Avoid unnecessary comparison in partition_equal

[Voultapher]

The branchy Hoare partition partition_equal as part of slice::sort_unstable has a bug that makes it perform a comparison of the last element twice.

Measuring inputs with a Zipfian distribution with characterizing exponent s == 1.0, yields a ~0.05% reduction in the total number of comparisons performed.

[rustbot]

r? @Mark-Simulacrum

(rustbot has picked a reviewer for you, use r? to override)

[Voultapher]

This is not meant as a big perf improvement, more as a bug fix.

[lqd]

That may not be hot in the compiler, but maybe it would show up in the runtime benchmarks: @bors try @rust-timer queue

[bors]

⌛ Trying commit e501add with merge c2c287e...

[Voultapher]

That may not be hot in the compiler, but maybe it would show up in the runtime benchmarks: @bors try @rust-timer queue

I very much doubt it, afaic the rustc runtime benchmarks don't have sort specific stuff, and even then it's such a minuscule change it shouldn't have a noticeable impact. If my goal was perf here, I'd replace the whole partition_equal with a call to partition with flipped comparator. According to the pdqsort author having a branchy equal partition was a bespoke decision that doesn't hold up anymore. I have some larger sort perf stuff in the works, this is just a small bugfix I cam across while working on that.

[bors]

☀️ Try build successful - checks-actions
Build commit: c2c287e (c2c287eb8ca6fd5d9f4af7614d6e1c1562350b96)

[rust-timer]

Finished benchmarking commit (c2c287e): 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-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.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	2.9%	[2.9%, 2.9%]	1
Improvements ✅ (primary)	-3.7%	[-3.7%, -3.7%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-3.7%	[-3.7%, -3.7%]	1

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)	2.9%	[2.9%, 2.9%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Binary size

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)	0.0%	[0.0%, 0.0%]	4
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.2%	[-0.2%, -0.1%]	2
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.0%	[-0.2%, 0.0%]	6

Bootstrap: missing data
Artifact size: 304.42 MiB -> 304.41 MiB (-0.00%)

[Mark-Simulacrum]

@bors r+

[bors]

📌 Commit e501add has been approved by Mark-Simulacrum

It is now in the queue for this repository.

[bors]

⌛ Testing commit e501add with merge 8ba8254...

[rust-log-analyzer]

The job aarch64-gnu failed! Check out the build log: (web) (plain)

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

   Compiling rayon v1.7.0
[RUSTC-TIMING] rayon_core test:false 1.667
[RUSTC-TIMING] tar test:false 2.087
[RUSTC-TIMING] rayon test:false 3.587
##[error]The runner has received a shutdown signal. This can happen when the runner service is stopped, or a manually started runner is canceled.
##[group]Clock drift check
  local time: Sun Nov  5 00:01:53 UTC 2023
  local time: Sun Nov  5 00:01:53 UTC 2023
Session terminated, killing shell... ...killed.
##[error]The operation was canceled.
Cleaning up orphan processes

[bors]

💔 Test failed - checks-actions

[Voultapher]

It's not clear to me if this is a true positive CI failure ##[error]The runner has received a shutdown signal. This can happen when the runner service is stopped, or a manually started runner is canceled..

[Mark-Simulacrum]

@bors retry

[bors]

⌛ Testing commit e501add with merge 5103173...

[bors]

☀️ Test successful - checks-actions
Approved by: Mark-Simulacrum
Pushing 5103173 to master...

[rust-timer]

Finished benchmarking commit (5103173): 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)	0.7%	[0.5%, 1.0%]	4
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-1.5%	[-4.0%, -0.5%]	5
Improvements ✅ (secondary)	-1.1%	[-1.1%, -1.1%]	2
All ❌✅ (primary)	-0.5%	[-4.0%, 1.0%]	9

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)	0.4%	[0.4%, 0.4%]	1
Regressions ❌ (secondary)	0.4%	[0.4%, 0.4%]	1
Improvements ✅ (primary)	-0.4%	[-0.4%, -0.4%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.0%	[-0.4%, 0.4%]	2

Binary size

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)	0.0%	[0.0%, 0.1%]	6
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.0%	[0.0%, 0.1%]	6

Bootstrap: 638.861s -> 635.766s (-0.48%)
Artifact size: 304.51 MiB -> 304.41 MiB (-0.03%)