132.5 ± 1.9 | | 1135.5 ± 14.8 | | 48,693,569 | yes |
| | clear-before-constant-folding | 126.3 ± 2.0 | -4.68% (-4.51) | 1073.1 ± 10.8 | -5.50% (-6.83) | 48,693,569 | yes |
| | clear-before-transform | 127.4 ± 2.8 | -3.92% (-3.06) | 1084.7 ± 19.8 | -4.47% (-4.10) | 48,693,569 | yes |
| | clearscope-before-constant-folding | 124.7 ± 4.3 | -5.89% (-3.33) | 1057.6 ± 37.0 | -6.86% (-3.91) | 48,693,569 | yes |
| | clearscope-before-transform | 131.0 ± 4.9 | -1.20% (-0.61) | 1114.8 ± 50.1 | -1.82% (-0.79) | 48,693,569 | yes |
| app3-android-prod | unpatched | 215.3 ± 4.2 | | 3479.3 ± 35.7 | | 114,719,789 | yes |
| | clear-before-constant-folding | 210.9 ± 3.2 | -2.05% (-1.9) | 3371.2 ± 61.2 | -3.11% (-3.4) | 114,719,789 | yes |
| | clear-before-transform | 212.1 ± 5.3 | -1.48% (-1.1) | 3415.7 ± 91.2 | -1.83% (-1.5) | 114,719,789 | yes |
| | clearscope-before-constant-folding | 211.2 ± 4.6 | -1.91% (-1.5) | 3382.7 ± 71.0 | -2.78% (-2.7) | 114,719,789 | yes |
| | clearscope-before-transform | 226.0 ± 20.2 | +4.99% (+1.2) | 3633.0 ± 339.7 | +4.42% (+1.0) | 114,719,789 | yes |
| app3-android-dev | unpatched | 192.3 ± 7.0 | | 2953.4 ± 130.4 | | 289,411,642 | yes |
| | clear-before-constant-folding | 184.4 ± 4.2 | -4.12% (-2.2) | 2777.0 ± 78.3 | -5.97% (-2.6) | 289,411,642 | yes |
| | clear-before-transform | 185.8 ± 6.7 | -3.39% (-1.5) | 2826.0 ± 102.1 | -4.31% (-1.7) | 289,411,642 | yes |
| | clearscope-before-constant-folding | 184.4 ± 8.1 | -4.09% (-1.6) | 2803.5 ± 113.8 | -5.08% (-1.9) | 289,411,642 | yes |
| | clearscope-before-transform | 185.6 ± 5.7 | -3.46% (-1.7) | 2823.2 ± 92.1 | -4.41% (-1.8) | 289,411,642 | yes |

5 measured runs per variant, each a cold build (`--reset-cache --no-use-saved-state`, `--max-workers=16`) on a 72-core Linux machine. CPU is user + sys of the build and its workers. Values are mean ± sd; t is Welch's t vs unpatched (at 5 runs, |t| < 2.3 is not significant). "Same as unpatched" compares bundle hashes.

app1 and app2 were measured with an earlier revision of the script that ran variants in a fixed order (unpatched first) and printed population sd; their t values were computed afterwards from the same per-run timings. app3 was measured with the current revision: rotating variant order, sample sd, and byte identity checked on every run.

## Analysis

1. All four candidates produce bundles byte-identical to unpatched in all 6 bundle/mode combinations (for app3, in every run), so each one fixes the stale-scope deletion that raw `cloneInputAst: false` causes in the app1 prod bundle.
2. Not cloning saves CPU: every candidate uses less CPU than unpatched everywhere except `clearscope-before-transform` on app3 prod. The size of the effect is similar across apps: for the two before-constant-folding candidates, 2.8–6.9% CPU and 1.5–6.3% wall. Wall-time deltas are mostly within noise at 5 runs; CPU deltas are mostly significant.
3. Where the clear goes matters more than which clear is used. Clearing before the main pass discards the scopes the Babel transformer just built, so the main pass re-crawls every module, in dev too. The two "before-constant-folding" variants never clear in dev and are faster there on app1 (177–178s vs 182–183s) and app2 (125–126s vs 127–131s); on app3 dev all four are within noise (184.4–185.8s).
4. `clearScope()` vs `clear()` is within noise. In dev the two before-constant-folding variants run identical code, and they still differ by up to 1.4% wall (app3 dev: 184.4s vs 184.4s, 2777s vs 2804s CPU). In prod, `clearScope()` is ahead on app2 (143.8s vs 148.1s), level on app1 and slightly behind on app3 (-2.78% vs -3.11% CPU). `clearScope()` discards less, since it keeps the path cache.
5. `clearscope-before-transform` is erratic: two of its five app3 prod builds took 245s and 251s, using about 15% more CPU, while the unpatched builds next to them were normal. It also had the largest spread on app1 prod.
6. `clearscope-before-constant-folding` has the best average and a significant CPU saving in 5 of 6 combinations (all except app3 dev, t -1.9); `clear-before-constant-folding` is close behind, also significant in 5 of 6 (all except app2 prod, t -1.67).

Averaged over app1, app2 and app3, prod and dev (6 combinations):

| Candidate | Avg wall Δ | Avg CPU Δ |
|---|--:|--:|
| `clearscope-before-constant-folding` (this diff) | -3.85% | -5.05% |
| `clear-before-constant-folding` | -3.12% | -4.54% |
| `clear-before-transform` | -2.39% | -3.56% |
| `clearscope-before-transform` | -0.80% | -1.91% |

This diff implements `clearscope-before-constant-folding`. On app3, the largest app, it saves about 2% wall and 3% CPU in prod, and 4% wall and 5% CPU in dev.

Caveats: 5 runs per variant; the app1 and app2 run used a fixed variant order (unpatched first in each round), while the app3 run rotated it; most wall-time deltas are within noise.

Changelog: [Performance] `metro-transform-worker` no longer deep-clones each module's AST before the main Babel pass, reducing cold build CPU time by about 5%.

Differential Revision: D123636983
@vzaidman

vzaidman commented Oct 27, 2026

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@robhogan I wanted to experiment with it in #2033. Do you mind approving that one instead?
I want to see the difference between my benchmarking script and what will be observed in the experiment. if you want I can ship it as true by default.

@vzaidman @facebook-github-bot
Add benchmark-patches script to compare bundling speed of patches (#2032
db4810a 
)

Summary:

Adds `scripts/benchmark-patches/benchmark-patches.js`, a script that runs a named benchmark unpatched and with each of a set of patches applied, and compares the results. The first benchmark, `bundling_speed`, measures how the patches affect Metro build speed and output on real bundles.

## Why

Performance changes in Metro are small (a few percent) and noisy (builds vary by ±2–7s run to run), so ad-hoc measurements easily produce wrong conclusions. While evaluating `cloneInputAst: false` in `metro-transform-worker`, two separate one-off investigations reached contradicting results: one measured the eventual fix as +3.4% slower on Meta real app 1, the other as no wall-time change. Both were artifacts of the method: a baseline measured hours earlier than the variants, no warmup build, 1–3 runs per variant, no variance or significance reported, a "prod" build without `--minify`, and an unbounded worker count.

This script gives a standardised way to benchmark patches that can be reviewed and improved once, instead of bespoke tests that each need to be validated. Different people can run the same benchmark on the same patches, locally or on a remote machine, and validate each other's results the same way: a diff's numbers are reproduced with one command.

## How

For every benchmark, `benchmark-patches.js`:
- measures the baseline in the same batch as the patches, and interleaves runs in a rotating order, so machine drift and position effects don't land on one variant;
- does one unmeasured warmup run per target;
- compares any number of candidate patches in one run: each is a diff, relative to the repository root, saved as `*.patch` in `patches/`;
- applies and reverts each patch around its run, including on Ctrl-C, so the working copy is left clean;
- keeps the last run's outputs and the results table in `dist/` for inspection.

Each benchmark is a module in `benchmarks/<name>.js` that exports `HELP`, `parseTargets`, `measure`, `describe` and `formatTable`, and is chosen explicitly by name on the command line. Other benchmarks, such as source map generation speed, can be added the same way and reuse the patch handling, run scheduling and output layout.

`bundling_speed` (`benchmarks/bundling_speed.js`):
- cold builds only (`--reset-cache --no-use-saved-state`), a fixed `--max-workers=16`, and real prod flags (`--minify`);
- every bundle is built in both prod and dev;
- CPU (user + sys, including workers) is reported next to wall time, as mean ± sd with Welch's t vs unpatched, so effects within noise are visible as such;
- every run's bundle is hashed and compared with the unpatched one. A speed-up that changes output (raw `cloneInputAst: false` silently deleted a live function in the Meta real app 1 prod bundle) shows up as `NO` instead of a win;
- the bundles and build logs of the last run stay in `dist/<variant>/`.

`dist/` and `patches/*.patch` are ignored by `.gitignore` files. Run `node scripts/benchmark-patches/benchmark-patches.js` for usage.

## Example: measuring D123636983 with `bundling_speed`

D123636983 (`cloneInputAst: false` in the worker's main Babel pass, plus a Babel scope-cache clear) was chosen with this script. Each candidate fix was saved as a patch:
```
scripts/benchmark-patches/patches/clear-before-constant-folding.patch
scripts/benchmark-patches/patches/clear-before-transform.patch
scripts/benchmark-patches/patches/clearscope-before-constant-folding.patch
scripts/benchmark-patches/patches/clearscope-before-transform.patch
```
and compared against the unpatched tree on three Meta production apps, referred to as Meta real apps 1–3 (`app1`–`app3` in the table):
```
node scripts/benchmark-patches/benchmark-patches.js bundling_speed --runs=5 <app1 entry>:android <app2 entry>:ios
```
That is 5 variants × 4 bundles (app1 android, app2 ios, each in prod and dev) × 5 runs = 100 measured builds, plus 4 warmup builds, about 5 hours. The same patches were then run on app3 android (46,534 modules; 50 measured builds, about 3 hours). The analysis of these results is in D123636983.

Variants (all set `cloneInputAst: false` in the worker's main Babel pass):
- `clear-before-transform`: `traverse.cache.clear()` before the main pass.
- `clear-before-constant-folding`: `traverse.cache.clear()` before the prod-only constant-folding pass.
- `clearscope-before-transform`: `traverse.cache.clearScope()` before the main pass.
- `clearscope-before-constant-folding`: `traverse.cache.clearScope()` before the prod-only constant-folding pass. This is D123636983's change.

| Bundle | Variant | Wall (s) | Wall Δ (t) | CPU (s) | CPU Δ (t) | Size (bytes) | Same as unpatched |
|---|---|--:|--:|--:|--:|--:|---|
| app1-android-prod | unpatched | 207.5 ± 4.6 | | 2956.1 ± 74.0 | | 37,357,607 | yes |
| | clear-before-constant-folding | 204.4 ± 5.0 | -1.48% (-0.91) | 2854.2 ± 41.9 | -3.45% (-2.40) | 37,357,607 | yes |
| | clear-before-transform | 203.5 ± 4.1 | -1.91% (-1.29) | 2856.4 ± 42.4 | -3.37% (-2.34) | 37,357,607 | yes |
| | clearscope-before-constant-folding | 204.4 ± 3.9 | -1.48% (-1.02) | 2853.2 ± 23.5 | -3.48% (-2.65) | 37,357,607 | yes |
| | clearscope-before-transform | 210.0 ± 6.6 | +1.20% (+0.61) | 2917.8 ± 87.5 | -1.30% (-0.67) | 37,357,607 | yes |
| app1-android-dev | unpatched | 183.4 ± 6.4 | | 2454.7 ± 66.2 | | 95,688,124 | yes |
| | clear-before-constant-folding | 178.2 ± 5.6 | -2.83% (-1.21) | 2304.4 ± 82.6 | -6.12% (-2.84) | 95,688,124 | yes |
| | clear-before-transform | 182.6 ± 4.1 | -0.42% (-0.19) | 2391.7 ± 66.4 | -2.57% (-1.34) | 95,688,124 | yes |
| | clearscope-before-constant-folding | 177.2 ± 3.8 | -3.37% (-1.65) | 2320.7 ± 59.9 | -5.46% (-3.00) | 95,688,124 | yes |
| | clearscope-before-transform | 182.0 ± 4.1 | -0.73% (-0.34) | 2367.3 ± 70.9 | -3.56% (-1.80) | 95,688,124 | yes |
| app2-ios-prod | unpatched | 153.5 ± 2.4 | | 1502.7 ± 34.0 | | 18,768,596 | yes |
| | clear-before-constant-folding | 148.1 ± 3.2 | -3.53% (-2.72) | 1456.3 ± 43.6 | -3.09% (-1.67) | 18,768,596 | yes |
| | clear-before-transform | 148.5 ± 2.9 | -3.23% (-2.61) | 1430.8 ± 20.5 | -4.79% (-3.62) | 18,768,596 | yes |
| | clearscope-before-constant-folding | 143.8 ± 3.5 | -6.33% (-4.56) | 1402.6 ± 26.1 | -6.66% (-4.67) | 18,768,596 | yes |
| | clearscope-before-transform | 144.9 ± 2.5 | -5.57% (-4.88) | 1431.0 ± 16.4 | -4.77% (-3.80) | 18,768,596 | yes |
| app2-ios-dev | unpatched | 132.5 ± 1.9 | | 1135.5 ± 14.8 | | 48,693,569 | yes |
| | clear-before-constant-folding | 126.3 ± 2.0 | -4.68% (-4.51) | 1073.1 ± 10.8 | -5.50% (-6.83) | 48,693,569 | yes |
| | clear-before-transform | 127.4 ± 2.8 | -3.92% (-3.06) | 1084.7 ± 19.8 | -4.47% (-4.10) | 48,693,569 | yes |
| | clearscope-before-constant-folding | 124.7 ± 4.3 | -5.89% (-3.33) | 1057.6 ± 37.0 | -6.86% (-3.91) | 48,693,569 | yes |
| | clearscope-before-transform | 131.0 ± 4.9 | -1.20% (-0.61) | 1114.8 ± 50.1 | -1.82% (-0.79) | 48,693,569 | yes |
| app3-android-prod | unpatched | 215.3 ± 4.2 | | 3479.3 ± 35.7 | | 114,719,789 | yes |
| | clear-before-constant-folding | 210.9 ± 3.2 | -2.05% (-1.9) | 3371.2 ± 61.2 | -3.11% (-3.4) | 114,719,789 | yes |
| | clear-before-transform | 212.1 ± 5.3 | -1.48% (-1.1) | 3415.7 ± 91.2 | -1.83% (-1.5) | 114,719,789 | yes |
| | clearscope-before-constant-folding | 211.2 ± 4.6 | -1.91% (-1.5) | 3382.7 ± 71.0 | -2.78% (-2.7) | 114,719,789 | yes |
| | clearscope-before-transform | 226.0 ± 20.2 | +4.99% (+1.2) | 3633.0 ± 339.7 | +4.42% (+1.0) | 114,719,789 | yes |
| app3-android-dev | unpatched | 192.3 ± 7.0 | | 2953.4 ± 130.4 | | 289,411,642 | yes |
| | clear-before-constant-folding | 184.4 ± 4.2 | -4.12% (-2.2) | 2777.0 ± 78.3 | -5.97% (-2.6) | 289,411,642 | yes |
| | clear-before-transform | 185.8 ± 6.7 | -3.39% (-1.5) | 2826.0 ± 102.1 | -4.31% (-1.7) | 289,411,642 | yes |
| | clearscope-before-constant-folding | 184.4 ± 8.1 | -4.09% (-1.6) | 2803.5 ± 113.8 | -5.08% (-1.9) | 289,411,642 | yes |
| | clearscope-before-transform | 185.6 ± 5.7 | -3.46% (-1.7) | 2823.2 ± 92.1 | -4.41% (-1.8) | 289,411,642 | yes |

5 measured runs per variant, each a cold build (`--reset-cache --no-use-saved-state`, `--max-workers=16`) on a 72-core Linux machine. CPU is user + sys of the build and its workers. Values are mean ± sd; t is Welch's t vs unpatched (at 5 runs, |t| < 2.3 is not significant). "Same as unpatched" compares bundle hashes.

app1 and app2 were measured with an earlier revision of the script that ran variants in a fixed order (unpatched first) and printed population sd; their t values were computed afterwards from the same per-run timings. app3 was measured with this revision: rotating variant order, sample sd, and output identity checked on every run.

Changelog: [Internal]

Differential Revision: D122997558
@vzaidman @facebook-github-bot
Experiment: skip AST cloning in the transform worker (#2033)
37f8b65 
Summary:

Adds `transformer.unstable_disableInputAstCloning` (default `false`), so the change in D123636983 can be tried out before it ships.

When the option is on, `transformJS`:
- runs the main Babel pass with `cloneInputAst: false` instead of `true`;
- calls `traverse.cache.clearScope()` before the prod-only constant-folding pass, so that pass doesn't reuse stale scopes from earlier passes over the same AST (#641).

The transformer config is part of Metro's transform cache key, so builds with and without the option don't share cache entries.

D123636983 makes the behavior unconditional, removes the option, and adds tests; its summary has the root cause and the measurements.

Changelog: [Internal]

Differential Revision: D123839895
@vzaidman @facebook-github-bot
Stop cloning the AST in the transform worker's main Babel pass (#2034)
355fcc4 
Summary:

`transformJS` runs the main Babel pass with `cloneInputAst: true`, deep-copying every module's AST although nothing reads the original afterwards. It was kept because `false` broke production bundles (#641). This diff turns cloning off and fixes the underlying bug, saving about 4% wall time and 5% CPU on average in cold builds of three Meta production apps, with byte-identical bundles. D123839895 puts the same change behind an option; this diff makes it unconditional and removes the option.

## Root cause

`babel/traverse` caches paths and scopes globally, keyed by AST node. When a later pass runs on the same, uncloned nodes, Babel reuses the scope crawled by an earlier pass instead of crawling again. Code inserted after that crawl, such as a helper Babel adds while lowering ESM or a call introduced by a plugin, is missing from the cached bindings. The prod-only constant-folding pass deletes every function declaration whose binding is unreferenced, so it deletes live functions while their calls remain:
- `import * as ns` (or duplicate imports from one module) without `experimentalImportSupport`: the inlined `_interopRequireWildcard` helper is deleted.
- In the Meta real app 1 bundle, `stringifySafe` is deleted from a module that still calls it.

The `_interopRequireDefault` symptom from #641 no longer reproduces only because that helper now comes from `babel/runtime` as a `var`, which constant folding never deletes; the bug itself is still present.

## Fix

- `cloneInputAst: false` in the main pass.
- `traverse.cache.clearScope()` right before the prod-only constant-folding pass, so that pass crawls fresh scopes. Dev builds skip that pass and pay nothing.
- `traverse` is imported from `babel/core`, which re-exports the instance Babel uses internally, so the clear always hits the cache Babel reads, and no new dependency is needed.
- Removes the `transformer.unstable_disableInputAstCloning` option from D123839895.
- Tests in `index-test.js`:
  - `keeps functions referenced by earlier passes in production`: a prod transform whose Babel transformer adds a call to `stringifySafe` after the scope crawl. It fails if the clear is removed.
  - `Babel keeps stale scope bindings when reusing an AST`: asserts the Babel behavior the clear works around. When Babel stops reusing stale scopes, it fails, and its comment and the comment above the clear say to delete both.
- The test file now uses the real constant-folding plugin instead of a no-op mock; all existing tests and snapshots are unchanged.

## Measurements

Measured with `scripts/benchmark-patches/benchmark-patches.js bundling_speed` from D122997558: 4 candidate fixes against the unpatched tree, on three Meta production apps, referred to as Meta real apps 1–3 (`app1`–`app3` below): app1 android and app2 ios, and separately app3 android (46,534 modules), each in prod and dev, 5 cold builds each.

Variants (all set `cloneInputAst: false` in the worker's main Babel pass):
- `clear-before-transform`: `traverse.cache.clear()` before the main pass.
- `clear-before-constant-folding`: `traverse.cache.clear()` before the prod-only constant-folding pass.
- `clearscope-before-transform`: `traverse.cache.clearScope()` before the main pass.
- `clearscope-before-constant-folding`: `traverse.cache.clearScope()` before the prod-only constant-folding pass. This is exactly this diff's `index.js` change relative to D122997558.

| Bundle | Variant | Wall (s) | Wall Δ (t) | CPU (s) | CPU Δ (t) | Size (bytes) | Same as unpatched |
|---|---|--:|--:|--:|--:|--:|---|
| app1-android-prod | unpatched | 207.5 ± 4.6 | | 2956.1 ± 74.0 | | 37,357,607 | yes |
| | clear-before-constant-folding | 204.4 ± 5.0 | -1.48% (-0.91) | 2854.2 ± 41.9 | -3.45% (-2.40) | 37,357,607 | yes |
| | clear-before-transform | 203.5 ± 4.1 | -1.91% (-1.29) | 2856.4 ± 42.4 | -3.37% (-2.34) | 37,357,607 | yes |
| | clearscope-before-constant-folding | 204.4 ± 3.9 | -1.48% (-1.02) | 2853.2 ± 23.5 | -3.48% (-2.65) | 37,357,607 | yes |
| | clearscope-before-transform | 210.0 ± 6.6 | +1.20% (+0.61) | 2917.8 ± 87.5 | -1.30% (-0.67) | 37,357,607 | yes |
| app1-android-dev | unpatched | 183.4 ± 6.4 | | 2454.7 ± 66.2 | | 95,688,124 | yes |
| | clear-before-constant-folding | 178.2 ± 5.6 | -2.83% (-1.21) | 2304.4 ± 82.6 | -6.12% (-2.84) | 95,688,124 | yes |
| | clear-before-transform | 182.6 ± 4.1 | -0.42% (-0.19) | 2391.7 ± 66.4 | -2.57% (-1.34) | 95,688,124 | yes |
| | clearscope-before-constant-folding | 177.2 ± 3.8 | -3.37% (-1.65) | 2320.7 ± 59.9 | -5.46% (-3.00) | 95,688,124 | yes |
| | clearscope-before-transform | 182.0 ± 4.1 | -0.73% (-0.34) | 2367.3 ± 70.9 | -3.56% (-1.80) | 95,688,124 | yes |
| app2-ios-prod | unpatched | 153.5 ± 2.4 | | 1502.7 ± 34.0 | | 18,768,596 | yes |
| | clear-before-constant-folding | 148.1 ± 3.2 | -3.53% (-2.72) | 1456.3 ± 43.6 | -3.09% (-1.67) | 18,768,596 | yes |
| | clear-before-transform | 148.5 ± 2.9 | -3.23% (-2.61) | 1430.8 ± 20.5 | -4.79% (-3.62) | 18,768,596 | yes |
| | clearscope-before-constant-folding | 143.8 ± 3.5 | -6.33% (-4.56) | 1402.6 ± 26.1 | -6.66% (-4.67) | 18,768,596 | yes |
| | clearscope-before-transform | 144.9 ± 2.5 | -5.57% (-4.88) | 1431.0 ± 16.4 | -4.77% (-3.80) | 18,768,596 | yes |
| app2-ios-dev | unpatched | 132.5 ± 1.9 | | 1135.5 ± 14.8 | | 48,693,569 | yes |
| | clear-before-constant-folding | 126.3 ± 2.0 | -4.68% (-4.51) | 1073.1 ± 10.8 | -5.50% (-6.83) | 48,693,569 | yes |
| | clear-before-transform | 127.4 ± 2.8 | -3.92% (-3.06) | 1084.7 ± 19.8 | -4.47% (-4.10) | 48,693,569 | yes |
| | clearscope-before-constant-folding | 124.7 ± 4.3 | -5.89% (-3.33) | 1057.6 ± 37.0 | -6.86% (-3.91) | 48,693,569 | yes |
| | clearscope-before-transform | 131.0 ± 4.9 | -1.20% (-0.61) | 1114.8 ± 50.1 | -1.82% (-0.79) | 48,693,569 | yes |
| app3-android-prod | unpatched | 215.3 ± 4.2 | | 3479.3 ± 35.7 | | 114,719,789 | yes |
| | clear-before-constant-folding | 210.9 ± 3.2 | -2.05% (-1.9) | 3371.2 ± 61.2 | -3.11% (-3.4) | 114,719,789 | yes |
| | clear-before-transform | 212.1 ± 5.3 | -1.48% (-1.1) | 3415.7 ± 91.2 | -1.83% (-1.5) | 114,719,789 | yes |
| | clearscope-before-constant-folding | 211.2 ± 4.6 | -1.91% (-1.5) | 3382.7 ± 71.0 | -2.78% (-2.7) | 114,719,789 | yes |
| | clearscope-before-transform | 226.0 ± 20.2 | +4.99% (+1.2) | 3633.0 ± 339.7 | +4.42% (+1.0) | 114,719,789 | yes |
| app3-android-dev | unpatched | 192.3 ± 7.0 | | 2953.4 ± 130.4 | | 289,411,642 | yes |
| | clear-before-constant-folding | 184.4 ± 4.2 | -4.12% (-2.2) | 2777.0 ± 78.3 | -5.97% (-2.6) | 289,411,642 | yes |
| | clear-before-transform | 185.8 ± 6.7 | -3.39% (-1.5) | 2826.0 ± 102.1 | -4.31% (-1.7) | 289,411,642 | yes |
| | clearscope-before-constant-folding | 184.4 ± 8.1 | -4.09% (-1.6) | 2803.5 ± 113.8 | -5.08% (-1.9) | 289,411,642 | yes |
| | clearscope-before-transform | 185.6 ± 5.7 | -3.46% (-1.7) | 2823.2 ± 92.1 | -4.41% (-1.8) | 289,411,642 | yes |

5 measured runs per variant, each a cold build (`--reset-cache --no-use-saved-state`, `--max-workers=16`) on a 72-core Linux machine. CPU is user + sys of the build and its workers. Values are mean ± sd; t is Welch's t vs unpatched (at 5 runs, |t| < 2.3 is not significant). "Same as unpatched" compares bundle hashes.

app1 and app2 were measured with an earlier revision of the script that ran variants in a fixed order (unpatched first) and printed population sd; their t values were computed afterwards from the same per-run timings. app3 was measured with the current revision: rotating variant order, sample sd, and byte identity checked on every run.

## Analysis

1. All four candidates produce bundles byte-identical to unpatched in all 6 bundle/mode combinations (for app3, in every run), so each one fixes the stale-scope deletion that raw `cloneInputAst: false` causes in the app1 prod bundle.
2. Not cloning saves CPU: every candidate uses less CPU than unpatched everywhere except `clearscope-before-transform` on app3 prod. The size of the effect is similar across apps: for the two before-constant-folding candidates, 2.8–6.9% CPU and 1.5–6.3% wall. Wall-time deltas are mostly within noise at 5 runs; CPU deltas are mostly significant.
3. Where the clear goes matters more than which clear is used. Clearing before the main pass discards the scopes the Babel transformer just built, so the main pass re-crawls every module, in dev too. The two "before-constant-folding" variants never clear in dev and are faster there on app1 (177–178s vs 182–183s) and app2 (125–126s vs 127–131s); on app3 dev all four are within noise (184.4–185.8s).
4. `clearScope()` vs `clear()` is within noise. In dev the two before-constant-folding variants run identical code, and they still differ by up to 1.4% wall (app3 dev: 184.4s vs 184.4s, 2777s vs 2804s CPU). In prod, `clearScope()` is ahead on app2 (143.8s vs 148.1s), level on app1 and slightly behind on app3 (-2.78% vs -3.11% CPU). `clearScope()` discards less, since it keeps the path cache.
5. `clearscope-before-transform` is erratic: two of its five app3 prod builds took 245s and 251s, using about 15% more CPU, while the unpatched builds next to them were normal. It also had the largest spread on app1 prod.
6. `clearscope-before-constant-folding` has the best average and a significant CPU saving in 5 of 6 combinations (all except app3 dev, t -1.9); `clear-before-constant-folding` is close behind, also significant in 5 of 6 (all except app2 prod, t -1.67).

Averaged over app1, app2 and app3, prod and dev (6 combinations):

| Candidate | Avg wall Δ | Avg CPU Δ |
|---|--:|--:|
| `clearscope-before-constant-folding` (this diff) | -3.85% | -5.05% |
| `clear-before-constant-folding` | -3.12% | -4.54% |
| `clear-before-transform` | -2.39% | -3.56% |
| `clearscope-before-transform` | -0.80% | -1.91% |

This diff implements `clearscope-before-constant-folding`. On app3, the largest app, it saves about 2% wall and 3% CPU in prod, and 4% wall and 5% CPU in dev.

Caveats: 5 runs per variant; the app1 and app2 run used a fixed variant order (unpatched first in each round), while the app3 run rotated it; most wall-time deltas are within noise.

Changelog: [Performance] `metro-transform-worker` no longer deep-clones each module's AST before the main Babel pass, reducing cold build CPU time by about 5%.

Differential Revision: D123636983
@robhogan robhogan mentioned this pull request Oct 28, 2026
Merged
@robhogan robhogan mentioned this pull request Nov 6, 2026
Merged
robhogan added a commit that referenced this pull request Nov 6, 2026
@robhogan
Publish 0.88.0
8f39a9e 
Summary:
Publish Metro 0.88.0 from `main`.

This is a draft until we choose the cut. These breaking PRs are still open, and can land first or wait for the next minor:

 - #2186 (`globalPrefix` for all module system globals)
 - #2158 (`/symbolicate` stops reading `req.rawBody`)
 - #2157 (remove `functionMap` from the Babel transformer result)
 - #1990 (keep the file extension in async bundle paths, approved and held for 0.88)
 - #1985 and #2156 (drafts, decoded source maps)

Approved non-breaking PRs that could also make it: #2185, #2040, #2034 and #2150 (which turns a missing watch folder into an immediate startup error, rather than a 240s hang). I'll rebase this and extend the notes as they land.

Changelog: [Internal]

Test plan:
```
node scripts/updateVersion.js 0.88.0
sl diff --stat
```

Only the 17 `packages/*/package.json` manifests change, and every `0.87.3` reference is replaced.

Notable breakage in third parties, for the notes or a heads-up before React Native adopts this:

 - `@rnx-kit/metro-config` 2.2.6 adds `path.dirname(resolver.emptyModulePath)` as a watch folder when there are none, which is now `metro:runtime/modules`, so the dev server never serves a bundle for a single-package app.
 - `@tamagui/metro-plugin` 3.x refuses `metro-resolver` outside 0.84-0.87, and then imports the removed `metro-resolver/private/createDefaultContext`.
 - `@rnx-kit/metro-resolver-symlinks` calls the removed `context.redirectModulePath`.

End to end (AI-driven), with the packed tarballs against 0.87.3 from npm, in a React Native 0.89.0-rc.0 template app with a few popular dependencies: dev and release bundles, source maps and assets for iOS and Android are byte-identical, except where #2174 changes resolution. `rxjs/operators` now resolves to the same ESM build as `rxjs`, dropping 190 duplicate modules, and `require('nanoid/non-secure')` gets nanoid's CJS build. Release builds reach the first screen on a Galaxy A16 and the iOS simulator (built before #2184 landed, which leaves the prelude and module system byte-identical). The dev server passes the same checks on both, with and without Watchman (edits, added and deleted files, HMR, `/symbolicate`, assets and lazy `import()`), and Fast Refresh works on the simulator.

Draft release notes (from the 14 commits in v0.87.3...main - internal-only changes omitted):

```
 - **[Breaking]**: Resolver: Read `"exports"` from the package root, and apply `"browser"` redirections to the file it resolves to, by default. Set `resolver.unstable_resolveExportsFromPackageRoot` and `resolver.unstable_applyBrowserSpecAfterExports` to `false` for the previous behavior (#2174 by @robhogan)
 - **[Breaking]**: Default `transformer.asyncRequireModulePath` and `resolver.emptyModulePath` to `metro:runtime/...`. A custom `resolver.resolveRequest` must delegate `metro:` specifiers it doesn't handle to `context.resolveRequest`, and code that reads either option as a file path must resolve it instead (#2180 by @robhogan)
 - **[Breaking]**: Resolve the module system via `metro:runtime/polyfills/require`. A custom `resolver.resolveRequest` must delegate `metro:` specifiers it doesn't handle to `context.resolveRequest` (#2184 by @robhogan)
 - **[Breaking]**: Remove the deprecated `redirectModulePath` from `ResolutionContext`. A custom resolver should drop any call to it and delegate to `context.resolveRequest` or `resolve`, which already apply `"browser"` redirections and return `{type: 'empty'}` for `false` (#2181 by @robhogan)
 - **[Breaking]**: Remove the deprecated `doesFileExist` from `ResolutionContext`, and the `DoesFileExist` type from `metro-resolver` (use `fileSystemLookup`) (#2181 by @robhogan)
 - **[Breaking]**: `ResolutionContext.isESMImport` is required (#2182 by @robhogan)
 - **[Breaking]**: Source and JSON files containing the reserved string `_$$_METRO_`, including in comments and string literals, now fail to transform (#2155 by @robhogan)
 - **[Breaking]**: Remove the deprecated, always-`true` `hot` option from `getTransformOptions` (#2159 by @robhogan)
 - **[Feature]**: Custom serializers receive the reserved string as `options.reservedString` (#2155 by @robhogan)
 - **[Fix]**: Fall back to file-based resolution, instead of throwing a `TypeError`, when a package's `"exports"` field is not a string, array or object (#2187 by @kwy404)
 - **[Fix]**: Fix relative symlinks on a different Windows drive than the project root (#2161 by @ramya-2196)
 - **[Performance]**: ~5-10% faster resolution (#2181 by @robhogan)

> NOTE: Experimental features are not covered by semver and can change at any time.

 - **[Experimental]**: Remove `unstable_incrementalResolution` from `ResolutionContext`, and `metro-resolver/private/createDefaultContext` (#2181 by @robhogan)

 **Full Changelog**: v0.87.3...v0.88.0
```
robhogan added a commit that referenced this pull request Nov 6, 2026
@robhogan
Publish 0.88.0
f65e63c 
Summary:
Publish Metro 0.88.0 from `main`.

This is a draft until we choose the cut. These breaking PRs are still open, and can land first or wait for the next minor:

 - #2158 (`/symbolicate` stops reading `req.rawBody`)
 - #2157 (remove `functionMap` from the Babel transformer result)
 - #1990 (keep the file extension in async bundle paths, approved and held for 0.88)
 - #1985 and #2156 (drafts, decoded source maps)

Approved non-breaking PRs that could also make it: #2185, #2040, #2034 and #2150 (which turns a missing watch folder into an immediate startup error, rather than a 240s hang). I'll rebase this and extend the notes as they land.

Changelog: [Internal]

Test plan:
```
node scripts/updateVersion.js 0.88.0
sl diff --stat
```

Only the 17 `packages/*/package.json` manifests change, and every `0.87.3` reference is replaced.

Notable breakage in third parties, for the notes or a heads-up before React Native adopts this:

 - `@rnx-kit/metro-config` 2.2.6 adds `path.dirname(resolver.emptyModulePath)` as a watch folder when there are none, which is now `metro:runtime/modules`, so the dev server never serves a bundle for a single-package app.
 - `@tamagui/metro-plugin` 3.x refuses `metro-resolver` outside 0.84-0.87, and then imports the removed `metro-resolver/private/createDefaultContext`.
 - `@rnx-kit/metro-resolver-symlinks` calls the removed `context.redirectModulePath`.

End to end (AI-driven), with the packed tarballs against 0.87.3 from npm, in a React Native 0.89.0-rc.0 template app with a few popular dependencies: dev and release bundles, source maps and assets for iOS and Android are byte-identical, except where #2174 changes resolution. `rxjs/operators` now resolves to the same ESM build as `rxjs`, dropping 190 duplicate modules, and `require('nanoid/non-secure')` gets nanoid's CJS build. With #2186, only the module system and `asyncRequire` differ besides those. Release builds reach the first screen on a Galaxy A16 and the iOS simulator. The dev server passes the same checks on both, with and without Watchman (edits, added and deleted files, HMR, `/symbolicate`, assets and lazy `import()`), and Fast Refresh works on the simulator.

Draft release notes (from the 15 commits in v0.87.3...main - internal-only changes omitted):

```
 - **[Breaking]**: Resolver: Read `"exports"` from the package root, and apply `"browser"` redirections to the file it resolves to, by default. Set `resolver.unstable_resolveExportsFromPackageRoot` and `resolver.unstable_applyBrowserSpecAfterExports` to `false` for the previous behavior (#2174 by @robhogan)
 - **[Breaking]**: Default `transformer.asyncRequireModulePath` and `resolver.emptyModulePath` to `metro:runtime/...`. A custom `resolver.resolveRequest` must delegate `metro:` specifiers it doesn't handle to `context.resolveRequest`, and code that reads either option as a file path must resolve it instead (#2180 by @robhogan)
 - **[Breaking]**: Resolve the module system via `metro:runtime/polyfills/require`. A custom `resolver.resolveRequest` must delegate `metro:` specifiers it doesn't handle to `context.resolveRequest` (#2184 by @robhogan)
 - **[Breaking]**: `transformer.globalPrefix` now also prefixes `__r`, `__c`, `__registerSegment` and `__accept`, and the default run statement. Code that calls these by name must apply the prefix (#2186 by @robhogan)
 - **[Breaking]**: Remove the deprecated `redirectModulePath` from `ResolutionContext`. A custom resolver should drop any call to it and delegate to `context.resolveRequest` or `resolve`, which already apply `"browser"` redirections and return `{type: 'empty'}` for `false` (#2181 by @robhogan)
 - **[Breaking]**: Remove the deprecated `doesFileExist` from `ResolutionContext`, and the `DoesFileExist` type from `metro-resolver` (use `fileSystemLookup`) (#2181 by @robhogan)
 - **[Breaking]**: `ResolutionContext.isESMImport` is required (#2182 by @robhogan)
 - **[Breaking]**: Source and JSON files containing the reserved string `_$$_METRO_`, including in comments and string literals, now fail to transform (#2155 by @robhogan)
 - **[Breaking]**: Remove the deprecated, always-`true` `hot` option from `getTransformOptions` (#2159 by @robhogan)
 - **[Feature]**: Custom serializers receive the reserved string as `options.reservedString` (#2155 by @robhogan)
 - **[Fix]**: Fall back to file-based resolution, instead of throwing a `TypeError`, when a package's `"exports"` field is not a string, array or object (#2187 by @kwy404)
 - **[Fix]**: Fix relative symlinks on a different Windows drive than the project root (#2161 by @ramya-2196)
 - **[Performance]**: ~5-10% faster resolution (#2181 by @robhogan)

> NOTE: Experimental features are not covered by semver and can change at any time.

 - **[Experimental]**: Remove `unstable_incrementalResolution` from `ResolutionContext`, and `metro-resolver/private/createDefaultContext` (#2181 by @robhogan)

 **Full Changelog**: v0.87.3...v0.88.0
```

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@robhogan robhogan robhogan approved these changes
@huntie huntie Awaiting requested review from huntie

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@vzaidman @robhogan