windows: Elide division-by-zero checks in Instant::now() by mathisbot · Pull Request #157328 · rust-lang/rust

mathisbot · 2026-06-02T19:01:12Z

This PR teaches LLVM that the frequency of the performance counter is non null so it is able to remove division by zero checks.
This removes the panic path in mul_div_u64 and should make calls to Instant::now() (very slightly) faster.

As seen in the assembly (see godbolt below), telling LLVM that the frequency is non zero suffices to get the optimization, but I don't know if it could be a great idea to also update the signature of mul_div_u64?

MSDN page for QueryPerformanceFrequency:

On systems that run Windows XP or later, the function will always succeed when given valid parameters and will thus never return zero.

Godbolt: https://rust.godbolt.org/z/xr6x8MrPE

rustbot · 2026-06-02T19:01:17Z

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rustbot · 2026-06-02T19:01:18Z

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Owners of files modified in this PR: @ChrisDenton

this removes the panic path when dividing by the frequency. this also makes calls to Instant::now() faster.

ChrisDenton · 2026-06-02T22:16:35Z

Thanks!

@bors r+ rollup

rust-bors · 2026-06-02T22:16:38Z

📌 Commit c2b9f39 has been approved by ChrisDenton

It is now in the queue for this repository.

windows: Elide division-by-zero checks in Instant::now() This PR teaches LLVM that the frequency of the performance counter is non null so it is able to remove division by zero checks. This removes the panic path in `mul_div_u64` and should make calls to `Instant::now()` (very slightly) faster. As seen in the assembly (see godbolt below), telling LLVM that the frequency is non zero suffices to get the optimization, but I don't know if it could be a great idea to also update the signature of `mul_div_u64`? MSDN page for [QueryPerformanceFrequency](https://learn.microsoft.com/en-us/windows/win32/api/profileapi/nf-profileapi-queryperformancefrequency): > On systems that run Windows XP or later, the function will always succeed when given valid parameters and will thus never return zero. Godbolt: https://rust.godbolt.org/z/xr6x8MrPE

…ChrisDenton windows: Elide division-by-zero checks in Instant::now() This PR teaches LLVM that the frequency of the performance counter is non null so it is able to remove division by zero checks. This removes the panic path in `mul_div_u64` and should make calls to `Instant::now()` (very slightly) faster. As seen in the assembly (see godbolt below), telling LLVM that the frequency is non zero suffices to get the optimization, but I don't know if it could be a great idea to also update the signature of `mul_div_u64`? MSDN page for [QueryPerformanceFrequency](https://learn.microsoft.com/en-us/windows/win32/api/profileapi/nf-profileapi-queryperformancefrequency): > On systems that run Windows XP or later, the function will always succeed when given valid parameters and will thus never return zero. Godbolt: https://rust.godbolt.org/z/xr6x8MrPE

jhpratt · 2026-06-02T23:17:59Z

@bors yield

for encompassing rollup

rust-bors · 2026-06-02T23:18:02Z

Auto build was cancelled. Cancelled workflows:

https://github.com/rust-lang/rust/actions/runs/26853736360

The next pull request likely to be tested is #157340.

Rollup of 2 pull requests Successful merges: - #157328 (windows: Elide division-by-zero checks in Instant::now()) - #157336 (Enable `clippy::mem_replace_with_default`)

windows: Elide division-by-zero checks in Instant::now() This PR teaches LLVM that the frequency of the performance counter is non null so it is able to remove division by zero checks. This removes the panic path in `mul_div_u64` and should make calls to `Instant::now()` (very slightly) faster. As seen in the assembly (see godbolt below), telling LLVM that the frequency is non zero suffices to get the optimization, but I don't know if it could be a great idea to also update the signature of `mul_div_u64`? MSDN page for [QueryPerformanceFrequency](https://learn.microsoft.com/en-us/windows/win32/api/profileapi/nf-profileapi-queryperformancefrequency): > On systems that run Windows XP or later, the function will always succeed when given valid parameters and will thus never return zero. Godbolt: https://rust.godbolt.org/z/xr6x8MrPE

jhpratt · 2026-06-02T23:32:40Z

same thing

@bors yield

rust-bors · 2026-06-02T23:32:43Z

Auto build was cancelled. Cancelled workflows:

https://github.com/rust-lang/rust/actions/runs/26854009597

The next pull request likely to be tested is #157340.

Rollup of 2 pull requests Successful merges: - #157328 (windows: Elide division-by-zero checks in Instant::now()) - #157336 (Enable `clippy::mem_replace_with_default`)

windows: Elide division-by-zero checks in Instant::now() This PR teaches LLVM that the frequency of the performance counter is non null so it is able to remove division by zero checks. This removes the panic path in `mul_div_u64` and should make calls to `Instant::now()` (very slightly) faster. As seen in the assembly (see godbolt below), telling LLVM that the frequency is non zero suffices to get the optimization, but I don't know if it could be a great idea to also update the signature of `mul_div_u64`? MSDN page for [QueryPerformanceFrequency](https://learn.microsoft.com/en-us/windows/win32/api/profileapi/nf-profileapi-queryperformancefrequency): > On systems that run Windows XP or later, the function will always succeed when given valid parameters and will thus never return zero. Godbolt: https://rust.godbolt.org/z/xr6x8MrPE

rust-bors · 2026-06-02T23:46:29Z

💔 Test for c09f2f7 failed: CI. Failed job:

auto - dist-powerpc64le-linux-gnu (web logs, enhanced plaintext logs)

jhpratt · 2026-06-02T23:52:37Z

not related to this PR. The tree is currently closed for that reason

@bors retry

rust-log-analyzer · 2026-06-03T00:32:40Z

A job failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

rustbot assigned ChrisDenton Jun 2, 2026

rustbot added O-windows Operating system: Windows S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-libs Relevant to the library team, which will review and decide on the PR/issue. labels Jun 2, 2026

ChrisDenton reviewed Jun 2, 2026

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Comment thread library/std/src/sys/pal/windows/time.rs Outdated

windows: remove division-by-zero checks for performance counters

c2b9f39

this removes the panic path when dividing by the frequency. this also makes calls to Instant::now() faster.

mathisbot force-pushed the opt-std-windows-time branch from fd3a114 to c2b9f39 Compare June 2, 2026 21:50

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-review Status: Awaiting review from the assignee but also interested parties. labels Jun 2, 2026

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jhpratt mentioned this pull request Jun 2, 2026

Rollup of 2 pull requests #157340

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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-review Status: Awaiting review from the assignee but also interested parties. labels Jun 2, 2026

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