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

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.

cjgillot · 2025-09-24T01:02:27Z

@saethlin this new version implements a much cleaner (and very different algorithm) that the previous one. The perf results are much more balanced.

This version opens the way to tractable dfa threading if we want to go that way (for instance if the loop is annotated with loop_match)

bors · 2025-09-27T13:18:34Z

☔ The latest upstream changes (presumably #146829) made this pull request unmergeable. Please resolve the merge conflicts.

rustbot · 2025-10-04T16:52:58Z

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.

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 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

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

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

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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

cjgillot force-pushed the jump-threading-single branch from 1b74a55 to 3a5c954 Compare September 24, 2025 00:25

cjgillot force-pushed the jump-threading-single branch from 3a5c954 to f82bdcb Compare September 27, 2025 16:03

cjgillot marked this pull request as ready for review September 27, 2025 16:07

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 Sep 27, 2025

cjgillot added 9 commits October 4, 2025 16:23

Fortify test.

758b89f

Add tests.

72ff82c

Extend value_analysis API.

dc063c5

Use a simpler condition set in jump threading.

2d30fba

Do not flood if state is empty.

b891d80

Skip process_constant if state has no matching value.

d911a1c

Maintain a graph of fulfilled conditions.

a888333

Simplify condition graph.

e5b514d

Filter costly chains after simplification.

eab0865

cjgillot force-pushed the jump-threading-single branch from f82bdcb to eab0865 Compare October 4, 2025 16:52

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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