[sandbox] Recover fast-path offset performance in ToDirectStringAssembler [v8/v8 : main]
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Samuel Groß (Gerrit)
Jun 9, 2026, 9:43:42 AM (2 days ago) Jun 9
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Maksim Ivanov (Gerrit)
Jun 9, 2026, 10:00:49 AM (2 days ago) Jun 9
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Maksim Ivanov voted and added 1 comment![]( "Open in Gerrit")
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| Code-Review | +1 |
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File-level comment, Patchset 2 (Latest):
Maksim Ivanov . resolved
thanks for following up on the regression!
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Michael Lippautz (Gerrit)
Jun 9, 2026, 10:41:54 AM (2 days ago) Jun 9
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Michael Lippautz voted Code-Review+1![]( "Open in Gerrit")
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Samuel Groß (Gerrit)
Jun 10, 2026, 4:27:15 AM (yesterday) Jun 10
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Samuel Groß voted Commit-Queue+2![]( "Open in Gerrit")
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v8-scoped@luci-project-accounts.iam.gserviceaccount.com (Gerrit)
Jun 10, 2026, 5:01:29 AM (24 hours ago) Jun 10
to Samuel Groß, Michael Lippautz, Maksim Ivanov, v8-re...@googlegroups.com
v8-s...@luci-project-accounts.iam.gserviceaccount.com submitted the change![]( "Open in Gerrit")
Change information
Commit message:
[sandbox] Recover fast-path offset performance in ToDirectStringAssembler
crrev.com/c/7900075 introduced 32-bit offset accumulation in
ToDirectStringAssembler to prevent out-of-sandbox reads via sliced
string chains. However, changing var_offset_ to a 32-bit Int32T forced
CodeStubAssembler and optimized code to create a 32-bit loop Phi.
Consequently, even on the hot fast-path for flat strings, calling
offset() introduced a 32-bit to 64-bit zero-extension inside hot inner
loops (e.g. StringCharCodeAt), causing a JetStream2 regression.
This CL restores var_offset_ to a 64-bit IntPtrT while maintaining
32-bit wrapping arithmetic on the sliced string slow-path. On the
fast-path (flat strings), var_offset_ remains a clean 64-bit zero
constant or native 64-bit Phi with no conversion overhead. On the
slow-path, 32-bit wrapping addition limits accumulated offsets to
4 GB before zero-extending back to IntPtrT.
TAG=agy
CONV=4c4055d1-3009-41e9-98f5-c10ca2d24ecf
Bug: 521657359
Change-Id: I49f5dbf69409474b815686202008df8bfed42e29
Reviewed-by: Maksim Ivanov <em...@google.com>
Reviewed-by: Michael Lippautz <mlip...@chromium.org>
Commit-Queue: Samuel Groß <sa...@chromium.org>
Cr-Commit-Position: refs/heads/main@{#107876}
Files:
- M src/codegen/code-stub-assembler.cc
- M src/codegen/code-stub-assembler.h
Change size: S
Delta: 2 files changed, 8 insertions(+), 6 deletions(-)
Branch: refs/heads/main
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Code-Review: +1 by Michael Lippautz, +1 by Maksim Ivanov
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