[EXPERIMENT] Check if isolating Matrix::unspecialize dodges regressions

[Zalathar]

We know from #161496 that DenseBitSet::insert is extremely sensitive to seemingly-irrelevant changes.

This is a perf experiment to see whether isolating Matrix::unspecialize from those changes avoids the regression.

[Zalathar]

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: ee3df1b (ee3df1bf2c2ef71bdbf802b6fefa8f503eabbe35)
Base parent: fb6531d (fb6531d550e0075b9eb9a51464f404805eec87d9)

[rust-timer]

Finished benchmarking commit (ee3df1b): comparison URL.

Overall result: ❌ regressions - please read:

Benchmarking means the PR may be perf-sensitive. It's automatically marked not fit for rolling up. Overriding is possible but disadvised: it risks changing compiler perf.

Next, please: If you can, justify the regressions found in this try perf run in writing along with @rustbot label: +perf-regression-triaged. If not, fix the regressions and do another perf run. Neutral or positive results will clear the label automatically.

@bors rollup=never rustc-perf
@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
Regressions ❌ (secondary)	0.8%	[0.6%, 1.5%]	8
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

This perf run didn't have relevant results for this metric.

Cycles

Results (primary 2.4%, secondary 0.9%)

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

	mean	range	count
Regressions ❌ (primary)	2.4%	[2.4%, 2.5%]	2
Regressions ❌ (secondary)	2.5%	[2.5%, 2.5%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.1%	[-2.1%, -2.1%]	1
All ❌✅ (primary)	2.4%	[2.4%, 2.5%]	2

Binary size

Results (primary -0.1%, secondary -0.1%)

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)	-	-	0
Improvements ✅ (primary)	-0.1%	[-0.2%, -0.0%]	34
Improvements ✅ (secondary)	-0.1%	[-0.2%, -0.0%]	38
All ❌✅ (primary)	-0.1%	[-0.2%, -0.0%]	34

Bootstrap: 469.882s -> 469.633s (-0.05%)
Artifact size: 400.35 MiB -> 400.20 MiB (-0.04%)

[Zalathar]

Unfortunately, it's hard to say whether this result means that unspecialize is not the culprit, or whether it's merely so sensitive that even this change perturbs it.