Pre-compute MIR CFG caches for borrowck and other analyses by cjgillot · Pull Request #142540 · rust-lang/rust

cjgillot · 2025-06-15T14:35:45Z

I was puzzled that #142390 introduces additional computations of CFG traversals: borrowck computes them, right?

It turns out that borrowck clones the MIR body, so doesn't share its cache with other analyses.

This PR:

forces the computation of all caches in mir_promoted query;
modifies region renumbering to avoid dropping that cache.

rustbot · 2025-06-15T14:35:49Z

rustbot has assigned @fee1-dead.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

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rustbot · 2025-06-15T14:35:51Z

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

cjgillot · 2025-06-15T14:41:37Z

@bors2 try @rust-timer queue

rust-bors · 2025-06-15T14:41:40Z

⌛ Trying commit e7e8f6d with merge 18218cc…

To cancel the try build, run the command @bors2 try cancel.

Pre-compute MIR CFG caches for borrowck and other analyses I was puzzled that #142390 introduces additional computations of CFG traversals: borrowck computes them, right? It turns out that borrowck clones the MIR body, so doesn't share its cache with other analyses. This PR: - forces the computation of all caches in `mir_promoted` query; - modifies region renumbering to avoid dropping that cache.

rust-bors · 2025-06-15T17:08:29Z

☀️ Try build successful (CI)
Build commit: 18218cc (18218cc5c664c3521f4769f46ef5e5e10128c92c, parent: 75e7cf5f85aad82331a38deff24845b63eaf30f3)

rust-timer · 2025-06-15T18:49:48Z

Finished benchmarking commit (18218cc): 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)	0.5%	[0.2%, 2.9%]	68
Regressions ❌ (secondary)	0.7%	[0.2%, 1.8%]	47
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.5%	[0.2%, 2.9%]	68

Max RSS (memory usage)

Results (primary 1.7%, 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)	1.7%	[0.9%, 2.8%]	12
Regressions ❌ (secondary)	2.5%	[2.0%, 3.0%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-1.2%	[-1.2%, -1.2%]	1
All ❌✅ (primary)	1.7%	[0.9%, 2.8%]	12

Cycles

Results (primary 1.1%)

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

	mean	range	count
Regressions ❌ (primary)	1.1%	[0.7%, 1.4%]	2
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	1.1%	[0.7%, 1.4%]	2

Binary size

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

Bootstrap: 756.494s -> 756.086s (-0.05%)
Artifact size: 372.15 MiB -> 372.18 MiB (0.01%)

cjgillot · 2025-06-15T20:25:34Z

Perf deserves investigation. I really didn't expect mir_borrowck to get slower.

cjgillot · 2025-06-15T20:34:53Z

@bors2 try @rust-timer queue

rust-bors · 2025-06-15T20:34:56Z

⌛ Trying commit 3e77d40 with merge 98e2e6a…

To cancel the try build, run the command @bors2 try cancel.

Pre-compute MIR CFG caches for borrowck and other analyses I was puzzled that #142390 introduces additional computations of CFG traversals: borrowck computes them, right? It turns out that borrowck clones the MIR body, so doesn't share its cache with other analyses. This PR: - forces the computation of all caches in `mir_promoted` query; - modifies region renumbering to avoid dropping that cache.

rust-bors · 2025-06-15T23:01:48Z

☀️ Try build successful (CI)
Build commit: 98e2e6a (98e2e6aea9abc7e12f76e27d72aced95f45b5260, parent: 586ad391f5ee4519acc7cae340e34673bae762b1)

rust-timer · 2025-06-16T00:32:57Z

Finished benchmarking commit (98e2e6a): 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)	1.8%	[1.0%, 2.6%]	4
Regressions ❌ (secondary)	1.2%	[0.6%, 1.6%]	6
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	1.8%	[1.0%, 2.6%]	4

Max RSS (memory usage)

Results (primary 0.1%, secondary -2.5%)

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

	mean	range	count
Regressions ❌ (primary)	3.1%	[0.5%, 6.5%]	3
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-1.4%	[-1.8%, -0.8%]	6
Improvements ✅ (secondary)	-2.5%	[-2.7%, -2.2%]	2
All ❌✅ (primary)	0.1%	[-1.8%, 6.5%]	9

Cycles

Results (secondary 1.2%)

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)	1.2%	[1.2%, 1.2%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Binary size

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

Bootstrap: 755.712s -> 754.846s (-0.11%)
Artifact size: 372.06 MiB -> 372.09 MiB (0.01%)

cjgillot · 2025-06-16T12:33:37Z

Rebased onto #142542
@bors2 try @rust-timer queue

rust-bors · 2025-06-16T12:33:42Z

⌛ Trying commit ba39fdf with merge 0e5f13e…

To cancel the try build, run the command @bors2 try cancel.

rustbot · 2025-10-12T17:45:36Z

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 · 2025-10-12T17:45:48Z

@bors2 try @rust-timer queue

Pre-compute MIR CFG caches for borrowck and other analyses

rust-bors · 2025-10-12T19:57:55Z

☀️ Try build successful (CI)
Build commit: 8e7d148 (8e7d14809b3504ba2f2a9e8caa6ae58e9c5fe86b, parent: ff6dc928c5e33ce8e65c6911a790b9efcb5ef53a)

rust-timer · 2025-10-12T21:22:34Z

Finished benchmarking commit (8e7d148): 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)	1.5%	[0.3%, 2.6%]	5
Regressions ❌ (secondary)	1.5%	[1.1%, 1.8%]	4
Improvements ✅ (primary)	-0.2%	[-0.4%, -0.1%]	49
Improvements ✅ (secondary)	-0.2%	[-0.3%, -0.0%]	35
All ❌✅ (primary)	-0.0%	[-0.4%, 2.6%]	54

Max RSS (memory usage)

Results (primary -2.9%, secondary -0.7%)

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)	1.5%	[0.5%, 2.5%]	2
Improvements ✅ (primary)	-2.9%	[-6.8%, -0.8%]	3
Improvements ✅ (secondary)	-1.3%	[-2.1%, -0.6%]	8
All ❌✅ (primary)	-2.9%	[-6.8%, -0.8%]	3

Cycles

Results (secondary 0.2%)

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%	[2.1%, 2.4%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-4.0%	[-4.0%, -4.0%]	1
All ❌✅ (primary)	-	-	0

Binary size

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

Bootstrap: 471.991s -> 473.105s (0.24%)
Artifact size: 388.12 MiB -> 388.10 MiB (-0.01%)

cjgillot · 2025-10-13T01:55:43Z

I don't think I will do better here. This is an improvement on all but 2 benchmarks. I can't figure out why.
@rustbot ready

fee1-dead · 2025-10-17T19:36:55Z

I think this is fine. We're seeing enough wins for more common Rust code (serde, image, syn, etc) that I think it is fine to take the hit for the two outliers (tt-muncher and cranelift-codegen for some reason), I do think it could be because of cloning taking more instructions than what gets saved, but I don't think that is preventable.

@bors r+ rollup=never

bors · 2025-10-17T19:36:58Z

📌 Commit 42ff108 has been approved by fee1-dead

It is now in the queue for this repository.

bors · 2025-10-17T21:01:28Z

⌛ Testing commit 42ff108 with merge 28fad95...

bors · 2025-10-18T00:07:11Z

☀️ Test successful - checks-actions
Approved by: fee1-dead
Pushing 28fad95 to master...

github-actions · 2025-10-18T00:10:12Z

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 f464759 (parent) -> 28fad95 (this PR)

Test differences

Show 2 test diffs

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

Test dashboard

Run

cargo run --manifest-path src/ci/citool/Cargo.toml -- \
test-dashboard 28fad9598975af7734be05116b3c5fc74b720d87 --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-x86_64-apple: 7239.0s -> 6005.0s (-17.0%)
test-various: 4099.6s -> 4729.1s (15.4%)
x86_64-gnu-gcc: 3053.2s -> 3465.6s (13.5%)
x86_64-gnu-llvm-20-1: 3227.1s -> 3622.4s (12.2%)
i686-gnu-nopt-1: 7316.6s -> 8196.5s (12.0%)
i686-gnu-2: 5624.2s -> 6245.1s (11.0%)
x86_64-gnu-llvm-20-3: 6452.3s -> 7156.3s (10.9%)
x86_64-gnu-llvm-20: 2540.1s -> 2805.8s (10.5%)
dist-aarch64-linux: 6579.6s -> 5964.6s (-9.3%)
dist-apple-various: 3666.2s -> 4004.2s (9.2%)

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 · 2025-10-18T01:50:39Z

Finished benchmarking commit (28fad95): 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)	1.7%	[1.0%, 2.6%]	4
Regressions ❌ (secondary)	0.9%	[0.1%, 1.6%]	6
Improvements ✅ (primary)	-0.2%	[-0.3%, -0.1%]	15
Improvements ✅ (secondary)	-0.3%	[-0.4%, -0.2%]	13
All ❌✅ (primary)	0.2%	[-0.3%, 2.6%]	19

Max RSS (memory usage)

Results (primary -0.0%, secondary 0.5%)

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

	mean	range	count
Regressions ❌ (primary)	3.0%	[3.0%, 3.0%]	1
Regressions ❌ (secondary)	2.9%	[1.6%, 4.1%]	2
Improvements ✅ (primary)	-1.6%	[-2.1%, -1.0%]	2
Improvements ✅ (secondary)	-1.1%	[-1.9%, -0.6%]	3
All ❌✅ (primary)	-0.0%	[-2.1%, 3.0%]	3

Cycles

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

Binary size

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

Bootstrap: 475.372s -> 475.32s (-0.01%)
Artifact size: 390.39 MiB -> 390.35 MiB (-0.01%)

lqd · 2025-10-18T05:04:21Z

cranelift-codegen is one of the benchmarks where borrowck takes the longest, is it expected that this change impacts it so negatively?

Mark-Simulacrum · 2025-10-21T03:23:28Z

I think this is fine. We're seeing enough wins for more common Rust code
(serde, image, syn, etc) that I think it is fine to take the hit for the two
outliers (tt-muncher and cranelift-codegen for some reason), I do think it
could be because of cloning taking more instructions than what gets saved,
but I don't think that is preventable.

#142540 (comment)

I think I'm broadly in agreement with this summary of the results. Definitely would be interested in figuring out why we have the outliers we do, though, and seems plausible there's an opportunity waiting there...

rustbot assigned fee1-dead 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 perf-regression Performance regression. and removed S-waiting-on-perf Status: Waiting on a perf run to be completed. labels Jun 15, 2025

tmiasko reviewed Jun 15, 2025
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Comment threadcompiler/rustc_middle/src/mir/basic_blocks.rs Outdated

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rustbot removed the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Jun 16, 2025

cjgillot force-pushed the renumber-cfg branch from 3e77d40 to ba39fdfCompare June 16, 2025 12:33

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cjgillot force-pushed the renumber-cfg branch from 6f7584a to 42ff108Compare October 12, 2025 17:45

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rust-borsBot added a commit that referenced this pull request Oct 12, 2025
Auto merge of#142540- cjgillot:renumber-cfg, r=<try>
8e7d148
Pre-compute MIR CFG caches for borrowck and other analyses

rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Oct 12, 2025

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rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Oct 13, 2025

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 Oct 17, 2025

bors added the merged-by-bors This PR was explicitly merged by bors. label Oct 18, 2025

bors merged commit 28fad95 into rust-lang:masterOct 18, 2025
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rustbot added this to the 1.92.0 milestone Oct 18, 2025

cjgillot deleted the renumber-cfg branch October 18, 2025 04:08

Mark-Simulacrum added the perf-regression-triaged The performance regression has been triaged. label Oct 21, 2025

github-actionsBot mentioned this pull request Oct 21, 2025
Toolchain upgrade to nightly-2026-02-09 flux-rs/flux#1345
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Overall result: ❌ regressions - please read the text below

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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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Test differences

Job duration changes

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

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