GVN: Support unions. #146355

cjgillot · 2025-09-09T01:27:03Z

Supporting union values is not very hard, and allows to const-prop them.

r? @ghost for perf

cjgillot · 2025-09-09T01:27:20Z

GVN: Support unions.

rust-bors · 2025-09-09T03:49:13Z

☀️ Try build successful (CI)
Build commit: 366a987 (366a9875a4c25a2e49bf4797159bb426a8ffd701, parent: 9c27f27ea3bab79a2fec827ef3ae0009959d60f4)

rust-timer · 2025-09-09T08:15:45Z

Finished benchmarking commit (366a987): comparison URL.

Overall result: ❌✅ regressions and 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)	0.2%	[0.2%, 0.2%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.0%	[-0.0%, -0.0%]	2
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (primary 2.9%, 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)	2.9%	[2.9%, 2.9%]	1
Regressions ❌ (secondary)	3.6%	[0.7%, 5.2%]	10
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.9%	[-2.9%, -2.8%]	2
All ❌✅ (primary)	2.9%	[2.9%, 2.9%]	1

Cycles

Results (secondary 2.8%)

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)	2.8%	[2.4%, 3.2%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Binary size

Results (primary 0.1%)

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

	mean	range	count
Regressions ❌ (primary)	0.1%	[0.1%, 0.2%]	5
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.2%	[-0.2%, -0.2%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.1%	[-0.2%, 0.2%]	6

Bootstrap: 467.433s -> 467.12s (-0.07%)
Artifact size: 387.54 MiB -> 387.87 MiB (0.08%)

rustbot · 2025-09-14T21:40:08Z

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

cjgillot · 2025-09-19T00:15:13Z

r? compiler

jackh726 · 2025-09-26T04:32:53Z

@bors r+

bors · 2025-09-26T04:32:56Z

📌 Commit 6fba73e has been approved by jackh726

It is now in the queue for this repository.

GVN: Support unions. Supporting union values is not very hard, and allows to const-prop them. r? `@ghost` for perf

bors · 2025-09-26T11:00:16Z

⌛ Testing commit 6fba73e with merge df8b765...

bors · 2025-09-26T11:34:49Z

💔 Test failed - checks-actions

cjgillot · 2025-09-27T00:35:01Z

Oops, that was a delayed error message.
@bors r-

cjgillot · 2025-10-06T14:14:49Z

@bors r=jackh726

bors · 2025-10-06T14:14:52Z

📌 Commit 8c1dfbb has been approved by jackh726

It is now in the queue for this repository.

rustbot · 2025-10-06T19:32:47Z

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 · 2025-10-06T19:33:19Z

Rebase after #144477
@bors r=jackh726

bors · 2025-10-06T19:33:23Z

📌 Commit 96c3978 has been approved by jackh726

It is now in the queue for this repository.

bors · 2025-10-07T00:22:43Z

⌛ Testing commit 96c3978 with merge ff5be13...

bors · 2025-10-07T03:30:20Z

☀️ Test successful - checks-actions
Approved by: jackh726
Pushing ff5be13 to master...

github-actions · 2025-10-07T03:33:05Z

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 68ad253 (parent) -> ff5be13 (this PR)

Test differences

Show 6 test diffs

Stage 1

[mir-opt] tests/mir-opt/const_prop/union.rs: [missing] -> pass (J0)
[mir-opt] tests/mir-opt/dest-prop/union.rs: pass -> [missing] (J0)

Stage 2

[mir-opt] tests/mir-opt/const_prop/union.rs: [missing] -> pass (J1)
[mir-opt] tests/mir-opt/dest-prop/union.rs: pass -> [missing] (J1)

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

Job group index

Test dashboard

Run

cargo run --manifest-path src/ci/citool/Cargo.toml -- \
    test-dashboard ff5be13e2d9f398c38215958021e4c7970f88914 --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

x86_64-rust-for-linux: 2464.4s -> 3106.3s (26.0%)
dist-apple-various: 3450.2s -> 4119.6s (19.4%)
tidy: 214.5s -> 190.5s (-11.2%)
dist-aarch64-apple: 8034.2s -> 7225.1s (-10.1%)
armhf-gnu: 5165.4s -> 4700.4s (-9.0%)
i686-msvc-2: 7228.1s -> 7875.4s (9.0%)
dist-powerpc64le-linux-gnu: 5238.5s -> 5702.6s (8.9%)
aarch64-gnu-llvm-20-1: 3906.9s -> 3606.2s (-7.7%)
dist-various-1: 3665.3s -> 3935.4s (7.4%)
aarch64-msvc-2: 5104.5s -> 4738.0s (-7.2%)

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 · 2025-10-07T04:42:24Z

Finished benchmarking commit (ff5be13): comparison URL.

Overall result: ❌ regressions - no action needed

@rustbot label: -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)	0.1%	[0.1%, 0.1%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (primary -2.4%, secondary -0.3%)

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)	2.3%	[2.1%, 2.6%]	3
Improvements ✅ (primary)	-2.4%	[-2.4%, -2.4%]	1
Improvements ✅ (secondary)	-2.2%	[-3.3%, -0.6%]	4
All ❌✅ (primary)	-2.4%	[-2.4%, -2.4%]	1

Cycles

Results (secondary 1.9%)

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)	5.9%	[5.9%, 5.9%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.0%	[-2.0%, -2.0%]	1
All ❌✅ (primary)	-	-	0

Binary size

Results (primary 0.1%)

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

	mean	range	count
Regressions ❌ (primary)	0.3%	[0.2%, 0.3%]	2
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.0%	[-0.0%, -0.0%]	3
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.1%	[-0.0%, 0.3%]	5

Bootstrap: 473.761s -> 473.87s (0.02%)
Artifact size: 388.37 MiB -> 388.40 MiB (0.01%)

RalfJung · 2025-10-09T09:09:36Z

What exactly does this assume about unions? I see something involving an "active field" and my alarm bells go off; unions in Rust do not have a notion of active field, they are basically just syntactic sugar for transmutes.

cjgillot · 2025-10-09T11:44:38Z

The two simplifications we perform are:

constant folding, which may introduce constants with undef bytes, more often than transmutes;
Union { x: foo }.x simplified to foo.

We do no assume any behaviour if the projection field is not the construction field.

For the "active" terminology, I saw it inside rustc, so I thought it ok.

RalfJung · 2025-10-09T13:14:38Z

Okay, that sounds good, thanks.

scottmcm · 2025-10-10T18:36:54Z

compiler/rustc_mir_transform/src/gvn.rs

+    /// A union aggregate value.
+    Union(FieldIdx, VnIndex),


cc @saethlin -- I wonder if you can use this to solve your uninit-array problem now that GVN tracks unions?

Yes. Working on it.

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 9, 2025

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Auto merge of #146355 - cjgillot:gvn-union, r=<try>

366a987

GVN: Support unions.

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cjgillot force-pushed the gvn-union branch from de25069 to 6fba73e Compare September 14, 2025 21:38

cjgillot marked this pull request as ready for review September 14, 2025 21:40

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 14, 2025

cjgillot added A-mir-opt Area: MIR optimizations A-mir-opt-GVN Area: MIR opt Global Value Numbering (GVN) labels Sep 14, 2025

rustbot assigned jackh726 Sep 19, 2025

jackh726 approved these changes Sep 26, 2025

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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 26, 2025

bors added a commit that referenced this pull request Sep 26, 2025

Auto merge of #146355 - cjgillot:gvn-union, r=jackh726

df8b765

GVN: Support unions. Supporting union values is not very hard, and allows to const-prop them. r? `@ghost` for perf

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bors added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. labels Sep 27, 2025

cjgillot force-pushed the gvn-union branch from cfd82e3 to 55eadf5 Compare September 27, 2025 00:36

cjgillot force-pushed the gvn-union branch from 55eadf5 to 8c1dfbb Compare October 4, 2025 16:52

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cjgillot added 2 commits October 6, 2025 19:31

GVN: Support unions.

5407eb8

Repurpose defunct test.

96c3978

cjgillot force-pushed the gvn-union branch from 8c1dfbb to 96c3978 Compare October 6, 2025 19:32

bors added the merged-by-bors This PR was explicitly merged by bors. label Oct 7, 2025

bors merged commit ff5be13 into rust-lang:master Oct 7, 2025
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rustbot added this to the 1.92.0 milestone Oct 7, 2025

bors mentioned this pull request Oct 7, 2025

Extend GVN to perform local value numbering. #143333

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cjgillot deleted the gvn-union branch October 7, 2025 10:14

scottmcm reviewed Oct 10, 2025

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saethlin mentioned this pull request Oct 17, 2025

Fix MaybeUninit codegen using GVN #147827

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GVN: Support unions. #146355

GVN: Support unions. #146355

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Overall result: ❌✅ regressions and improvements - no action needed

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Test differences

Stage 1

Stage 2

Job duration changes

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Overall result: ❌ regressions - no action needed

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