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, 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 Conditions. The first commit replaces this state with a simpler &[Condition], and puts the corresponding ValueIndex inside Condition.

The third 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.

Later commits are perf tweaks.

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

rust-bors · 2025-09-13T17:39:14Z

💔 Test for 8577f38 failed: CI. Failed jobs:

try - dist-x86_64-linux (web logs, extended logs)

rustbot · 2025-09-13T18:22:34Z

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 · 2025-09-14T21:31:49Z

@bors try @rust-timer queue

Compute jump threading opportunities in a single pass

rust-bors · 2025-09-14T23:48:21Z

☀️ Try build successful (CI)
Build commit: 4983996 (4983996d8ea483eeff53efe5cca3d6c27e3e1ff7, parent: 52618eb338609df44978b0ca4451ab7941fd1c7a)

rust-timer · 2025-09-15T00:57:25Z

Finished benchmarking commit (4983996): 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)	0.6%	[0.1%, 1.6%]	32
Regressions ❌ (secondary)	0.3%	[0.1%, 1.7%]	19
Improvements ✅ (primary)	-0.4%	[-0.7%, -0.1%]	21
Improvements ✅ (secondary)	-0.7%	[-1.2%, -0.1%]	16
All ❌✅ (primary)	0.2%	[-0.7%, 1.6%]	53

Max RSS (memory usage)

Results (primary 0.4%, 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)	2.7%	[2.3%, 3.1%]	3
Regressions ❌ (secondary)	4.4%	[4.2%, 4.6%]	7
Improvements ✅ (primary)	-1.8%	[-4.1%, -0.5%]	3
Improvements ✅ (secondary)	-2.3%	[-4.5%, -0.9%]	6
All ❌✅ (primary)	0.4%	[-4.1%, 3.1%]	6

Cycles

Results (primary 2.6%, 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)	2.6%	[2.6%, 2.6%]	1
Regressions ❌ (secondary)	3.4%	[2.2%, 6.3%]	12
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.0%	[-4.1%, -2.1%]	5
All ❌✅ (primary)	2.6%	[2.6%, 2.6%]	1

Binary size

Results (primary -0.1%, secondary -0.6%)

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%, 1.5%]	26
Regressions ❌ (secondary)	0.1%	[0.0%, 0.1%]	8
Improvements ✅ (primary)	-0.2%	[-1.2%, -0.0%]	46
Improvements ✅ (secondary)	-0.7%	[-13.0%, -0.0%]	53
All ❌✅ (primary)	-0.1%	[-1.2%, 1.5%]	72

Bootstrap: 474.182s -> 472.053s (-0.45%)
Artifact size: 390.15 MiB -> 389.62 MiB (-0.13%)

bors · 2025-09-17T14:42:04Z

☔ The latest upstream changes (presumably #142915) made this pull request unmergeable. Please resolve the merge conflicts.

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

rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Sep 13, 2025

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cjgillot added 8 commits September 13, 2025 18:11

Fortify test.

9935f07

Use a simpler condition set in jump threading.

d790561

Only traverse MIR once.

b680ee0

Compute loop headers in postorder.

0f069aa

Compute block costs lazily.

6cbd215

Do not flood if state is empty.

da61d88

Bless coverage.

8d2ee44

Skip process_constant if state has no matching value.

79b6e67

cjgillot force-pushed the jump-threading-single branch from 7ba59e7 to 282d317 Compare September 13, 2025 18:22

cjgillot marked this pull request as draft September 13, 2025 18:34

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cjgillot added 2 commits September 14, 2025 18:20

Maintain a graph of fulfilled conditions.

eb8e141

Simplify condition graph.

394cb02

cjgillot force-pushed the jump-threading-single branch from 282d317 to 394cb02 Compare September 14, 2025 20:05

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Fix links.

1b74a55

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rust-bors bot added a commit that referenced this pull request Sep 14, 2025

Auto merge of #142821 - cjgillot:jump-threading-single, r=<try>

4983996

Compute jump threading opportunities in a single pass

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

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

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