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| author | Josh Stone <jistone@redhat.com> | 2024-10-11 14:41:02 -0700 |
|---|---|---|
| committer | Josh Stone <jistone@redhat.com> | 2024-10-11 16:22:43 -0700 |
| commit | 5365b3f7be079008d7cac03b821f7c65563ee5ed (patch) | |
| tree | b797d50e60b0c4feeabf72027a0a7d25c78839ce | |
| parent | f4966590d8edd5f493a1aab04016b94a75494329 (diff) | |
| download | rust-5365b3f7be079008d7cac03b821f7c65563ee5ed.tar.gz rust-5365b3f7be079008d7cac03b821f7c65563ee5ed.zip | |
Avoid superfluous UB checks in `IndexRange`
`IndexRange::len` is justified as an overall invariant, and `take_prefix` and `take_suffix` are justified by local branch conditions. A few more UB-checked calls remain in cases that are only supported locally by `debug_assert!`, which won't do anything in distributed builds, so those UB checks may still be useful. We generally expect core's `#![rustc_preserve_ub_checks]` to optimize away in user's release builds, but the mere presence of that extra code can sometimes inhibit optimization, as seen in #131563.
| -rw-r--r-- | library/core/src/ops/index_range.rs | 11 | ||||
| -rw-r--r-- | tests/codegen/issues/issue-101082.rs | 5 |
2 files changed, 12 insertions, 4 deletions
diff --git a/library/core/src/ops/index_range.rs b/library/core/src/ops/index_range.rs index 64214eae377..dce3514a159 100644 --- a/library/core/src/ops/index_range.rs +++ b/library/core/src/ops/index_range.rs @@ -45,7 +45,8 @@ impl IndexRange { #[inline] pub const fn len(&self) -> usize { // SAFETY: By invariant, this cannot wrap - unsafe { self.end.unchecked_sub(self.start) } + // Using the intrinsic because a UB check here impedes LLVM optimization. (#131563) + unsafe { crate::intrinsics::unchecked_sub(self.end, self.start) } } /// # Safety @@ -82,7 +83,8 @@ impl IndexRange { let mid = if n <= self.len() { // SAFETY: We just checked that this will be between start and end, // and thus the addition cannot overflow. - unsafe { self.start.unchecked_add(n) } + // Using the intrinsic avoids a superfluous UB check. + unsafe { crate::intrinsics::unchecked_add(self.start, n) } } else { self.end }; @@ -100,8 +102,9 @@ impl IndexRange { pub fn take_suffix(&mut self, n: usize) -> Self { let mid = if n <= self.len() { // SAFETY: We just checked that this will be between start and end, - // and thus the addition cannot overflow. - unsafe { self.end.unchecked_sub(n) } + // and thus the subtraction cannot overflow. + // Using the intrinsic avoids a superfluous UB check. + unsafe { crate::intrinsics::unchecked_sub(self.end, n) } } else { self.start }; diff --git a/tests/codegen/issues/issue-101082.rs b/tests/codegen/issues/issue-101082.rs index 550d267a98f..4be1b6cb168 100644 --- a/tests/codegen/issues/issue-101082.rs +++ b/tests/codegen/issues/issue-101082.rs @@ -1,4 +1,9 @@ //@ compile-flags: -O +//@ revisions: host x86-64-v3 + +// This particular CPU regressed in #131563 +//@[x86-64-v3] only-x86_64 +//@[x86-64-v3] compile-flags: -Ctarget-cpu=x86-64-v3 #![crate_type = "lib"] |