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Optimize checked_ilog and pow when base is a power of two #147250

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Optimize checked_ilog and pow when base is a power of two #147250

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@Kmeakin Kmeakin commented Oct 2, 2025 •
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Optimize checked_ilog and pow when the base is a power of two

@rustbot rustbot added 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 Oct 2, 2025
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@Kmeakin Kmeakin force-pushed the km/optimize-ilog-base-power-of-two branch from 8f19ce6 to e5ca8cd Compare October 2, 2025 01:31
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workingjubilee commented Oct 2, 2025 •
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does this affect the codegen in practical circumstances? I would expect even a fairly weak optimizer to perform this optimization, making us hand-coding it irrelevant and potentially even harmful because we introduce more conditional logic to chew through.

@Kmeakin Kmeakin changed the title Optimize checked_ilog when base is a power of two Optimize checked_ilog and pow when base is a power of two Oct 2, 2025
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Kmeakin commented Oct 2, 2025 •
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does this affect the codegen in practical circumstances?

It replaces a loop by some bit manipulations. Whether anyone actually calls either function with a compile-time known, power of 2 base is another question. But it can't hurt, since it is guarded by is_statically_known.

I would expect even a fairly weak optimizer to perform this optimization, making us hand-coding it irrelevant

No, in either function, LLVM is not able to see that the loop is performing a log/pow and apply the identities: https://godbolt.org/z/6vMsxc9Kh

and potentially even harmful because we introduce more conditional logic to chew through.

They're guarded by is_statically_known so will be ignored if the base is not known at compile-time

@Kmeakin Kmeakin force-pushed the km/optimize-ilog-base-power-of-two branch from e5ca8cd to 2faa397 Compare October 2, 2025 02:29
Kmeakin added 2 commits October 2, 2025 03:31
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Optimize checked_ilog when base is a power of two
f612451 
if base == 2 ** k, then
log(base, n) == log(2, n) / k
@Kmeakin
Optimize pow when self is a power of two
abb7f32 
if base == 2 ** k, then
   (2 ** k) ** n
== 2 ** (k * n)
== 1 << (k * n)
@Kmeakin Kmeakin force-pushed the km/optimize-ilog-base-power-of-two branch from 2faa397 to abb7f32 Compare October 2, 2025 02:38
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No, in either function, LLVM is not able to see that the loop is performing a log/pow and apply the identities: https://godbolt.org/z/6vMsxc9Kh

...then I'm kinda surprised! Nice catch.

scottmcm
scottmcm reviewed Oct 2, 2025
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library/core/src/num/uint_macros.rs
Comment on lines 1574 to +1576
if base == 2 {
return self.checked_ilog2();
} else if base == 10 {
}
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Seems like there's no need for the base == 2 case any more? The is_power_of_two one would handle it.

scottmcm
scottmcm reviewed Oct 2, 2025
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library/core/src/num/uint_macros.rs
Comment on lines +1580 to +1583
if base.is_power_of_two() {
let k = base.ilog2();
return Some(try_opt!(self.checked_ilog2()) / k);
}
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I think this is the a problem because it'll apply for base == 1, then divide by zero?

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scottmcm commented Oct 2, 2025

does this affect the codegen in practical circumstances?

Would be good to have codegen tests to demonstrate what this is doing -- especially since that way there's a way for people to try removing the special cases later if they think that LLVM no longer needs them.

@scottmcm scottmcm added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Oct 2, 2025
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