Use llvm.memset.p0i8.* to initialize all same-bytes arrays

[oli-obk]

It doesn't affect tests, LLVM seems smart enough for it, but then I wonder why we have the zero case at all (it was introduced in #43488, maybe LLVM wasn't smart enough then). So let's run perf to see if there's any build time effect, and if no, I'll remove the zero special case and also run perf.

[rustbot]

r? @saethlin

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[oli-obk]

@bors try @rust-timer queue

[oli-obk]

@bors try @rust-timer queue

[bors]

⌛ Trying commit 3ca023e with merge 7ed3665...

[bors]

☀️ Try build successful - checks-actions
Build commit: 7ed3665 (7ed366534d90da69be91456384caf3738966680a)

[rust-timer]

Finished benchmarking commit (7ed3665): comparison URL.

Overall result: ❌ regressions - 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 is the most reliable metric that we have; it was used to determine the overall result at the top of this comment. However, even this metric can sometimes exhibit noise.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	0.3%	[0.3%, 0.3%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (primary 5.2%, secondary -2.5%)

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)	5.2%	[2.0%, 8.3%]	2
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.5%	[-2.5%, -2.5%]	1
All ❌✅ (primary)	5.2%	[2.0%, 8.3%]	2

Cycles

Results (primary -1.9%, secondary -3.3%)

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)	-1.9%	[-3.4%, -0.6%]	12
Improvements ✅ (secondary)	-3.3%	[-3.8%, -2.2%]	3
All ❌✅ (primary)	-1.9%	[-3.4%, -0.6%]	12

Binary size

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

Bootstrap: 763.722s -> 763.396s (-0.04%)
Artifact size: 325.68 MiB -> 325.71 MiB (0.01%)