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]

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[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.