ffi: shrink trampoline placement probe window - #64969
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AllocateCodeNear() searched 1024 pages in each direction for a free page next to the native target, one MAP_FIXED_NOREPLACE mmap per candidate. Every probe is expected to fail, so an exhausted window cost up to 2048 failing syscalls before taking the plain-mmap fallback that was already there, all to enable a jmp rel32 instead of movabs+jmp. Resolving a fast-eligible signature plateaued at ~790us, against ~28us for one that allocates no trampoline. Reduce the window to 16 pages per direction, bounding the exhausted case to 32 probes while still covering typical per-library symbol counts. Placement stays opportunistic: the caller already emits the absolute form when EmitJmpRel32() reports the target is out of range. Signed-off-by: Trivikram Kamat <16024985+trivikr@users.noreply.github.com> Assisted-by: claude:opus-5
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Codecov Report✅ All modified and coverable lines are covered by tests. Additional details and impacted files@@ Coverage Diff @@
## main #64969 +/- ##
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Coverage 90.27% 90.27%
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Files 762 762
Lines 247515 247515
Branches 46683 46681 -2
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Hits 223447 223447
- Misses 15485 15491 +6
+ Partials 8583 8577 -6 🚀 New features to boost your workflow:
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Fixes: #64968
AllocateCodeNear()searched1024pages in each direction for a free page next to the native target, oneMAP_FIXED_NOREPLACEmmap per candidate. Every probe is expected to fail, so an exhausted window cost up to 2048 failing syscalls before taking the plain-mmap fallback that was already there, all to enable a jmp rel32 instead of movabs+jmp. Resolving a fast-eligible signature plateaued at~790us, against~28usfor one that allocates no trampoline.Reduce the window to
16pages per direction, bounding the exhausted case to32probes while still covering typical per-library symbol counts. Placement stays opportunistic: the caller already emits the absolute form whenEmitJmpRel32()reports the target is out of range.Add
benchmark/ffi/get-function.js, which measures resolution rather than call throughput, so this path has coverage. Its fast arm reports 1,852 ops/sec at the old window and 37,836 at the new one, with the fast-ineligible control arm flat at ~80,000.Assisted-by: claude:opus-5