Compute jump threading opportunities in a single pass #142821

cjgillot · 2025-06-21T11:07:53Z

The current implementation of jump threading walks MIR CFG backwards from each SwitchInt terminator. This PR replaces this by a single postorder traversal of MIR. In theory, we could do a full fixpoint dataflow analysis, but this has low returns as we forbid threading through a loop header.

The second commit in this PR modifies the carried state to a lighter data structure. The current implementation uses some kind of IndexVec<ValueIndex, &[Condition]>. This is needlessly heavy, as the state rarely ever carries more than a few Conditions. The first commit replaces this state with a simpler &[Condition], and puts the corresponding ValueIndex inside Condition.

The three later commits are perf tweaks.

The sixth commit is the main change. Instead of carrying the goto target inside the condition, we maintain a set of conditions associated with each block, and their consequences in following blocks. Think: if this condition is fulfilled in this block, then that condition is fulfilled in that block. This makes the threading algorithm much easier to implement, without the extra bookkeeping of ThreadingOpportunity we had.

Later commits modify that algorithm to shrink the set of duplicated blocks. By propagating fulfilled conditions down the CFG, and trimming costly threads.

rustbot · 2025-06-21T11:07:57Z

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

rustbot · 2025-06-21T11:07:59Z

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

oli-obk · 2025-06-21T11:58:07Z

@bors try @rust-timer queue

Compute jump threading opportunities in a single pass The current implementation of jump threading walks MIR CFG backwards from each `SwitchInt` terminator. This PR replaces this by a single postorder traversal of MIR. In theory, we could do a full fixpoint dataflow analysis, but this has low returns as we forbid threading through a loop header, and we do not merge TOs yet. The second commit in this PR modifies the carried state to a lighter data structure. The current implementation uses some kind of `IndexVec<ValueIndex, &[Condition]>`. This is needlessly heavy, as the state rarely ever carries more than a few `Condition`s. The first commit replaces this state with a simpler `&[Condition]`, and puts the corresponding `ValueIndex` inside `Condition`. The last commit is the main change. It needs a fair amount of data structure tweaks, as each condition now needs to carry its chain of blocks with it.

bors · 2025-06-21T11:59:20Z

⌛ Trying commit bdf9d85 with merge d27b44e...

bors · 2025-06-21T14:28:28Z

☀️ Try build successful - checks-actions
Build commit: d27b44e (d27b44ea37402dbd9462aa3b8eee9ff6d5e4c007)

rust-timer · 2025-06-21T17:52:49Z

Finished benchmarking commit (d27b44e): 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)	0.5%	[0.1%, 3.1%]	54
Regressions ❌ (secondary)	1.4%	[0.2%, 6.0%]	17
Improvements ✅ (primary)	-0.4%	[-0.8%, -0.2%]	8
Improvements ✅ (secondary)	-0.4%	[-0.6%, -0.2%]	8
All ❌✅ (primary)	0.4%	[-0.8%, 3.1%]	62

Max RSS (memory usage)

Results (primary -1.9%, secondary -3.6%)

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

	mean	range	count
Regressions ❌ (primary)	5.5%	[5.5%, 5.5%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-3.7%	[-4.7%, -2.7%]	4
Improvements ✅ (secondary)	-3.6%	[-3.6%, -3.6%]	1
All ❌✅ (primary)	-1.9%	[-4.7%, 5.5%]	5

Cycles

Results (primary 1.6%, secondary 2.4%)

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

	mean	range	count
Regressions ❌ (primary)	1.6%	[0.9%, 2.9%]	3
Regressions ❌ (secondary)	2.4%	[1.7%, 3.1%]	4
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	1.6%	[0.9%, 2.9%]	3

Binary size

Results (primary -0.1%, 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)	0.2%	[0.0%, 1.7%]	24
Regressions ❌ (secondary)	0.2%	[0.0%, 0.6%]	8
Improvements ✅ (primary)	-0.3%	[-0.8%, -0.0%]	49
Improvements ✅ (secondary)	-0.6%	[-13.0%, -0.1%]	86
All ❌✅ (primary)	-0.1%	[-0.8%, 1.7%]	73

Bootstrap: 689.042s -> 688.964s (-0.01%)
Artifact size: 371.85 MiB -> 371.64 MiB (-0.06%)

rustbot · 2025-06-23T08:56:01Z

Some changes occurred in coverage tests.

cc @Zalathar

lcnr · 2025-06-23T11:40:12Z

r? wg-mir-opt

rustbot · 2025-06-23T11:40:15Z

Failed to set assignee to vertexclique: invalid assignee

Note: Only org members with at least the repository "read" role, users with write permissions, or people who have commented on the PR may be assigned.

lcnr · 2025-07-03T13:16:03Z

oh, there are people in the wg which can't actually be assigned for review 😅
r? wg-mir-opt

oli-obk

I can yolo-review it (check that the general design makes sense and appears to be doing what it is supposed to), but I am certain I cannot antagonistically review it in the way that we should be reviewing mir opts to make sure we don't have a misoptimization. I have tried the last two weeks but I don't think I am a good reviewer for such work

oli-obk · 2025-07-02T12:38:55Z

compiler/rustc_mir_dataflow/src/value_analysis.rs

 rustc_index::newtype_index!(
    /// This index uniquely identifies a tracked place and therefore a slot in [`State`].
    ///
    /// It is an implementation detail of this module.


this comment is now outdated

oli-obk · 2025-07-07T12:58:45Z

r? mir

bors · 2025-11-13T03:17:15Z

☔ The latest upstream changes (presumably #148789) made this pull request unmergeable. Please resolve the merge conflicts.

rustbot · 2025-11-17T23:41:38Z

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.

saethlin · 2025-12-01T20:34:41Z

I've been back and forth over this so many times that I think I've built enough confidence in it, even though I'll admit it is at the edge of my grasp.

This PR is really well-organized and well-described.

@bors r+

bors · 2025-12-01T20:34:44Z

📌 Commit 223620f has been approved by saethlin

It is now in the queue for this repository.

bors · 2025-12-01T23:44:53Z

⌛ Testing commit 223620f with merge 4ad239f...

bors · 2025-12-02T02:53:46Z

☀️ Test successful - checks-actions
Approved by: saethlin
Pushing 4ad239f to main...

github-actions · 2025-12-02T02:57:20Z

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 1d60f9e (parent) -> 4ad239f (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 4ad239f4156aa4e7df5ac9eb90ff0ab3d0089d1c --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: 3676.2s -> 4508.3s (+22.6%)
x86_64-gnu-stable: 7524.2s -> 6449.8s (-14.3%)
i686-gnu-2: 6044.0s -> 5198.3s (-14.0%)
aarch64-msvc-2: 5566.9s -> 4862.6s (-12.7%)
armhf-gnu: 5377.9s -> 4762.9s (-11.4%)
x86_64-gnu-llvm-21-1: 3711.3s -> 3288.7s (-11.4%)
pr-check-1: 1886.8s -> 1680.3s (-10.9%)
i686-gnu-nopt-1: 8273.0s -> 7388.2s (-10.7%)
x86_64-rust-for-linux: 2990.9s -> 2672.5s (-10.6%)
x86_64-gnu-llvm-20-2: 6249.3s -> 5630.9s (-9.9%)

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 · 2025-12-02T04:14:09Z

Finished benchmarking commit (4ad239f): 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)	0.4%	[0.1%, 0.8%]	18
Regressions ❌ (secondary)	0.2%	[0.1%, 1.0%]	19
Improvements ✅ (primary)	-0.7%	[-1.4%, -0.3%]	8
Improvements ✅ (secondary)	-0.3%	[-0.8%, -0.1%]	27
All ❌✅ (primary)	0.1%	[-1.4%, 0.8%]	26

Max RSS (memory usage)

Results (primary 1.5%, 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)	3.8%	[1.1%, 5.8%]	3
Regressions ❌ (secondary)	2.5%	[0.8%, 5.0%]	3
Improvements ✅ (primary)	-5.4%	[-5.4%, -5.4%]	1
Improvements ✅ (secondary)	-2.4%	[-2.4%, -2.4%]	1
All ❌✅ (primary)	1.5%	[-5.4%, 5.8%]	4

Cycles

Results (primary 3.5%, 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)	3.5%	[3.5%, 3.5%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.2%	[-2.7%, -1.7%]	5
All ❌✅ (primary)	3.5%	[3.5%, 3.5%]	1

Binary size

Results (primary -0.2%, 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)	0.2%	[0.0%, 0.9%]	7
Regressions ❌ (secondary)	0.1%	[0.0%, 0.1%]	6
Improvements ✅ (primary)	-0.3%	[-0.8%, -0.0%]	27
Improvements ✅ (secondary)	-0.5%	[-0.7%, -0.0%]	56
All ❌✅ (primary)	-0.2%	[-0.8%, 0.9%]	34

Bootstrap: 471.751s -> 469.27s (-0.53%)
Artifact size: 386.95 MiB -> 386.74 MiB (-0.06%)

rustbot assigned lcnr Jun 21, 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 21, 2025

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

cjgillot force-pushed the jump-threading-single branch from bdf9d85 to 3f66e3a Compare June 22, 2025 10:00

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

cjgillot force-pushed the jump-threading-single branch from 325fee6 to b541dc6 Compare July 2, 2025 10:56

rustbot assigned oli-obk and unassigned lcnr Jul 3, 2025

oli-obk reviewed Jul 7, 2025

View reviewed changes

rustbot assigned saethlin and unassigned oli-obk Jul 7, 2025

cjgillot force-pushed the jump-threading-single branch from 31e6085 to 95e10dd Compare October 17, 2025 01:49

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cjgillot force-pushed the jump-threading-single branch from 95e10dd to 87ac07f Compare October 19, 2025 02:12

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cjgillot added 9 commits November 16, 2025 01:37

Fortify test.

ee16aed

Add tests.

bd8db45

Extend value_analysis API.

4f24d70

Use a simpler condition set in jump threading.

d67e3e6

Do not flood if state is empty.

f12ab27

Skip process_constant if state has no matching value.

acf3b6a

Maintain a graph of fulfilled conditions.

f59dfc1

Simplify condition graph.

2a63fde

Filter costly chains after simplification.

223620f

cjgillot force-pushed the jump-threading-single branch from 87ac07f to 223620f Compare November 17, 2025 23:41

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 Dec 1, 2025

bors added the merged-by-bors This PR was explicitly merged by bors. label Dec 2, 2025

bors merged commit 4ad239f into rust-lang:main Dec 2, 2025
12 checks passed

rustbot added this to the 1.93.0 milestone Dec 2, 2025

bors mentioned this pull request Dec 2, 2025

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

Open

Mark-Simulacrum added the perf-regression-triaged The performance regression has been triaged. label Dec 3, 2025

cjgillot deleted the jump-threading-single branch December 3, 2025 03:42

Compute jump threading opportunities in a single pass #142821

Compute jump threading opportunities in a single pass #142821

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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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