Hint that memchr returns an in-bounds index by SomeFlyingThing · Pull Request #159784 · rust-lang/rust

SomeFlyingThing · 2026-07-23T17:33:23Z

memchr_aligned always returns an index within the input slice, but that fact was not visible to callers after inlining. Record the invariant with assert_unchecked so LLVM can simplify callers such as str::find(char)

In x86_64 codegen, find(char) shrank from 463 to 377 bytes.

rustbot · 2026-07-23T17:42:06Z

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petrochenkov · 2026-07-24T10:06:41Z

r? libs

Darksonn · 2026-07-24T12:00:43Z

@rustbot author

rustbot · 2026-07-24T12:00:47Z

Reminder, once the PR becomes ready for a review, use @rustbot ready.

SomeFlyingThing · 2026-07-27T16:52:30Z

@rustbot ready

Darksonn

Thanks

@bors r+

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rust-bors · 2026-07-28T06:54:45Z

📌 Commit 49c1f02 has been approved by Darksonn

It is now in the queue for this repository.

🌲 The tree is currently closed for pull requests below priority 100. This pull request will be tested once the tree is reopened.

Reason for tree closure: spurious failures

…result-bounds, r=Darksonn Hint that memchr returns an in-bounds index `memchr_aligned` always returns an index within the input slice, but that fact was not visible to callers after inlining. Record the invariant with `assert_unchecked` so LLVM can simplify callers such as `str::find(char)` In x86_64 codegen, `find(char)` shrank from 463 to 377 bytes.

LLVM 21 preserves the bounds assumption but does not eliminate the aggregate phi that LLVM 22 removes. Check each version's supported optimization and restore the shared postcondition so direct callers can eliminate bounds checks.

Darksonn · 2026-08-05T21:47:43Z

@bors try jobs=x86_64-gnu-llvm-21-3

…ds, r=<try> Hint that memchr returns an in-bounds index try-job: x86_64-gnu-llvm-21-3

rust-bors · 2026-08-05T23:33:13Z

☀️ Try build successful (CI)
Build commit: 1f2a0e0 (1f2a0e0b4e63b2c685326de1b26cd5203a165bb2)
Base parent: 7608eb7 (7608eb7b07eaf93f16d7cf5bcb2098eca87503df)

Darksonn · 2026-08-06T07:59:41Z

@bors r+ rollup=iffy

rust-bors · 2026-08-06T07:59:45Z

📌 Commit cd8a97b has been approved by Darksonn

It is now in the queue for this repository.

…result-bounds, r=Darksonn Hint that memchr returns an in-bounds index `memchr_aligned` always returns an index within the input slice, but that fact was not visible to callers after inlining. Record the invariant with `assert_unchecked` so LLVM can simplify callers such as `str::find(char)` In x86_64 codegen, `find(char)` shrank from 463 to 377 bytes.

JonathanBrouwer · 2026-08-06T09:57:37Z

💔 I suspect this PR failed tests as part of a rollup
@bors r-

After fixing the problem, consider running a try job for the failed job before re-approving.

Link to failure: #160615 (comment)

rust-bors · 2026-08-06T09:57:44Z

This pull request was unapproved.

This PR was contained in a rollup (#160615), which was unapproved.

View changes since this unapproval

Darksonn · 2026-08-06T12:40:33Z

I think we should take this test out. It's too fragile.

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Darksonn

@bors r+

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rust-bors · 2026-08-07T07:12:03Z

📌 Commit 08e7eaf has been approved by Darksonn

It is now in the queue for this repository.

…result-bounds, r=Darksonn Hint that memchr returns an in-bounds index `memchr_aligned` always returns an index within the input slice, but that fact was not visible to callers after inlining. Record the invariant with `assert_unchecked` so LLVM can simplify callers such as `str::find(char)` In x86_64 codegen, `find(char)` shrank from 463 to 377 bytes.

…uwer Rollup of 21 pull requests Successful merges: - #159784 (Hint that memchr returns an in-bounds index) - #150885 (Revive L4Re target) - #159643 (Add support for splatted function pointers) - #160433 (delegation: add support for wrapping of the return value with `From::from`) - #160530 (refactor handling of target features in Session) - #160606 (bootstrap: Store and use an explicit CheckKind in `check::Rustc`) - #160628 (fix ICE in `suggest_add_reference_to_arg` for non-callable items) - #160634 (miri subtree update) - #157641 (Do not promote extern statics) - #158904 (Fix FutureDropPoll shim for by-move async closures) - #160103 (Add regression test for GAT bound mismatched type error) - #160335 (dlopen offload) - #160445 (codegen: classify localized MSVC linker progress as linker_info) - #160499 (rustc_resolve: move diagnostic attribute linting to attr parsing) - #160504 (cleanup borrowck, improve c-variadic handling) - #160577 (expand: Feature gate AST-based attribute macros on expressions and statements) - #160587 (Add regression test for associated type outlives bound at call site) - #160625 (platform-support/netbsd.md: No longer mention 8.x, due to EoL.) - #160636 (derive(Diagnostic): link to proper docs) - #160644 (Clean up some manual debug impls) - #160649 (move naked function ui tests)

…result-bounds, r=Darksonn Hint that memchr returns an in-bounds index `memchr_aligned` always returns an index within the input slice, but that fact was not visible to callers after inlining. Record the invariant with `assert_unchecked` so LLVM can simplify callers such as `str::find(char)` In x86_64 codegen, `find(char)` shrank from 463 to 377 bytes.

…uwer Rollup of 20 pull requests Successful merges: - #159784 (Hint that memchr returns an in-bounds index) - #150885 (Revive L4Re target) - #159643 (Add support for splatted function pointers) - #160433 (delegation: add support for wrapping of the return value with `From::from`) - #160530 (refactor handling of target features in Session) - #160606 (bootstrap: Store and use an explicit CheckKind in `check::Rustc`) - #160628 (fix ICE in `suggest_add_reference_to_arg` for non-callable items) - #160634 (miri subtree update) - #157641 (Do not promote extern statics) - #158904 (Fix FutureDropPoll shim for by-move async closures) - #160103 (Add regression test for GAT bound mismatched type error) - #160335 (dlopen offload) - #160445 (codegen: classify localized MSVC linker progress as linker_info) - #160499 (rustc_resolve: move diagnostic attribute linting to attr parsing) - #160504 (cleanup borrowck, improve c-variadic handling) - #160577 (expand: Feature gate AST-based attribute macros on expressions and statements) - #160587 (Add regression test for associated type outlives bound at call site) - #160625 (platform-support/netbsd.md: No longer mention 8.x, due to EoL.) - #160636 (derive(Diagnostic): link to proper docs) - #160644 (Clean up some manual debug impls)

Hint that memchr returns an in-bounds index

0439f4e

SomeFlyingThing marked this pull request as ready for review July 23, 2026 17:42

rustbot added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Jul 23, 2026

rustbot assigned petrochenkov Jul 23, 2026

rustbot removed the S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. label Jul 23, 2026

rustbot assigned Darksonn and unassigned petrochenkov Jul 24, 2026

Hint that memrchr returns an in-bounds index

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Darksonn reviewed Jul 24, 2026

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Comment thread library/core/src/slice/memchr.rs Outdated

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Cover memchr fast path with bounds assertion

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Fix memchr result CI checks

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JonathanBrouwer mentioned this pull request Jul 28, 2026

Rollup of 5 pull requests #160070

Closed

Handle LLVM 21 in memchr result codegen test

cd8a97b

LLVM 21 preserves the bounds assumption but does not eliminate the aggregate phi that LLVM 22 removes. Check each version's supported optimization and restore the shared postcondition so direct callers can eliminate bounds checks.

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rust-bors Bot pushed a commit that referenced this pull request Aug 5, 2026

Auto merge of #159784 - SomeFlyingThing:agent/hint-memchr-result-boun…

1f2a0e0

…ds, r=<try> Hint that memchr returns an in-bounds index try-job: x86_64-gnu-llvm-21-3

rust-bors Bot added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Aug 6, 2026

JonathanBrouwer mentioned this pull request Aug 6, 2026

Rollup of 17 pull requests #160615

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Remove fragile memchr codegen test

08e7eaf

SomeFlyingThing requested a review from Darksonn August 6, 2026 17:59

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Rollup of 21 pull requests #160678

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JonathanBrouwer mentioned this pull request Aug 7, 2026

Rollup of 20 pull requests #160682

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