Speedup heapsort by 1.8x by making it branchless

[Voultapher]

slice::sort_unstable will fall back to heapsort if it repeatedly fails to find a good pivot. By making the core child update code branchless it is much faster. On Zen3 sorting 10k u64 and forcing the sort to pick heapsort, results in:

455us -> 278us

455us -> 249us

[rustbot]

r? @scottmcm

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[rustbot]

Hey! It looks like you've submitted a new PR for the library teams!

If this PR contains changes to any rust-lang/rust public library APIs then please comment with @rustbot label +T-libs-api -T-libs to tag it appropriately. If this PR contains changes to any unstable APIs please edit the PR description to add a link to the relevant API Change Proposal or create one if you haven't already. If you're unsure where your change falls no worries, just leave it as is and the reviewer will take a look and make a decision to forward on if necessary.

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[Voultapher]

r? thomcc

[Voultapher]

r? @scottmcm

[scottmcm]

Thanks for benching it!

@bors r+

(Ok to rollup because this is in the fallback sorting/selecting path, so is unlikely to matter for compiler perf.)

[bors]

📌 Commit ee0376c has been approved by scottmcm

It is now in the queue for this repository.

[thomcc]

@bors rollup=never

[thomcc]

@scottmcm Not sure I agree — that's true but compiler is huge and it's hard to say what matters. Better safe than sorry IMO.

[scottmcm]

@thomcc Sure, no objections, especially since the queue isn't too long right now.

[bors]

⌛ Testing commit ee0376c with merge b7089e0...

[bors]

☀️ Test successful - checks-actions
Approved by: scottmcm
Pushing b7089e0 to master...

[bors]

☀️ Test successful - checks-actions
Approved by: scottmcm
Pushing b7089e0 to master...

[rust-timer]

Finished benchmarking commit (b7089e0): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -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.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	1.6%	[1.6%, 1.6%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.3%	[-3.3%, -3.3%]	1
All ❌✅ (primary)	-	-	0

Cycles

This benchmark run did not return any relevant results for this metric.