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

[Zalathar]

• Successor to [EXPERIMENT] Check if isolating Matrix::unspecialize dodges regressions #161632
• See also [EXPERIMENT] RawBitSet experiments #153589

[Zalathar]

@bors try @rust-timer queue

[panstromek]

Maybe I was wrong in the original assesment and the culprit is a different insert call in match checking - there's another one in ConstructorSet::split:

rust/compiler/rustc_pattern_analysis/src/constructor.rs

Lines 1106 to 1108 in 05320a2

	for idx in seen.iter().filter_map(|c| c.as_variant()) {
	seen_set.insert(idx);
	}

This was pretty cold in cachegrind, so I didn't pay attention to it very much, but at this point it feels like whatever you do there's some spooky regression at a distance, so maybe it's not as innocent as it looks.

(and there's no other insert call in match checking as far as I know, including transitively called ones)

[rust-bors]

☀️ Try build successful (CI)
Build commit: 0fd30f7 (0fd30f71aa360d59aa9283318317b761e764e41c)
Base parent: da51146 (da5114692c9ebe46b869488c5f34f92eb10b98c1)

[rust-timer]

Finished benchmarking commit (0fd30f7): comparison URL.

Overall result: no relevant changes - no action needed

Benchmarking means the PR may be perf-sensitive. Consider adding rollup=never if this change is not fit for rolling up.

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

Instruction count

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

Max RSS (memory usage)

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

Cycles

Results (primary -0.3%, secondary -4.6%)

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)	-	-	0
Improvements ✅ (primary)	-2.6%	[-2.6%, -2.6%]	1
Improvements ✅ (secondary)	-4.6%	[-4.6%, -4.6%]	1
All ❌✅ (primary)	-0.3%	[-2.6%, 2.0%]	2

Binary size

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

Bootstrap: 469.244s -> 469.33s (0.02%)
Artifact size: 400.19 MiB -> 400.28 MiB (0.02%)

[Zalathar]

Let's see if only perturbing ConstructorSet::split causes regressions:

@bors try @rust-timer queue

[rust-timer]

Awaiting bors try build completion.

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

[rust-bors]

☀️ Try build successful (CI)
Build commit: 362939d (362939d9b1e7906cedbecdee745c0ed9eec3a41d)
Base parent: 347a1db (347a1dbf428ab92f284e3afafacfa7eb46341a5d)

[rust-timer]

Queued 362939d with parent 347a1db, future comparison URL.
There are currently 2 preceding artifacts in the queue.
It will probably take at least ~3.0 hours until the benchmark run finishes.