Remove fewer Storage calls in CopyProp and GVN #142531

ohadravid · 2025-06-15T07:51:38Z

Modify the CopyProp and GVN MIR optimization passes to remove fewer Storage{Live,Dead} calls, allowing for better optimizations by LLVM - see #141649.

Details

The idea is to use a new MaybeUninitializedLocals analysis and remove only the storage calls of locals that are maybe-uninit when accessed in a new location.

rustbot · 2025-06-15T07:51:44Z

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

matthiaskrgr · 2025-06-15T09:37:15Z

@bors try @rust-timer queue

…try> Remove fewer Storage calls in `copy_prop` Modify the `copy_prop` MIR optimization pass to remove fewer `Storage{Live,Dead}` calls, allowing for better optimizations by LLVM - see #141649. ### Details This is my attempt to fix the mentioned issue (this is the first part, I also implemented a similar solution for GVN in [this branch](https://github.com/rust-lang/rust/compare/master...ohadravid:rust:better-storage-calls-gvn-v2?expand=1)). The idea is to use the `MaybeStorageDead` analysis and remove only the storage calls of `head`s that are maybe-storage-dead when the associated `local` is accessed (or, conversely, keep the storage of `head`s that are for-sure alive in _every_ relevant access). When combined with the GVN change, the final example in the issue (#141649 (comment)) is optimized as expected by LLVM. I also measured the effect on a few functions in `rav1d` (where I originally saw the issue) and observed reduced stack usage in several of them. This is my first attempt at working with MIR optimizations, so it's possible this isn't the right approach — but all tests pass, and the resulting diffs appear correct. r? tmiasko since he commented on the issue and pointed to these passes.

bors · 2025-06-15T09:38:28Z

⌛ Trying commit d24d035 with merge ef7d206...

bors · 2025-06-15T12:05:29Z

☀️ Try build successful - checks-actions
Build commit: ef7d206 (ef7d20666974f0dac45b03e051f2e283f9d9f090)

rust-timer · 2025-06-15T13:31:55Z

Finished benchmarking commit (ef7d206): comparison URL.

Overall result: ❌ regressions - 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)	0.3%	[0.2%, 0.4%]	8
Regressions ❌ (secondary)	0.3%	[0.2%, 0.4%]	7
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.3%	[0.2%, 0.4%]	8

Max RSS (memory usage)

Results (primary 0.7%, 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)	3.5%	[1.8%, 5.0%]	5
Regressions ❌ (secondary)	3.4%	[3.4%, 3.4%]	1
Improvements ✅ (primary)	-3.9%	[-6.5%, -2.0%]	3
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.7%	[-6.5%, 5.0%]	8

Cycles

Results (primary -0.6%, 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)	3.8%	[3.8%, 3.8%]	1
Improvements ✅ (primary)	-0.6%	[-0.6%, -0.6%]	1
Improvements ✅ (secondary)	-4.1%	[-4.1%, -4.1%]	1
All ❌✅ (primary)	-0.6%	[-0.6%, -0.6%]	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)	0.2%	[0.0%, 0.8%]	10
Regressions ❌ (secondary)	0.1%	[0.0%, 0.1%]	5
Improvements ✅ (primary)	-0.2%	[-0.8%, -0.0%]	8
Improvements ✅ (secondary)	-0.2%	[-0.2%, -0.2%]	1
All ❌✅ (primary)	0.0%	[-0.8%, 0.8%]	18

Bootstrap: 757.399s -> 756.065s (-0.18%)
Artifact size: 372.20 MiB -> 372.12 MiB (-0.02%)

ohadravid · 2025-06-15T14:55:18Z

@matthiaskrgr - I updated the impl to stop re-checking once a head is found to be maybe-dead, which should be a bit better

matthiaskrgr · 2025-06-15T15:06:54Z

@bors try @rust-timer queue

bors · 2025-06-15T15:08:08Z

⌛ Trying commit 905e968 with merge c0a2949...

…try> Remove fewer Storage calls in `copy_prop` Modify the `copy_prop` MIR optimization pass to remove fewer `Storage{Live,Dead}` calls, allowing for better optimizations by LLVM - see #141649. ### Details This is my attempt to fix the mentioned issue (this is the first part, I also implemented a similar solution for GVN in [this branch](https://github.com/rust-lang/rust/compare/master...ohadravid:rust:better-storage-calls-gvn-v2?expand=1)). The idea is to use the `MaybeStorageDead` analysis and remove only the storage calls of `head`s that are maybe-storage-dead when the associated `local` is accessed (or, conversely, keep the storage of `head`s that are for-sure alive in _every_ relevant access). When combined with the GVN change, the final example in the issue (#141649 (comment)) is optimized as expected by LLVM. I also measured the effect on a few functions in `rav1d` (where I originally saw the issue) and observed reduced stack usage in several of them. This is my first attempt at working with MIR optimizations, so it's possible this isn't the right approach — but all tests pass, and the resulting diffs appear correct. r? tmiasko since he commented on the issue and pointed to these passes.

cjgillot · 2025-06-15T15:45:26Z

Should this check happen in Replacer::visit_local, and move the replacement of storage statements to a dedicated cleanup visitor?

bors · 2025-06-15T17:41:36Z

☀️ Try build successful - checks-actions
Build commit: c0a2949 (c0a294957df10fc3880e1677c72c0cf122485509)

ohadravid · 2025-06-15T18:12:43Z

Should this check happen in Replacer::visit_local

I'm not sure how to make this work: using ResultsCursor requires a &body, but it's not possible to have that while running a MutVisitor since it requires a &mut body.

Is there a different way to do this?

rust-timer · 2025-06-15T20:15:45Z

Finished benchmarking commit (c0a2949): 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)	0.3%	[0.2%, 0.4%]	9
Regressions ❌ (secondary)	0.3%	[0.2%, 0.4%]	7
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.2%	[-0.2%, -0.2%]	1
All ❌✅ (primary)	0.3%	[0.2%, 0.4%]	9

Max RSS (memory usage)

Results (primary -0.1%, secondary -1.3%)

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

	mean	range	count
Regressions ❌ (primary)	4.2%	[3.4%, 5.8%]	4
Regressions ❌ (secondary)	3.1%	[3.1%, 3.1%]	1
Improvements ✅ (primary)	-4.4%	[-6.6%, -1.8%]	4
Improvements ✅ (secondary)	-5.8%	[-5.8%, -5.8%]	1
All ❌✅ (primary)	-0.1%	[-6.6%, 5.8%]	8

Cycles

Results (secondary -1.0%)

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)	2.3%	[2.3%, 2.3%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.6%	[-2.6%, -2.5%]	2
All ❌✅ (primary)	-	-	0

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)	0.2%	[0.0%, 0.8%]	10
Regressions ❌ (secondary)	0.1%	[0.0%, 0.1%]	5
Improvements ✅ (primary)	-0.2%	[-0.8%, -0.0%]	8
Improvements ✅ (secondary)	-0.2%	[-0.2%, -0.2%]	1
All ❌✅ (primary)	-0.0%	[-0.8%, 0.8%]	18

Bootstrap: 756.494s -> 757.685s (0.16%)
Artifact size: 372.15 MiB -> 372.11 MiB (-0.01%)

compiler/rustc_mir_transform/src/copy_prop.rs

bors · 2025-07-06T23:50:46Z

☔ The latest upstream changes (presumably #143509) made this pull request unmergeable. Please resolve the merge conflicts.

bors · 2025-07-12T17:43:45Z

☔ The latest upstream changes (presumably #143624) made this pull request unmergeable. Please resolve the merge conflicts.

bors · 2025-07-24T07:46:53Z

☔ The latest upstream changes (presumably #116707) made this pull request unmergeable. Please resolve the merge conflicts.

wesleywiser · 2025-09-18T14:53:08Z

@cjgillot Do you want to re-review? @tmiasko signed off on the CopyProp changes.

bors · 2025-10-12T06:19:40Z

☔ The latest upstream changes (presumably #145513) made this pull request unmergeable. Please resolve the merge conflicts.

wesleywiser · 2025-11-06T15:27:43Z

@cjgillot I think this can be reviewed when you get a chance, mir-opt tests just need to be re-blessed before merging.

bors · 2025-11-23T17:43:37Z

☔ The latest upstream changes (presumably #147804) made this pull request unmergeable. Please resolve the merge conflicts.

wesleywiser · 2025-12-11T15:15:56Z

r? mir-opt

saethlin · 2025-12-27T16:37:16Z

@ohadravid Can you add a test under tests/codegen-llvm/ that demonstrates that this change actually improves codegen?

ohadravid · 2026-01-08T20:47:40Z

@saethlin Done! I added a new test under issues/ - it shows that the relevant example from the issue correctly produces llvm.lifetime calls.

I also checked that the beta compiler fails this test, so this is an improvement. I compared the compiled code as well, and the stack usage is reduced as expected - godbolt.

LMK if this should be checked in a test as well.

If this looks OK, I’ll rebase and fix the conflicts ⛏️

saethlin · 2026-01-09T01:43:21Z

I meant an actual optimization improvement, not the existence of the information that LLVM ought to be able to use to do an optimization. Can you test that the desired stack reuse actually happens?

ohadravid · 2026-01-09T10:05:26Z

Added another test under assembly-llvm which shows that stack usage is reduced. Is this what you had in mind? Should I also keep the llvm.lifetime test?

rustbot · 2026-01-09T13:44:46Z

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

rustbot assigned tmiasko Jun 15, 2025

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Jun 15, 2025