Enable DestinationPropagation by default #142915

cjgillot · 2025-06-23T16:49:29Z

This PR proposes to perform destination propagation on MIR. Most of the pass was fully rewritten by @JakobDegen in #96451.

This pass is quite heavy, as it needs to perform and save the results of a full liveness dataflow analysis. This accounts for ~50% of the pass' runtime.

Perf sees a few decent savings in later llvm passes, but also sizeable régressions when there are no savings to balance this pass' runtime.

cjgillot · 2025-06-23T16:49:42Z

@bors try @rust-timer queue

Enable DestinationPropagation by default r? `@ghost`

bors · 2025-06-23T16:50:54Z

⌛ Trying commit b835228 with merge 2989fd8...

bors · 2025-06-23T19:21:53Z

☀️ Try build successful - checks-actions
Build commit: 2989fd8 (2989fd868755417871c65b8864528f20125af5bc)

rust-timer · 2025-06-24T06:32:17Z

Finished benchmarking commit (2989fd8): 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)	0.7%	[0.2%, 3.1%]	27
Regressions ❌ (secondary)	0.6%	[0.2%, 2.5%]	36
Improvements ✅ (primary)	-0.4%	[-1.7%, -0.1%]	60
Improvements ✅ (secondary)	-0.7%	[-2.9%, -0.2%]	71
All ❌✅ (primary)	-0.0%	[-1.7%, 3.1%]	87

Max RSS (memory usage)

Results (primary 0.7%, secondary 1.6%)

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

	mean	range	count
Regressions ❌ (primary)	5.6%	[1.7%, 10.5%]	5
Regressions ❌ (secondary)	1.6%	[1.4%, 2.2%]	9
Improvements ✅ (primary)	-4.3%	[-5.9%, -2.5%]	5
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.7%	[-5.9%, 10.5%]	10

Cycles

Results (primary 0.3%, 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)	1.0%	[0.5%, 1.4%]	5
Regressions ❌ (secondary)	3.1%	[1.8%, 4.4%]	3
Improvements ✅ (primary)	-1.5%	[-1.7%, -1.3%]	2
Improvements ✅ (secondary)	-2.6%	[-3.5%, -1.5%]	8
All ❌✅ (primary)	0.3%	[-1.7%, 1.4%]	7

Binary size

Results (primary -0.0%, 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)	0.2%	[0.0%, 0.4%]	22
Regressions ❌ (secondary)	0.4%	[0.0%, 2.0%]	94
Improvements ✅ (primary)	-0.1%	[-1.0%, -0.0%]	54
Improvements ✅ (secondary)	-1.0%	[-13.0%, -0.0%]	28
All ❌✅ (primary)	-0.0%	[-1.0%, 0.4%]	76

Bootstrap: 688.878s -> 687.306s (-0.23%)
Artifact size: 371.92 MiB -> 371.62 MiB (-0.08%)

rustbot · 2025-06-25T19:10:42Z

r? @petrochenkov

rustbot has assigned @petrochenkov.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

rustbot · 2025-06-25T19:10:43Z

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

petrochenkov · 2025-06-25T20:36:20Z

r? mir

oli-obk

r=me

compiler/rustc_mir_transform/src/dest_prop.rs

cjgillot · 2025-07-01T14:14:27Z

There is a pathological behaviour with cranelift-codegen I'd like to fix first. There are many locals that can be merged with _0. The current version needs to recompute all candidates and all conflicts each time it merges _0 with something.

cjgillot · 2025-07-02T10:10:38Z

@bors try @rust-timer queue

bors · 2025-07-02T10:11:55Z

⌛ Trying commit 9298dfa with merge 00ffd2b...

Enable DestinationPropagation by default This PR proposes to perform destination propagation on MIR. Most of the pass was fully rewritten by `@JakobDegen` in #96451. This pass is quite heavy, as it needs to perform and save the results of a full liveness dataflow analysis. This accounts for ~50% of the pass' runtime. Perf sees a few decent savings in later llvm passes, but also sizeable régressions when there are no savings to balance this pass' runtime.

bors · 2025-07-02T12:27:32Z

☀️ Try build successful - checks-actions
Build commit: 00ffd2b (00ffd2bf2b816d1f0df05611fccdd7dad0a8db22)

Enable DestinationPropagation by default

rust-bors · 2025-09-09T03:38:04Z

☀️ Try build successful (CI)
Build commit: 58abde7 (58abde714788a58aaf63d472020334b16a6bc8bf, parent: 9c27f27ea3bab79a2fec827ef3ae0009959d60f4)

rust-timer · 2025-09-09T04:48:53Z

Finished benchmarking commit (58abde7): 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%	[0.1%, 1.2%]	62
Regressions ❌ (secondary)	0.4%	[0.0%, 1.6%]	98
Improvements ✅ (primary)	-0.5%	[-2.0%, -0.1%]	29
Improvements ✅ (secondary)	-0.9%	[-2.5%, -0.1%]	46
All ❌✅ (primary)	0.0%	[-2.0%, 1.2%]	91

Max RSS (memory usage)

Results (primary 1.8%)

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

	mean	range	count
Regressions ❌ (primary)	3.3%	[2.2%, 4.5%]	2
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-1.3%	[-1.3%, -1.3%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	1.8%	[-1.3%, 4.5%]	3

Cycles

Results (primary -2.3%, secondary -4.3%)

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)	2.9%	[2.9%, 2.9%]	1
Improvements ✅ (primary)	-2.3%	[-2.3%, -2.3%]	1
Improvements ✅ (secondary)	-4.8%	[-7.8%, -1.9%]	13
All ❌✅ (primary)	-2.3%	[-2.3%, -2.3%]	1

Binary size

Results (primary -0.1%, 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)	0.1%	[0.0%, 0.4%]	21
Regressions ❌ (secondary)	0.3%	[0.0%, 1.1%]	57
Improvements ✅ (primary)	-0.1%	[-2.4%, -0.0%]	73
Improvements ✅ (secondary)	-0.6%	[-2.4%, -0.0%]	24
All ❌✅ (primary)	-0.1%	[-2.4%, 0.4%]	94

Bootstrap: 467.433s -> 465.765s (-0.36%)
Artifact size: 387.54 MiB -> 387.18 MiB (-0.09%)

lqd · 2025-09-09T20:25:04Z

It's possible destprop caused #146383, so we may want to hold on this PR until we have more information, or a fix.

saethlin · 2025-09-09T20:45:16Z

It's possible

I am certain that DestinationPropagation is the cause. I've added some more text to make that clear.

rustbot · 2025-09-16T22:50:04Z

This PR was rebased onto a different master 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.

cjgillot · 2025-09-16T23:46:40Z

#146383 is fixed.

saethlin · 2025-09-17T02:54:27Z

I don't think this is particularly risky; this pass has had a fair bit of fuzzing by Rustlantis because I run with -Zmir-opt-level=4.

@bors r+ rollup=never

bors · 2025-09-17T02:54:29Z

📌 Commit 44c1a00 has been approved by saethlin

It is now in the queue for this repository.

bors · 2025-09-17T10:44:26Z

⌛ Testing commit 44c1a00 with merge ce6daf3...

bors · 2025-09-17T13:55:44Z

☀️ Test successful - checks-actions
Approved by: saethlin
Pushing ce6daf3 to master...

github-actions · 2025-09-17T13:58:22Z

What is this?

This is an experimental post-merge analysis report that shows differences in test outcomes between the merged PR and its parent PR.

Comparing 2ebb126 (parent) -> ce6daf3 (this PR)

Test differences

Show 16 test diffs

Stage 1

[mir-opt] tests/mir-opt/dest-prop/nrvo_borrowed.rs: [missing] -> pass (J1)
[mir-opt] tests/mir-opt/dest-prop/nrvo_miscompile_111005.rs: [missing] -> pass (J1)
[mir-opt] tests/mir-opt/nrvo_miscompile_111005.rs: pass -> [missing] (J1)
[mir-opt] tests/mir-opt/nrvo_simple.rs: pass -> [missing] (J1)

Stage 2

[mir-opt] tests/mir-opt/dest-prop/nrvo_borrowed.rs: [missing] -> pass (J0)
[mir-opt] tests/mir-opt/dest-prop/nrvo_miscompile_111005.rs: [missing] -> pass (J0)
[mir-opt] tests/mir-opt/nrvo_miscompile_111005.rs: pass -> [missing] (J0)
[mir-opt] tests/mir-opt/nrvo_simple.rs: pass -> [missing] (J0)

Additionally, 8 doctest diffs were found. These are ignored, as they are noisy.

Job group index

Test dashboard

Run

cargo run --manifest-path src/ci/citool/Cargo.toml -- \
    test-dashboard ce6daf3d5a5bffb2a00264197f92dc31608df0da --output-dir test-dashboard

And then open test-dashboard/index.html in your browser to see an overview of all executed tests.

Job duration changes

dist-apple-various: 4430.5s -> 3369.0s (-24.0%)
aarch64-apple: 5511.1s -> 6624.5s (20.2%)
x86_64-rust-for-linux: 3107.5s -> 2645.6s (-14.9%)
x86_64-gnu-llvm-20-1: 3720.3s -> 3228.5s (-13.2%)
aarch64-gnu-llvm-20-1: 3787.9s -> 3328.0s (-12.1%)
i686-gnu-nopt-1: 8261.1s -> 7258.6s (-12.1%)
pr-check-1: 1537.1s -> 1358.2s (-11.6%)
i686-gnu-2: 6028.3s -> 5400.6s (-10.4%)
x86_64-gnu-debug: 7132.4s -> 6532.8s (-8.4%)
arm-android: 6173.2s -> 5670.9s (-8.1%)

How to interpret the job duration changes?

Job durations can vary a lot, based on the actual runner instance
that executed the job, system noise, invalidated caches, etc. The table above is provided
mostly for t-infra members, for simpler debugging of potential CI slow-downs.

rust-timer · 2025-09-17T15:06:49Z

Finished benchmarking commit (ce6daf3): comparison URL.

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

Our benchmarks found a performance regression caused by this PR.
This might be an actual regression, but it can also be just noise.

Next Steps:

If the regression was expected or you think it can be justified,
please write a comment with sufficient written justification, and add
@rustbot label: +perf-regression-triaged to it, to mark the regression as triaged.
If you think that you know of a way to resolve the regression, try to create
a new PR with a fix for the regression.
If you do not understand the regression or you think that it is just noise,
you can ask the @rust-lang/wg-compiler-performance working group for help (members of this group
were already notified of this PR).

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

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%	[0.1%, 1.1%]	62
Regressions ❌ (secondary)	0.4%	[0.1%, 1.6%]	98
Improvements ✅ (primary)	-0.4%	[-1.7%, -0.1%]	31
Improvements ✅ (secondary)	-0.7%	[-2.5%, -0.1%]	60
All ❌✅ (primary)	0.0%	[-1.7%, 1.1%]	93

Max RSS (memory usage)

Results (primary 1.5%, secondary -2.0%)

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

	mean	range	count
Regressions ❌ (primary)	2.8%	[0.9%, 4.6%]	2
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-1.1%	[-1.1%, -1.1%]	1
Improvements ✅ (secondary)	-2.0%	[-2.4%, -1.5%]	3
All ❌✅ (primary)	1.5%	[-1.1%, 4.6%]	3

Cycles

Results (primary -1.4%, secondary -0.3%)

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)	3.5%	[2.1%, 5.3%]	6
Improvements ✅ (primary)	-1.4%	[-1.4%, -1.4%]	1
Improvements ✅ (secondary)	-3.7%	[-6.1%, -1.3%]	7
All ❌✅ (primary)	-1.4%	[-1.4%, -1.4%]	1

Binary size

Results (primary -0.1%, 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)	0.1%	[0.0%, 0.4%]	25
Regressions ❌ (secondary)	0.3%	[0.0%, 1.0%]	62
Improvements ✅ (primary)	-0.1%	[-2.0%, -0.0%]	56
Improvements ✅ (secondary)	-1.0%	[-3.5%, -0.0%]	23
All ❌✅ (primary)	-0.1%	[-2.0%, 0.4%]	81

Bootstrap: 472.767s -> 472.939s (0.04%)
Artifact size: 390.62 MiB -> 387.93 MiB (-0.69%)

rustbot added the T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. label Jun 23, 2025

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rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Jun 23, 2025

bors added a commit that referenced this pull request Jun 23, 2025

Auto merge of #142915 - cjgillot:dest-prop-default, r=<try>

2989fd8

Enable DestinationPropagation by default r? `@ghost`

cjgillot added the A-mir-opt Area: MIR optimizations label Jun 23, 2025

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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 24, 2025

cjgillot marked this pull request as ready for review June 25, 2025 19:10

rustbot assigned petrochenkov Jun 25, 2025

rustbot added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Jun 25, 2025

rustbot assigned oli-obk and unassigned petrochenkov Jun 25, 2025

oli-obk approved these changes Jun 30, 2025

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cjgillot force-pushed the dest-prop-default branch from b835228 to 9298dfa Compare July 2, 2025 10:10

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Auto merge of #142915 - cjgillot:dest-prop-default, r=<try>

58abde7

Enable DestinationPropagation by default

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cjgillot force-pushed the dest-prop-default branch from a763af8 to 74be0a9 Compare September 14, 2025 15:01

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Enable DestinationPropagation by default.

44c1a00

cjgillot force-pushed the dest-prop-default branch from 74be0a9 to 44c1a00 Compare September 16, 2025 22:50

bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Sep 17, 2025

bors added the merged-by-bors This PR was explicitly merged by bors. label Sep 17, 2025

bors merged commit ce6daf3 into rust-lang:master Sep 17, 2025
11 checks passed

rustbot added this to the 1.92.0 milestone Sep 17, 2025

bors mentioned this pull request Sep 17, 2025

Do partial SsaLocals analysis in unoptimized builds #134051

Draft

cjgillot deleted the dest-prop-default branch September 17, 2025 14:17

bors mentioned this pull request Sep 17, 2025

Compute jump threading opportunities in a single pass #142821

Draft

github-actions bot mentioned this pull request Sep 18, 2025

Toolchain upgrade to nightly-2025-09-18 flux-rs/flux#1271

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Enable DestinationPropagation by default #142915

Enable DestinationPropagation by default #142915

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

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

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Test differences

Stage 1

Stage 2

Job duration changes

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

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