JumpThreading: Avoid computing dominators to identify loop headers. #146829

cjgillot · 2025-09-21T03:51:21Z

JumpThreading tries to avoid threading through loop headers to avoid creating irreducible CFGs.

However, computing dominators is expensive, and accounts up to 20 % of the runtime of the JumpThreading pass for some cases like serde.

This PR proposes to approximate according to the post-order traversal order. We define a "maybe" loop header as a block which is visited after its predecessor in post-order.

cjgillot · 2025-09-21T03:52:37Z

@bors try @rust-timer queue

JumpThreading: Avoid computing dominators to identify loop headers.

rust-bors · 2025-09-21T06:05:50Z

☀️ Try build successful (CI)
Build commit: a055c23 (a055c23b64184e483c6cd8de31e2cf3f87d4e410, parent: dd7fda570040e8a736f7d8bc28ddd1b444aabc82)

rust-timer · 2025-09-21T07:16:15Z

Finished benchmarking commit (a055c23): comparison URL.

Overall result: ✅ improvements - no action needed

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.

@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
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.2%	[-0.2%, -0.1%]	2
Improvements ✅ (secondary)	-0.2%	[-0.2%, -0.2%]	2
All ❌✅ (primary)	-0.2%	[-0.2%, -0.1%]	2

Max RSS (memory usage)

Results (primary -2.7%, secondary 2.2%)

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)	2.2%	[2.2%, 2.2%]	1
Improvements ✅ (primary)	-2.7%	[-2.7%, -2.7%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-2.7%	[-2.7%, -2.7%]	1

Cycles

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

Binary size

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

Bootstrap: 472.936s -> 470.628s (-0.49%)
Artifact size: 389.95 MiB -> 389.92 MiB (-0.01%)

rustbot · 2025-09-21T14:41:51Z

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

rustbot · 2025-09-21T14:41:52Z

r? @lcnr

rustbot has assigned @lcnr.
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

lcnr · 2025-09-26T09:03:40Z

r? wg-mir-opt

compiler/rustc_mir_transform/src/jump_threading.rs

rustbot · 2025-09-26T20:46:53Z

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.

dianqk

LGTM. Thanks!
@bors r+

View changes since this review

dianqk · 2025-09-27T00:35:15Z

@bors r+

bors · 2025-09-27T00:35:17Z

📌 Commit d13a5b0 has been approved by dianqk

It is now in the queue for this repository.

bors · 2025-09-27T09:32:43Z

⌛ Testing commit d13a5b0 with merge ade8487...

bors · 2025-09-27T12:37:54Z

☀️ Test successful - checks-actions
Approved by: dianqk
Pushing ade8487 to master...

github-actions · 2025-09-27T12:41:37Z

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 959b450 (parent) -> ade8487 (this PR)

Test differences

No test diffs found

Test dashboard

Run

cargo run --manifest-path src/ci/citool/Cargo.toml -- \
    test-dashboard ade84871f718ea20a6460d28e82290353b4bf3d2 --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

aarch64-apple: 6284.6s -> 4566.8s (-27.3%)
tidy: 227.0s -> 179.4s (-21.0%)
dist-x86_64-apple: 5724.3s -> 6779.2s (18.4%)
x86_64-gnu-llvm-20: 2979.2s -> 2443.5s (-18.0%)
i686-gnu-2: 6534.5s -> 5451.0s (-16.6%)
i686-gnu-1: 8371.2s -> 7018.3s (-16.2%)
x86_64-gnu-distcheck: 7360.6s -> 6281.3s (-14.7%)
dist-aarch64-apple: 7095.8s -> 6149.4s (-13.3%)
aarch64-gnu-llvm-20-2: 2468.9s -> 2150.3s (-12.9%)
pr-check-2: 2748.7s -> 2398.7s (-12.7%)

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 · 2025-09-27T13:48:38Z

Finished benchmarking commit (ade8487): 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
Regressions ❌ (secondary)	0.3%	[0.3%, 0.3%]	1
Improvements ✅ (primary)	-0.2%	[-0.3%, -0.2%]	2
Improvements ✅ (secondary)	-0.2%	[-0.3%, -0.2%]	2
All ❌✅ (primary)	-0.2%	[-0.3%, -0.2%]	2

Max RSS (memory usage)

Results (primary 0.5%, secondary 2.7%)

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

	mean	range	count
Regressions ❌ (primary)	2.4%	[2.4%, 2.4%]	1
Regressions ❌ (secondary)	2.7%	[2.7%, 2.7%]	1
Improvements ✅ (primary)	-1.3%	[-1.3%, -1.3%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.5%	[-1.3%, 2.4%]	2

Cycles

Results (secondary -2.7%)

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)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.7%	[-2.7%, -2.7%]	1
All ❌✅ (primary)	-	-	0

Binary size

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

Bootstrap: 472.089s -> 469.268s (-0.60%)
Artifact size: 388.09 MiB -> 388.12 MiB (0.01%)

Mark-Simulacrum · 2025-09-29T02:30:15Z

Regression is just bimodality.

rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Sep 21, 2025

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rust-bors bot added a commit that referenced this pull request Sep 21, 2025

Auto merge of #146829 - cjgillot:jump-threading-loop-dominator, r=<try>

a055c23

JumpThreading: Avoid computing dominators to identify loop headers.

rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Sep 21, 2025

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rustbot removed the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Sep 21, 2025

cjgillot marked this pull request as ready for review September 21, 2025 14:41

rustbot added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Sep 21, 2025

rustbot assigned lcnr Sep 21, 2025

rustbot removed the S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. label Sep 21, 2025

JumpThreading: Avoid computing dominators to identify loop headers.

1a37374

rustbot assigned dianqk and unassigned lcnr Sep 26, 2025

dianqk reviewed Sep 26, 2025

View reviewed changes

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

Handle self-loops too.

d13a5b0

cjgillot force-pushed the jump-threading-loop-dominator branch from 33ff3be to d13a5b0 Compare September 26, 2025 20:46

dianqk approved these changes Sep 27, 2025

View reviewed changes

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 27, 2025

bors added the merged-by-bors This PR was explicitly merged by bors. label Sep 27, 2025

bors merged commit ade8487 into rust-lang:master Sep 27, 2025
11 checks passed

rustbot added this to the 1.92.0 milestone Sep 27, 2025

This was referenced Sep 27, 2025

Compute jump threading opportunities in a single pass #142821

Open

JumpThreading: compute place and value indices on-demand #142881

Open

rustbot added the perf-regression Performance regression. label Sep 27, 2025

cjgillot deleted the jump-threading-loop-dominator branch September 27, 2025 13:57

Mark-Simulacrum removed the perf-regression Performance regression. label Sep 29, 2025

github-actions bot mentioned this pull request Sep 29, 2025

Toolchain upgrade to nightly-2025-10-09 flux-rs/flux#1271

Merged

JumpThreading: Avoid computing dominators to identify loop headers. #146829

JumpThreading: Avoid computing dominators to identify loop headers. #146829

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rust-timer commented Sep 21, 2025

Overall result: ✅ improvements - no action needed

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github-actions bot commented Sep 27, 2025

Test differences

Job duration changes

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rust-timer commented Sep 27, 2025

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

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