Enable MatchBranchSimplification

[saethlin]

This pass is one of the small number of benefits from -Zmir-opt-level=3 that has motivated rustc_codegen_cranelift to use it:

rust/compiler/rustc_codegen_cranelift/build_system/build_sysroot.rs

Lines 244 to 246 in 19ed0aa

	if channel == "release" {
	rustflags.push_str(" -Zmir-opt-level=3");
	}

Cranelift's motivation for this is runtime performance improvements in debug builds. Lifting this pass all the way to -Zmir-opt-level=1 seems to come without significant perf overhead, so that's what I'm suggesting here.

[saethlin]

@bors try @rust-timer queue

[bors]

⌛ Trying commit 4a46d791044568189a3087b08ef918f97b4f179d with merge 915cd328c0e78ffd700e8e29dca4916629ebdcef...

[bors]

☀️ Try build successful - checks-actions
Build commit: 915cd328c0e78ffd700e8e29dca4916629ebdcef (915cd328c0e78ffd700e8e29dca4916629ebdcef)

[bors]

☀️ Try build successful - checks-actions
Build commit: 915cd328c0e78ffd700e8e29dca4916629ebdcef (915cd328c0e78ffd700e8e29dca4916629ebdcef)

[rust-timer]

Finished benchmarking commit (915cd328c0e78ffd700e8e29dca4916629ebdcef): comparison URL.

Overall result: ❌✅ regressions and improvements - ACTION NEEDED

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. While you can manually mark this PR as fit for rollup, we strongly recommend not doing so since this PR may lead to changes in compiler perf.

Next Steps: If you can justify the regressions found in this try perf run, please indicate this with @rustbot label: +perf-regression-triaged along with sufficient written justification. If you cannot justify the regressions please fix the regressions and do another perf run. If the next run shows neutral or positive results, the label will be automatically removed.

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

Instruction count

This is a highly reliable metric that was used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	0.9%	[0.7%, 1.1%]	7
Regressions ❌ (secondary)	0.4%	[0.3%, 0.5%]	3
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.2%	[-0.2%, -0.2%]	1
All ❌✅ (primary)	0.9%	[0.7%, 1.1%]	7

Max RSS (memory usage)

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	3.8%	[2.2%, 6.5%]	3
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.0%	[-0.0%, -0.0%]	1
Improvements ✅ (secondary)	-3.0%	[-3.0%, -3.0%]	1
All ❌✅ (primary)	2.9%	[-0.0%, 6.5%]	4

Cycles

This benchmark run did not return any relevant results for this metric.

Binary size

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	0.1%	[0.0%, 0.2%]	4
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.1%	[-0.2%, -0.0%]	26
Improvements ✅ (secondary)	-0.2%	[-0.3%, -0.0%]	4
All ❌✅ (primary)	-0.1%	[-0.2%, 0.2%]	30

Bootstrap: 643.928s -> 643.82s (-0.02%)

[saethlin]

@bors try @rust-timer queue

[bors]

⌛ Trying commit a770220fd1b41a35ddfffa79b328730e4b030302 with merge 31cb4817bc7bb15715807f7109f203feacd9abce...

[bors]

☀️ Try build successful - checks-actions
Build commit: 31cb4817bc7bb15715807f7109f203feacd9abce (31cb4817bc7bb15715807f7109f203feacd9abce)

[rust-timer]

Finished benchmarking commit (31cb4817bc7bb15715807f7109f203feacd9abce): comparison URL.

Overall result: ❌ regressions - no action needed

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. While you can manually mark this PR as fit for rollup, we strongly recommend not doing so since this PR may lead to changes in compiler perf.

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

Instruction count

This is a highly reliable metric that was used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	0.7%	[0.7%, 0.7%]	1
Regressions ❌ (secondary)	0.4%	[0.4%, 0.5%]	3
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.7%	[0.7%, 0.7%]	1

Max RSS (memory usage)

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	7.1%	[7.1%, 7.1%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.0%	[-0.0%, -0.0%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	3.6%	[-0.0%, 7.1%]	2

Cycles

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.7%	[-0.7%, -0.7%]	1
Improvements ✅ (secondary)	-3.3%	[-4.2%, -2.3%]	2
All ❌✅ (primary)	-0.7%	[-0.7%, -0.7%]	1

Binary size

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	0.1%	[0.0%, 0.2%]	4
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.1%	[-0.2%, -0.0%]	26
Improvements ✅ (secondary)	-0.2%	[-0.3%, -0.0%]	4
All ❌✅ (primary)	-0.1%	[-0.2%, 0.2%]	30

Bootstrap: 644.897s -> 644.188s (-0.11%)

[rustbot]

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

[cjgillot]

Re-reading the pass, I found a possible bug. We do not check that bb_idx != first nor bb_idx != second, only that first != second. This will ICE at.
https://github.com/rust-lang/rust/blob/a770220fd1b41a35ddfffa79b328730e4b030302/compiler/rustc_mir_transform/src/match_branches.rs#L115

For instance: https://godbolt.org/z/dba95Y84P

Perf reports a slight improvement.
r=me with that bug fixed and a test.

[cjgillot]

@bors r+

[bors]

📌 Commit 7e04c93 has been approved by cjgillot

It is now in the queue for this repository.

[bors]

⌛ Testing commit 7e04c93 with merge f59d577...

[bors]

☀️ Test successful - checks-actions
Approved by: cjgillot
Pushing f59d577 to master...

[rust-timer]

Finished benchmarking commit (f59d577): comparison URL.

Overall result: ❌✅ regressions and improvements - ACTION NEEDED

Next Steps: If you can justify the regressions found in this perf run, please indicate this with @rustbot label: +perf-regression-triaged along with sufficient written justification. If you cannot justify the regressions please open an issue or create a new PR that fixes the regressions, add a comment linking to the newly created issue or PR, and then add the perf-regression-triaged label to this PR.

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

Instruction count

This is a highly reliable metric that was used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	0.5%	[0.3%, 0.7%]	5
Regressions ❌ (secondary)	0.6%	[0.2%, 0.8%]	10
Improvements ✅ (primary)	-0.3%	[-0.4%, -0.2%]	4
Improvements ✅ (secondary)	-0.3%	[-0.3%, -0.3%]	1
All ❌✅ (primary)	0.2%	[-0.4%, 0.7%]	9

Max RSS (memory usage)

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	6.3%	[6.3%, 6.3%]	1
Regressions ❌ (secondary)	2.6%	[2.6%, 2.6%]	1
Improvements ✅ (primary)	-0.0%	[-0.0%, -0.0%]	1
Improvements ✅ (secondary)	-1.7%	[-1.7%, -1.7%]	1
All ❌✅ (primary)	3.2%	[-0.0%, 6.3%]	2

Cycles

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

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

Binary size

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	0.1%	[0.0%, 0.2%]	4
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.1%	[-0.2%, -0.0%]	26
Improvements ✅ (secondary)	-0.2%	[-0.3%, -0.0%]	4
All ❌✅ (primary)	-0.1%	[-0.2%, 0.2%]	30

Bootstrap: 644.831s -> 647.057s (0.35%)

[saethlin]

That's definitely not what I expected. I wouldn't have suggested we merge that if I got a perf report like that before merge. I'll try to look into this today?

[saethlin]

The codegen for process_obligations has a few surprise movs in it that it didn't before. We also now inline <Rc<ObligationCauseCode> as Drop>::drop.

The perf regression is only in icount, no other metrics look concerning. So I think this is probably fine to leave as-is but I'm going to keep investigating because it seems plausible that this could have moved our generated code a bit in the wrong direction.

[saethlin]

There are two call sites for Rc<ObligationCauseCode> as Drop>::drop that show up in a perf-record profile. Only one of them gets inlined with this PR, and I think PGO has more to do with that change than a MIR opt change. So I think investigation here is basically a dead end.