Simplify const memchr.

[reitermarkus]

Extracted from #101607.

Removes the need for const_eval_select.

[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.

Examples of T-libs-api changes:

• Stabilizing library features
• Introducing insta-stable changes such as new implementations of existing stable traits on existing stable types
• Introducing new or changing existing unstable library APIs (excluding permanently unstable features / features without a tracking issue)
• Changing public documentation in ways that create new stability guarantees
• Changing observable runtime behavior of library APIs

[rust-highfive]

r? @thomcc

(rust-highfive has picked a reviewer for you, use r? to override)

[thomcc]

@bors try @rust-timer queue

[rust-timer]

Awaiting bors try build completion.

@rustbot label: +S-waiting-on-perf

[bors]

⌛ Trying commit 044e06b1137ca15470c4d49b2504de75800f256c with merge 9dc04300a5a305d2d428acbab5de557b568a1fe2...

[reitermarkus]

@rustbot label +const-hack

[thomcc]

I think pushing to the branch after queuing the perf job killed it...

[thomcc]

@bors try @rust-timer queue

[rust-timer]

Awaiting bors try build completion.

@rustbot label: +S-waiting-on-perf

[bors]

⌛ Trying commit db29de7 with merge 1910faf74b5eaa029de0304fb4e055b9aa3219a1...

[bors]

☀️ Try build successful - checks-actions
Build commit: 1910faf74b5eaa029de0304fb4e055b9aa3219a1 (1910faf74b5eaa029de0304fb4e055b9aa3219a1)

[rust-timer]

Queued 1910faf74b5eaa029de0304fb4e055b9aa3219a1 with parent 294f0ee, future comparison URL.

[rust-timer]

Finished benchmarking commit (1910faf74b5eaa029de0304fb4e055b9aa3219a1): comparison URL.

Overall result: ✅ improvements - 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-review -S-waiting-on-perf -perf-regression

Instruction count

This is a highly reliable metric that was used to determine the overall result at the top of this comment.

	mean1	range	count2
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.4%	[-0.4%, -0.4%]	3
All ❌✅ (primary)	-	-	0

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.

	mean1	range	count2
Regressions ❌ (primary)	3.1%	[3.0%, 3.1%]	2
Regressions ❌ (secondary)	7.4%	[7.4%, 7.4%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	3.1%	[3.0%, 3.1%]	2

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.

	mean1	range	count2
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	3.7%	[3.7%, 3.7%]	1
Improvements ✅ (primary)	-10.0%	[-10.0%, -10.0%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-10.0%	[-10.0%, -10.0%]	1

Footnotes

1. the arithmetic mean of the percent change ↩ ↩² ↩³

2. number of relevant changes ↩ ↩² ↩³

[thomcc]

Looks fine.

@bors r+

[bors]

📌 Commit db29de7 has been approved by thomcc

It is now in the queue for this repository.

[bors]

⌛ Testing commit db29de7 with merge b195f53...

[bors]

☀️ Test successful - checks-actions
Approved by: thomcc
Pushing b195f53 to master...

[rust-timer]

Finished benchmarking commit (b195f53): comparison URL.

Overall result: ✅ improvements - no action needed

@rustbot label: -perf-regression

Instruction count

This is a highly reliable metric that was used to determine the overall result at the top of this comment.

	mean1	range	count2
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.4%	[-0.5%, -0.4%]	3
All ❌✅ (primary)	-	-	0

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.

	mean1	range	count2
Regressions ❌ (primary)	1.9%	[1.7%, 2.2%]	3
Regressions ❌ (secondary)	5.1%	[5.1%, 5.1%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.5%	[-3.5%, -3.5%]	1
All ❌✅ (primary)	1.9%	[1.7%, 2.2%]	3

Cycles

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

Footnotes

1. the arithmetic mean of the percent change ↩ ↩²

2. number of relevant changes ↩ ↩²

[klensy]

Perf runs shows literally nothing if there no catastrophic regression\improvement, or fn not used a lot (not this case). Microbenches is probably good way to check this kind of changes.