Use code with reliable branchless code-gen for slice::sort merge

[Voultapher]

The recent LLVM 16 update changes code-gen to be not branchless anymore, in the slice::sort implementation merge function. This improves performance by 30% for random patterns, restoring the performance to the state with LLVM 15.

Fixes #111559

[rustbot]

r? @cuviper

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[cuviper]

LGTM, and I also confirmed the perf change locally.

@bors r+

[bors]

📌 Commit 42655ff has been approved by cuviper

It is now in the queue for this repository.

[bors]

⌛ Testing commit 42655ff with merge fe76e14...

[bors]

☀️ Test successful - checks-actions
Approved by: cuviper
Pushing fe76e14 to master...

[rust-timer]

Finished benchmarking commit (fe76e14): 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)	-	-	0
Improvements ✅ (primary)	-3.6%	[-3.6%, -3.6%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-3.6%	[-3.6%, -3.6%]	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)	4.4%	[1.5%, 12.2%]	12
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.3%	[-2.3%, -2.3%]	1
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
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.0%	[-0.1%, -0.0%]	21
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.0%	[-0.1%, -0.0%]	21

Bootstrap: 643.563s -> 645.412s (0.29%)