[EXPERIMENT] Check if isolating `Matrix::unspecialize` dodges regressions by Zalathar · Pull Request #161647 · rust-lang/rust

Zalathar · 2026-08-24T06:51:21Z

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

Zalathar · 2026-08-24T06:51:34Z

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

panstromek · 2026-08-24T07:39:24Z

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 · 2026-08-24T07:50:12Z

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

rust-timer · 2026-08-24T10:58:11Z

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 · 2026-08-24T11:49:52Z

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

@bors try @rust-timer queue

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

rust-bors · 2026-08-24T12:48:41Z

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

rust-timer · 2026-08-24T15:36:53Z

Finished benchmarking commit (362939d): 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)

Results (primary 7.2%)

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

	mean	range	count
Regressions ❌ (primary)	7.2%	[7.2%, 7.2%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	7.2%	[7.2%, 7.2%]	1

Cycles

Results (primary -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)	-	-	0
Improvements ✅ (primary)	-2.0%	[-2.0%, -2.0%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-2.0%	[-2.0%, -2.0%]	1

Binary size

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

Bootstrap: 468.927s -> 469.784s (0.18%)
Artifact size: 400.24 MiB -> 400.28 MiB (0.01%)

panstromek · 2026-08-24T19:54:26Z

All these results are quite puzzling. So I guess the only remaining explanation is that the regression is not in match checking after all?

panstromek · 2026-08-24T20:03:21Z

btw. The commit benchmarked here #161647 (comment) does nothing (insert2 is just the exact same function as insert). Was that intentional?

panstromek · 2026-08-24T20:16:48Z

Given the above comment, I think the only combination wee haven't tried is this one: 7f86311 (It seems to me that this was the original intent of this PR)

I kicked off a perf run in #161496 (comment). If this one doesn't regress, it (most likely means) that the regression is not in match checking code.

Zalathar · 2026-08-25T02:13:58Z

btw. The commit benchmarked here #161647 (comment) does nothing (insert2 is just the exact same function as insert). Was that intentional?

Yes, one of my perf runs was trying to check whether even an identical duplicate of insert was enough to trigger regressions.

(This made more sense when we thought that particular call site was the culprit. Either way, it didn’t produce an interesting result.)

Zalathar · 2026-08-25T03:58:59Z

Here's another experiment: What happens if we switch to the simpler insert2 only in other data structures that use DenseBitSet internally? (patch)

@bors try @rust-timer queue

Zalathar · 2026-08-28T04:12:51Z

Let's try using insert2 throughout rustc_mir_transform (patch).

@bors try @rust-timer queue

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

rust-bors · 2026-08-28T05:18:43Z

☀️ Try build successful (CI)
Build commit: 5509a41 (5509a41d07fe1dc00748b096ee54632336866015)
Base parent: e457a7b (e457a7b0d326d67b4322ef0d11bd715cfaeda48f)

rust-timer · 2026-08-28T06:01:43Z

Finished benchmarking commit (5509a41): 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)

Results (primary 0.1%)

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

	mean	range	count
Regressions ❌ (primary)	2.3%	[2.3%, 2.3%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-2.1%	[-2.1%, -2.1%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.1%	[-2.1%, 2.3%]	2

Cycles

Results (primary -2.2%, secondary 3.3%)

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)	3.3%	[3.3%, 3.3%]	1
Improvements ✅ (primary)	-2.2%	[-2.4%, -2.0%]	3
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-2.2%	[-2.4%, -2.0%]	3

Binary size

Results (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
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.0%	[-0.0%, -0.0%]	1
All ❌✅ (primary)	-	-	0

Bootstrap: 476.343s -> 477.044s (0.15%)
Artifact size: 402.74 MiB -> 402.70 MiB (-0.01%)

Zalathar · 2026-08-28T06:06:04Z

This benchmark result suggests that the sensitive call site is not in rustc_mir_transform.

Zalathar · 2026-08-28T06:17:51Z

Let's try using insert2 throughout rustc_mir_dataflow (patch).

@bors try @rust-timer queue

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

rust-bors · 2026-08-28T07:16:49Z

☀️ Try build successful (CI)
Build commit: 172ad2d (172ad2da9ee99b17aa55fda6190feb71f24460d0)
Base parent: d0f2ef5 (d0f2ef5e53039bd86fdcaa6e71860c4948880e04)

rust-timer · 2026-08-28T09:04:14Z

Finished benchmarking commit (172ad2d): 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)

Results (primary -2.4%, secondary 6.7%)

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)	6.7%	[6.7%, 6.7%]	1
Improvements ✅ (primary)	-2.4%	[-2.4%, -2.3%]	2
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-2.4%	[-2.4%, -2.3%]	2

Cycles

Results (primary -2.5%, secondary 3.4%)

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)	3.4%	[3.4%, 3.4%]	1
Improvements ✅ (primary)	-2.5%	[-2.5%, -2.5%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-2.5%	[-2.5%, -2.5%]	1

Binary size

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

Bootstrap: 475.629s -> 476.245s (0.13%)
Artifact size: 402.76 MiB -> 402.72 MiB (-0.01%)

- rustc_abi - rustc_codegen_ssa - rustc_hir_analysis - rustc_ty_utils

Zalathar · 2026-08-28T13:15:55Z

We're running out of candidates, but let's try using insert2 in all remaining places: (patch)

rustc_abi
rustc_codegen_ssa
rustc_hir_analysis
rustc_ty_utils

If this doesn't trigger the regression, then either I've missed something, or we have some spooky aggregate effects.

@bors try @rust-timer queue

rust-timer · 2026-08-28T13:15:58Z

Awaiting bors try build completion.

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

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

rust-bors · 2026-08-28T14:15:45Z

☀️ Try build successful (CI)
Build commit: a065bfc (a065bfc1682eaeb9f48e6ffd952a5e87c520ba34)
Base parent: c42ac5f (c42ac5fd59628ea7e2f52af5944c2aaac3f0e7f6)

rust-timer · 2026-08-28T14:15:49Z

Queued a065bfc with parent c42ac5f, future comparison URL.
There are currently 0 preceding artifacts in the queue.
It will probably take at least ~1.0 hours until the benchmark run finishes.

rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Aug 24, 2026