Derive source-map tuples from Babel's decodedMap, reduce cold build CPU ~2.5%

[robhogan]

Summary:
Metro's transform worker currently returns source maps from Babel's tranform result via result.rawMappings.map(toSegmentTuple). This used to be (as the name suggests) Babel's own source map representation, and was therefore free to access.

However, since babel/babel#14497 (babel/generator since v7.17.10), rawMappings is now a getter providing the old structure for backwards compatibility. Accessing result.rawMappings forces
babel/generator to run a second decode (allMappings) that allocates a flat
array of ~4-5 objects per segment.

The better alternative now is to use result.decodedMap, which is eagerly computed and free to access. To accommodate the different structure, we introduce
tuplesFromBabelDecodedMap (decoded source lines are 0-based -> +1, name indices
resolved against decodedMap.names).

Transformer output is byte-identical to result.rawMappings.map(toSegmentTuple), and is simply more efficient.

Microbenchmark

• Real babel/generator 7.29.1 over 133 modules / ~30.6K segments,
  --expose-gc, taking median of 11 repeats to discount GC outliers, etc.

Path	CPU (ms/pass)	Transient heap	Notes
New: generate() + decodedMap	19.2	13.9 MB	eager, already computed — free
Old: generate() + rawMappings	28.8	19.5 MB	triggers allMappings decode
Saving	−9.6 ms (−33%)	−5.6 MB (−29%)	per pass over 30.6K segments

E2E benchmark - large bundle, cold build

(AI driven benchmarks and analysis, real numbers)

• Interleaved, paired A/B: each of 12 rounds runs one cold build per cell —
  {baseline, this diff} x {child-process workers, worker threads}.
• Fresh Metro per build, transform cache wiped (cold), maxWorkers=16
• "Transform CPU" = total user+sys CPU across the
  whole worker process tree
• "tree RSS" = whole-tree resident set (captures
  workers in both modes)
• "graph heap" = main-isolate heapUsed post-build (the
  retained module graph).
• base/this-diff columns are medians; Δ is the paired
  mean with a 95% CI (Student-t, 11 df)
• "n.s." (not significant) = CI includes 0.

Child-process workers (Metro default; 12 paired rounds):

metric	baseline	this diff	Δ (95% CI)
transform CPU (s)	625	612	-16.6 (-2.6%) [-24.7, -8.5]
build wall (s)	65.9	65.6	-0.5 (-0.7%) n.s.
transient tree RSS (GB)	15.8	16.0	+0.06, n.s.
post-build tree RSS (GB)	15.1	15.1	+0.08, n.s.
graph heap, main isolate (GB)	1.59	1.59	~0, n.s.

Worker threads (unstable_workerThreads; 12 paired rounds):

metric	baseline	this diff	Δ (95% CI)
transform CPU (s)	664	653	-18.6 (-2.8%) [-37.5, +0.3]
build wall (s)	59.8	59.5	-1.2 (-1.9%) n.s.
transient RSS (GB)	13.2	12.7	-0.46 (-3.5%) [-0.81, -0.11]
post-build RSS (GB)	12.3	11.9	-0.45 (-3.7%) [-0.80, -0.10]
graph heap, main isolate (GB)	1.60	1.60	~0, n.s.

Takeaways:

• Transform CPU drops ~2.6-2.8%, equally in both worker modes — the point
  estimates (-16.6 s child-process, -18.6 s threads) agree to within 2 s and
  their CIs overlap almost entirely, so there is no real asymmetry. This is
  exactly what the mechanism predicts: the optimization runs inside the worker
  (consume decodedMap instead of forcing the rawMappings/allMappings
  decode), so the saving is identical whether the worker is a child process or a
  thread. (An earlier small-n pass suggested a child-process-only win; that was
  sampling noise — threads-mode CPU is just noisier, SD 30 s vs 13 s, which only
  widens its CI without moving the point estimate.)
• Build wall time is ~1-2% lower in both modes but within noise — the CPU saving
  is spread across 16 workers, so it moves the critical path little.
• Main-isolate post-build heap (the retained graph of stored tuples) is
  unchanged in every config — no memory regression, byte-identical output.

Changelog:

 - **[Performance]**: Use Babel's `decodedMap` for ~2.5% faster transforms

Reviewed By: huntie, GijsWeterings

Differential Revision: D108506323

[meta-codesync]

@robhogan has exported this pull request. If you are a Meta employee, you can view the originating Diff in D108506323.

[Reinaldotec]

Testing