GVN: Support unions.

[cjgillot]

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

r? @ghost for perf

[cjgillot]

@bors try @rust-timer queue

[rust-bors]

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

[rust-timer]

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]

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

[cjgillot]

r? compiler

[jackh726]

@bors r+

[bors]

📌 Commit 6fba73e has been approved by jackh726

It is now in the queue for this repository.

[bors]

⌛ Testing commit 6fba73e with merge df8b765...

[bors]

💔 Test failed - checks-actions

[cjgillot]

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

[cjgillot]

@bors r=jackh726

[bors]

📌 Commit 8c1dfbb has been approved by jackh726

It is now in the queue for this repository.

[rustbot]

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]

Rebase after #144477
@bors r=jackh726

[bors]

📌 Commit 96c3978 has been approved by jackh726

It is now in the queue for this repository.

[bors]

⌛ Testing commit 96c3978 with merge ff5be13...

[bors]

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

[github-actions]

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

• J0: x86_64-gnu-llvm-20-3
• J1: aarch64-apple, aarch64-gnu, aarch64-gnu-llvm-20-1, aarch64-msvc-1, arm-android, armhf-gnu, dist-i586-gnu-i586-i686-musl, i686-gnu-1, i686-gnu-nopt-1, i686-msvc-1, test-various, x86_64-gnu, x86_64-gnu-llvm-20, x86_64-gnu-llvm-20-2, x86_64-gnu-nopt, x86_64-gnu-stable, x86_64-mingw-1, x86_64-msvc-1

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

1. x86_64-rust-for-linux: 2464.4s -> 3106.3s (26.0%)
2. dist-apple-various: 3450.2s -> 4119.6s (19.4%)
3. tidy: 214.5s -> 190.5s (-11.2%)
4. dist-aarch64-apple: 8034.2s -> 7225.1s (-10.1%)
5. armhf-gnu: 5165.4s -> 4700.4s (-9.0%)
6. i686-msvc-2: 7228.1s -> 7875.4s (9.0%)
7. dist-powerpc64le-linux-gnu: 5238.5s -> 5702.6s (8.9%)
8. aarch64-gnu-llvm-20-1: 3906.9s -> 3606.2s (-7.7%)
9. dist-various-1: 3665.3s -> 3935.4s (7.4%)
10. 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]

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]

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]

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]

Okay, that sounds good, thanks.

[scottmcm]

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

[saethlin]

Yes. Working on it.