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

cjgillot · 2025-06-22T13:38:51Z

@ohadravid do you mind merging this PR and #142819? Both should use the same code to decide whether to keep or remove storage statements. And I fear that having 2 PRs mean that @tmiasko and I won't see each other ideas and give you diverging advice.

ohadravid · 2025-06-22T16:34:40Z

@cjgillot , @tmiasko - merged both PR here.

Current impls are based on the new MaybeUninitializedLocals analysis in both passes, with all the new tests cases passing.

Does GVN require an additional check against borrowed locals like mentioned in #142531 (comment)?

Both only do the more complex analysis when tcx.sess.emit_lifetime_markers(), so they shouldn't negatively affect check/debug builds, but the last perf run did show some changes to them as well.

And thank you both for reviewing these and explaining everything! 🙏

compiler/rustc_mir_transform/src/copy_prop.rs

tests/mir-opt/copy-prop/copy_prop_borrowed_storage_not_removed.rs

tests/mir-opt/copy-prop/copy_prop_storage_twice.rs

compiler/rustc_mir_dataflow/src/impls/initialized.rs

compiler/rustc_mir_transform/src/copy_prop.rs

tests/mir-opt/copy-prop/issue_141649.rs

tmiasko · 2025-06-25T06:56:32Z

I am not familiar with GVN, so I will leave review of that part to @cjgillot .

bors · 2025-06-25T18:23:13Z

☔ The latest upstream changes (presumably #142870) made this pull request unmergeable. Please resolve the merge conflicts.

… to remove fewer storage statements

…r storage statements

…chable blocks

ohadravid · 2025-06-29T08:52:24Z

@tmiasko - implemented all the changes 😄

I also updated the GVN code since they applied there are well (use a single storage_to_remove bitset, added a test and a fix for unreachable blocks, updated the base issue test to use MIR, added FileCheck annotations) @cjgillot

I can also split this PR if needed, and I'll polish the git history when you think this looks good enough 🧹

PS
I'm getting (again) CI errors on reordered storage statements in some tests (like tests/mir-opt/pre-codegen/derived_ord.rs), but I'm not sure why.

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

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rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Jun 15, 2025

matthiaskrgr mentioned this pull request Jun 15, 2025

ci: git Unknown option: -C #142534

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rustbot added perf-regression Performance regression. and removed S-waiting-on-perf Status: Waiting on a perf run to be completed. labels Jun 15, 2025

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rustbot removed the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Jun 15, 2025

tmiasko reviewed Jun 15, 2025

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compiler/rustc_mir_transform/src/copy_prop.rs Outdated Show resolved Hide resolved

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ohadravid force-pushed the better-storage-calls-copy-prop branch from fdcc8a6 to 26fc160 Compare June 22, 2025 16:30

ohadravid changed the title ~~Remove fewer Storage calls in copy_prop~~ Remove fewer Storage calls in CopyProp and GVN Jun 22, 2025

tmiasko reviewed Jun 25, 2025

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ohadravid added 6 commits June 29, 2025 07:35

Added a mir-opt test for generating storage statements for scoped locals

d8f39d3

Implement MaybeUninitializedLocals analysis for copy_prop mir-opt…

2679fff

… to remove fewer storage statements

Added gvn test for removed local storage annotations

ec52a2d

Use MaybeUninitializedLocals analysis in GVN mir-opt to remove fewe…

02434a2

…r storage statements

Use a single bitset to check the storage in copy_prop, only check rea…

ce5b7ec

…chable blocks

Added test for preserving head storage

e0def51

ohadravid force-pushed the better-storage-calls-copy-prop branch from 26fc160 to 5c21ce3 Compare June 29, 2025 05:40

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ohadravid force-pushed the better-storage-calls-copy-prop branch from 654c4a2 to ab1da60 Compare June 29, 2025 07:22

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ohadravid added 3 commits June 29, 2025 10:33

Move MaybeUninitializedLocals to the ssa module

13bbc32

Added FileCheck to copy prop storage tests

67b6ae3

Simplify GVN storage checker to use a single bitset

e69bef8

ohadravid force-pushed the better-storage-calls-copy-prop branch from ab1da60 to e69bef8 Compare June 29, 2025 07:34

Improve GVN storage tests

d59f79f

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ohadravid added 3 commits June 29, 2025 11:23

GVN unnit analysis shouldn't check unreachable blocks

228ad1e

Add more FileCheck statements in gvn storage tests

3a2a099

Added a GVN storage test for borrowed value

7c6b388

ohadravid requested a review from tmiasko June 29, 2025 08:52

Improved wording in comments and logs

43f5654

Remove fewer Storage calls in CopyProp and GVN #142531

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Overall result: ❌ regressions - please read the text below

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Overall result: ❌✅ regressions and improvements - please read the text below

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