Tweak SlicePartialEq to allow MIR-inlining the compare_bytes call

[scottmcm]

#150265 disabled this because it was a net perf win, but let's see if we can tweak the structure of this to allow more inlining on this side while still not MIR-inlining the loop when it's not just memcmp and thus hopefully preserving the perf win.

This should also allow MIR-inlining the length check, which was previously blocked, and thus might allow some obvious non-matches to optimize away as well.

[scottmcm]

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: 8018bcc (8018bcc8c2cb8bbdb7e2eee7163156d48c0bcc85, parent: f57eac1bf98cb5d578e3364b64365ec398c137df)

[rust-timer]

Finished benchmarking commit (8018bcc): comparison URL.

Overall result: ❌✅ regressions and improvements - please read the text below

Benchmarking this pull request means it may be perf-sensitive – we'll automatically label it not fit for rolling up. You can override this, but we strongly advise not to, due to possible changes in compiler perf.

Next Steps: If you can justify the regressions found in this try perf run, please do so in sufficient writing along with @rustbot label: +perf-regression-triaged. If not, please fix the regressions and do another perf run. If its results are neutral or positive, the label will be automatically removed.

@bors rollup=never
@rustbot label: -S-waiting-on-perf +perf-regression

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	0.5%	[0.3%, 1.1%]	7
Regressions ❌ (secondary)	0.4%	[0.4%, 0.4%]	1
Improvements ✅ (primary)	-0.8%	[-2.8%, -0.1%]	5
Improvements ✅ (secondary)	-0.9%	[-2.7%, -0.1%]	3
All ❌✅ (primary)	-0.0%	[-2.8%, 1.1%]	12

Max RSS (memory usage)

Results (primary 1.0%, secondary -2.7%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	3.9%	[2.0%, 10.8%]	6
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-4.9%	[-6.7%, -3.1%]	3
Improvements ✅ (secondary)	-2.7%	[-2.7%, -2.7%]	1
All ❌✅ (primary)	1.0%	[-6.7%, 10.8%]	9

Cycles

Results (primary 2.0%, secondary 2.7%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	2.0%	[2.0%, 2.0%]	1
Regressions ❌ (secondary)	2.7%	[2.0%, 3.2%]	3
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	2.0%	[2.0%, 2.0%]	1

Binary size

Results (primary 0.0%, secondary -0.0%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	0.4%	[0.1%, 0.7%]	4
Regressions ❌ (secondary)	0.3%	[0.1%, 0.5%]	4
Improvements ✅ (primary)	-0.3%	[-1.2%, -0.0%]	4
Improvements ✅ (secondary)	-1.2%	[-1.2%, -1.2%]	1
All ❌✅ (primary)	0.0%	[-1.2%, 0.7%]	8

Bootstrap: 473.812s -> 477.487s (0.78%)
Artifact size: 391.34 MiB -> 391.34 MiB (0.00%)

[scottmcm]

Hmm, so this recovered the syn loss from #150265 (comment), but isn't an obvious overall win.

I do like removing the second [rustc_no_mir_inline] that #150265 had added, though, so maybe it makes sense regardless.

cc @saethlin in case you have any thoughts here.

[rustbot]

r? @Mark-Simulacrum

rustbot has assigned @Mark-Simulacrum.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

[saethlin]

I don't have any strong opinions on this. It would be neat if we had better ways to learn about the impacts of MIR opts than squinting at the perf result.

[Mark-Simulacrum]

r=me with rebase if we want to go ahead.

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[rust-log-analyzer]

The job x86_64-gnu-tools failed! Check out the build log: (web) (plain enhanced) (plain)

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

REPOSITORY                                   TAG       IMAGE ID       CREATED      SIZE
ghcr.io/dependabot/dependabot-updater-core   latest    354d02aa29ac   7 days ago   783MB
=> Removing docker images...
Deleted Images:
untagged: ghcr.io/dependabot/dependabot-updater-core:latest
untagged: ghcr.io/dependabot/dependabot-updater-core@sha256:596da3f22bcbdff2c96fd7126001278022c834c1621c5efa2ad1a7794590636c
deleted: sha256:354d02aa29acf525570c732b6e006ecf138de6d63ca525d552eb4b24880ddc6c
deleted: sha256:8b7af0e426bc2cbeeacfd96b8354d3b80016991520977197e62090e47abaede8
deleted: sha256:cadf11ef1de7fdd5eab563757942353684047f09b212dc99d6ed48e8acf34d62
deleted: sha256:569b0caf9d5285db44ccd2629a3470139eea755be423a33a54d8a24cb3926bfa
deleted: sha256:f9dc5feb048d8f9fd43137e3998f59e9acfbd76c47a4e14984d109654119e282
---
tests/ui/double_parens.rs ... ok
tests/ui/drain_collect.fixed ... ok
tests/ui/duplicate_underscore_argument.rs ... ok
tests/ui/duplicated_attributes.rs ... ok
tests/ui/duration_suboptimal_units.rs ... ok
tests/ui/duration_suboptimal_units_days_weeks.rs ... ok
tests/ui/duration_subsec.rs ... ok
tests/ui/double_parens.fixed ... ok
tests/ui/duration_suboptimal_units_days_weeks.fixed ... ok
tests/ui/duration_suboptimal_units.fixed ... ok
tests/ui/duration_subsec.fixed ... ok
tests/ui/empty_docs.rs ... ok
tests/ui/elidable_lifetime_names.rs ... ok
tests/ui/else_if_without_else.rs ... ok
tests/ui/eager_transmute.rs ... ok
---
[WARNING] line 39: Delta is 0 for "x", maybe try to use `compare-elements-position` instead?

======== tests/rustdoc-gui/setting-go-to-only-result.goml ========

[ERROR] setting-go-to-only-result output:
Execution context was destroyed, most likely because of a navigation.
stack: Error: Execution context was destroyed, most likely because of a navigation.
    at rewriteError (/checkout/obj/build/x86_64-unknown-linux-gnu/test/rustdoc-gui/node_modules/puppeteer-core/lib/cjs/puppeteer/cdp/ExecutionContext.js:457:15)
    at async #evaluate (/checkout/obj/build/x86_64-unknown-linux-gnu/test/rustdoc-gui/node_modules/puppeteer-core/lib/cjs/puppeteer/cdp/ExecutionContext.js:389:60)
    at async ExecutionContext.evaluate (/checkout/obj/build/x86_64-unknown-linux-gnu/test/rustdoc-gui/node_modules/puppeteer-core/lib/cjs/puppeteer/cdp/ExecutionContext.js:277:16)
    at async IsolatedWorld.evaluate (/checkout/obj/build/x86_64-unknown-linux-gnu/test/rustdoc-gui/node_modules/puppeteer-core/lib/cjs/puppeteer/cdp/IsolatedWorld.js:100:16)
    at async CdpFrame.evaluate (/checkout/obj/build/x86_64-unknown-linux-gnu/test/rustdoc-gui/node_modules/puppeteer-core/lib/cjs/puppeteer/api/Frame.js:362:20)
    at async CdpPage.evaluate (/checkout/obj/build/x86_64-unknown-linux-gnu/test/rustdoc-gui/node_modules/puppeteer-core/lib/cjs/puppeteer/api/Page.js:826:20)
    at async /checkout/obj/build/x86_64-unknown-linux-gnu/test/rustdoc-gui/node_modules/browser-ui-test/src/index.js:432:28
    at async waitForConditionTrue (/checkout/obj/build/x86_64-unknown-linux-gnu/test/rustdoc-gui/node_modules/browser-ui-test/src/utils.js:209:13)
    at async runAllCommands (/checkout/obj/build/x86_64-unknown-linux-gnu/test/rustdoc-gui/node_modules/browser-ui-test/src/index.js:431:22)
    at async innerRunTestCode (/checkout/obj/build/x86_64-unknown-linux-gnu/test/rustdoc-gui/node_modules/browser-ui-test/src/index.js:714:21)



<= doc-ui tests done: 146 succeeded, 1 failed, 0 filtered out

[scottmcm]

"Execution context was destroyed, most likely because of a navigation."

[scottmcm]

The more I think about it the more I like removing the rustc_no_mir_inline that needed a giant comment, so let's do it.

@bors r=Mark-Simulacrum

[rust-bors]

📌 Commit 51de309 has been approved by Mark-Simulacrum

It is now in the queue for this repository.

[Zalathar]

Scheduling: Encourage a mixture of rollup and non-rollup PRs.

@bors p=5

[rust-bors]

☀️ Test successful - CI
Approved by: Mark-Simulacrum
Duration: 3h 10m 31s
Pushing 1e5065a to main...

[github-actions]

What is this?

This is an experimental post-merge analysis report that shows differences in test outcomes between the merged PR and its parent PR.

Comparing a234ae6 (parent) -> 1e5065a (this PR)

Test differences

No test diffs found

Test dashboard

Run

cargo run --manifest-path src/ci/citool/Cargo.toml -- \
    test-dashboard 1e5065a4d99e0e3ccf1a1719055308e7a20e8f36 --output-dir test-dashboard

And then open test-dashboard/index.html in your browser to see an overview of all executed tests.

Job duration changes

1. pr-check-1: 1921.4s -> 1644.9s (-14.4%)
2. aarch64-gnu-debug: 4603.4s -> 3957.1s (-14.0%)
3. x86_64-gnu-llvm-21-3: 7011.9s -> 6049.7s (-13.7%)
4. dist-apple-various: 5134.1s -> 4444.9s (-13.4%)
5. aarch64-gnu-llvm-20-1: 3908.8s -> 3403.2s (-12.9%)
6. armhf-gnu: 5435.4s -> 4744.4s (-12.7%)
7. i686-gnu-2: 6104.1s -> 5387.0s (-11.7%)
8. i686-gnu-1: 8456.9s -> 7489.8s (-11.4%)
9. x86_64-gnu-llvm-20: 4644.9s -> 4125.3s (-11.2%)
10. x86_64-gnu-gcc: 4038.9s -> 3663.1s (-9.3%)

How to interpret the job duration changes?

Job durations can vary a lot, based on the actual runner instance
that executed the job, system noise, invalidated caches, etc. The table above is provided
mostly for t-infra members, for simpler debugging of potential CI slow-downs.

[rust-timer]

Finished benchmarking commit (1e5065a): comparison URL.

Overall result: ❌✅ regressions and improvements - please read the text below

Our benchmarks found a performance regression caused by this PR.
This might be an actual regression, but it can also be just noise.

Next Steps:

• If the regression was expected or you think it can be justified,
  please write a comment with sufficient written justification, and add
  @rustbot label: +perf-regression-triaged to it, to mark the regression as triaged.
• If you think that you know of a way to resolve the regression, try to create
  a new PR with a fix for the regression.
• If you do not understand the regression or you think that it is just noise,
  you can ask the @rust-lang/wg-compiler-performance working group for help (members of this group
  were already notified of this PR).

@rustbot label: +perf-regression
cc @rust-lang/wg-compiler-performance

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	0.4%	[0.3%, 0.6%]	6
Regressions ❌ (secondary)	0.7%	[0.7%, 0.7%]	1
Improvements ✅ (primary)	-0.9%	[-2.3%, -0.1%]	7
Improvements ✅ (secondary)	-2.3%	[-2.3%, -2.3%]	1
All ❌✅ (primary)	-0.3%	[-2.3%, 0.6%]	13

Max RSS (memory usage)

Results (primary -7.2%, secondary 2.0%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	5.4%	[4.6%, 5.7%]	5
Improvements ✅ (primary)	-7.2%	[-10.8%, -3.6%]	2
Improvements ✅ (secondary)	-3.6%	[-6.9%, -2.0%]	3
All ❌✅ (primary)	-7.2%	[-10.8%, -3.6%]	2

Cycles

Results (primary 8.6%, secondary 4.8%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	9.5%	[3.2%, 17.3%]	12
Regressions ❌ (secondary)	4.8%	[2.5%, 6.3%]	4
Improvements ✅ (primary)	-2.0%	[-2.0%, -2.0%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	8.6%	[-2.0%, 17.3%]	13

Binary size

Results (primary -0.0%, secondary -0.0%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	0.1%	[0.0%, 0.6%]	25
Regressions ❌ (secondary)	0.1%	[0.0%, 0.2%]	9
Improvements ✅ (primary)	-0.3%	[-1.0%, -0.0%]	12
Improvements ✅ (secondary)	-1.0%	[-1.0%, -1.0%]	1
All ❌✅ (primary)	-0.0%	[-1.0%, 0.6%]	37

Bootstrap: 476.908s -> 474.687s (-0.47%)
Artifact size: 397.92 MiB -> 397.87 MiB (-0.01%)

[scottmcm]

The cycle regression looks like html5ever bimodality -- it went back to the previous level in #151550.

(It appears that #151646 and #151674 had similar spikes.)