Optimize EscapeIterInner
#125317
Optimize EscapeIterInner
#125317
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r? @Nilstrieb rustbot has assigned @Nilstrieb. Use |
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@bors try @rust-timer queue |
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Optimize `EscapeIterInner` This optimizes `EscapeIterInner` by using `MaybeUninit` for unused array elements instead of initializing them with `ascii::Char::Null`. Follow up to rust-lang#124307, CC `@reitermarkus`
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Finished benchmarking commit (a2f180d): comparison URL. Overall result: ❌ regressions - no action neededBenchmarking 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 Instruction countThis is a highly reliable metric that was used to determine the overall result at the top of this comment.
Max RSS (memory usage)Results (primary 3.0%)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.
CyclesResults (secondary -2.2%)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.
Binary sizeResults (primary -0.1%, secondary -0.0%)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.
Bootstrap: 670.321s -> 670.058s (-0.04%) |
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| self.data.get_unchecked(usize::from(self.alive.start)..usize::from(self.alive.end)) | ||
| let data = self.data.get_unchecked(self.alive.start as usize..self.alive.end as usize); |
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Why did you change usize::fromto as here? I prefer the from conversions
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Do you have any benchmarks or at least assembly examples to show that this makes the code faster? If you do, r=me after addressing my other question (and either changing it back or not). The unsafe here is not very complex but I don't want to merge it without any proof of at least some improvements. |
| // The element type ensures this is always ASCII, and thus also valid UTF-8. | ||
| data: [ascii::Char; N], | ||
| // Invariant: all elements inside the range indexed by `alive` are initialized | ||
| data: [MaybeUninit<ascii::Char>; N], |
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Note that, for small N, it's not obvious at all to me that this is necessarily a win. Zero-initializing a [ascii::Char; 8] for example is just a single 0_u64, and having it be MaybeUninit instead comes with the cost of it not getting noundef when it's passed around, reducing optimization possibilities for what LLVM can do with it.
For large, especially heap-allocated buffers it can help to avoid initializing them, but making all this code safety-critical (EscapeIterInner::ascii now has to be considered for understanding if it's UB, for example) is not at all obvious that it's a good tradeoff.
(We really need unsafe structs so that potential UB stops hiding in struct literals.)
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What exactly is the status of this? Is it actually worth it to make these changes? |
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@joboet any updates on this? thanks |
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On second thought I agree with Scott's assessment and am closing this. |
This optimizes
EscapeIterInnerby usingMaybeUninitfor unused array elements instead of initializing them withascii::Char::Null.Follow up to #124307, CC @reitermarkus