Don't use usub.with.overflow intrinsic by nikic · Pull Request #103299 · rust-lang/rust (original) (raw)
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The canonical form of a usub.with.overflow check in LLVM are separate sub + icmp instructions, rather than a usub.with.overflow intrinsic. Using usub.with.overflow will generally result in worse optimization potential.
The backend will attempt to form usub.with.overflow when it comes to actual instruction selection. This is not fully reliable, but I believe this is a better tradeoff than using the intrinsic in IR.
Fixes #103285.
The canonical form of a usub.with.overflow check in LLVM are separate sub + icmp instructions, rather than a usub.with.overflow intrinsic. Using usub.with.overflow will generally result in worse optimization potential.
The backend will attempt to form usub.with.overflow when it comes to actual instruction selection. This is not fully reliable, but I believe this is a better tradeoff than using the intrinsic in IR.
Fixes rust-lang#103285.
Area: The testsuite used to check the correctness of rustc
Relevant to the compiler team, which will review and decide on the PR/issue.
labels
r? @wesleywiser
(rust-highfive has picked a reviewer for you, use r? to override)
Awaiting bors try build completion.
@rustbot label: +S-waiting-on-perf
⌛ Trying commit 7833012 with merge 9e5787ab4583e67d16150a014f6c118dfa47ab43...
☀️ Try build successful - checks-actions
Build commit: 9e5787ab4583e67d16150a014f6c118dfa47ab43 (9e5787ab4583e67d16150a014f6c118dfa47ab43)
Finished benchmarking commit (9e5787ab4583e67d16150a014f6c118dfa47ab43): comparison URL.
Overall result: no relevant changes - no action needed
Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. While you can manually mark this PR as fit for rollup, we strongly recommend not doing so since this PR may lead to changes in compiler perf.
@bors rollup=never
@rustbot label: +S-waiting-on-review -S-waiting-on-perf -perf-regression
Instruction count
This benchmark run did not return any relevant results for this metric.
Max RSS (memory usage)
Results
This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.
| mean1 | range | count2 | |
|---|---|---|---|
| Regressions ❌ (primary) | - | - | 0 |
| Regressions ❌ (secondary) | 2.5% | [2.5%, 2.5%] | 1 |
| Improvements ✅ (primary) | - | - | 0 |
| Improvements ✅ (secondary) | - | - | 0 |
| All ❌✅ (primary) | - | - | 0 |
Cycles
Results
This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.
| mean1 | range | count2 | |
|---|---|---|---|
| Regressions ❌ (primary) | - | - | 0 |
| Regressions ❌ (secondary) | - | - | 0 |
| Improvements ✅ (primary) | - | - | 0 |
| Improvements ✅ (secondary) | -2.3% | [-2.3%, -2.3%] | 1 |
| All ❌✅ (primary) | - | - | 0 |
Footnotes
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🎉
📌 Commit 7833012 has been approved by wesleywiser
It is now in the queue for this repository.
Status: Waiting on bors to run and complete tests. Bors will change the label on completion.
and removed S-waiting-on-review
Status: Awaiting review from the assignee but also interested parties.
labels
Finished benchmarking commit (f42b6fa): comparison URL.
Overall result: ❌ regressions - ACTION NEEDED
Next Steps: If you can justify the regressions found in this perf run, please indicate this with @rustbot label: +perf-regression-triaged along with sufficient written justification. If you cannot justify the regressions please open an issue or create a new PR that fixes the regressions, add a comment linking to the newly created issue or PR, and then add the perf-regression-triaged label to this PR.
@rustbot label: +perf-regression
cc @rust-lang/wg-compiler-performance
Instruction count
This is a highly reliable metric that was used to determine the overall result at the top of this comment.
| mean | range | count | |
|---|---|---|---|
| Regressions ❌ (primary) | 1.3% | [1.2%, 1.4%] | 2 |
| Regressions ❌ (secondary) | 3.6% | [3.2%, 4.1%] | 6 |
| Improvements ✅ (primary) | - | - | 0 |
| Improvements ✅ (secondary) | - | - | 0 |
| All ❌✅ (primary) | 1.3% | [1.2%, 1.4%] | 2 |
Max RSS (memory usage)
This benchmark run did not return any relevant results for this metric.
Cycles
Results
This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.
| mean | range | count | |
|---|---|---|---|
| Regressions ❌ (primary) | - | - | 0 |
| Regressions ❌ (secondary) | - | - | 0 |
| Improvements ✅ (primary) | - | - | 0 |
| Improvements ✅ (secondary) | -3.0% | [-3.2%, -2.8%] | 2 |
| All ❌✅ (primary) | - | - | 0 |
This is benchmark noise.
@rustbot label: +perf-regression-triaged
Aaron1011 pushed a commit to Aaron1011/rust that referenced this pull request
Don't use usub.with.overflow intrinsic
The canonical form of a usub.with.overflow check in LLVM are separate sub + icmp instructions, rather than a usub.with.overflow intrinsic. Using usub.with.overflow will generally result in worse optimization potential.
The backend will attempt to form usub.with.overflow when it comes to actual instruction selection. This is not fully reliable, but I believe this is a better tradeoff than using the intrinsic in IR.
Fixes rust-lang#103285.
bors added a commit to rust-lang-ci/rust that referenced this pull request
Make checked ops emit unchecked LLVM operations where feasible
For things with easily pre-checked overflow conditions -- shifts and unsigned subtraction -- write then checked methods in such a way that we stop emitting wrapping versions of them.
For example, today <https://rust.godbolt.org/z/qM9YK8Txb> neither
a.checked_sub(b).unwrap()nor
a.checked_sub(b).unwrap_unchecked()actually optimizes to sub nuw. After this PR they do.
bors added a commit to rust-lang-ci/rust that referenced this pull request
…bilee
Make checked ops emit unchecked LLVM operations where feasible
For things with easily pre-checked overflow conditions -- shifts and unsigned subtraction -- write the checked methods in such a way that we stop emitting wrapping versions of them.
For example, today <https://rust.godbolt.org/z/qM9YK8Txb> neither
a.checked_sub(b).unwrap()nor
a.checked_sub(b).unwrap_unchecked()actually optimizes to sub nuw. After this PR they do.
bors added a commit to rust-lang-ci/rust that referenced this pull request
fmease added a commit to fmease/rust that referenced this pull request
rust-timer added a commit to rust-lang-ci/rust that referenced this pull request
