GVN: Don't assume nested shared references are read-only. by dianqk · Pull Request #157346 · rust-lang/rust

dianqk · 2026-06-03T00:39:42Z

Fixes #155884.

#150485 only checks the type, whether it is a reference, which is not enough. The PR extends to other types.

cc @RalfJung @saethlin

r? @cjgillot

rustbot · 2026-06-03T00:39:45Z

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

dianqk · 2026-06-03T04:07:31Z

@bors try @rust-timer queue

GVN: Don't assume nested shared references are read-only.

rust-bors · 2026-06-03T06:19:09Z

☀️ Try build successful (CI)
Build commit: 0eeeb18 (0eeeb181f41359e276c1dae9eec938e457300cec, parent: d595fce01043347bf7f80e85b76dcc41b59a3e6e)

rust-timer · 2026-06-03T07:00:08Z

Finished benchmarking commit (0eeeb18): comparison URL.

Overall result: ❌✅ regressions and improvements - please read:

Benchmarking means the PR may be perf-sensitive. It's automatically marked not fit for rolling up. Overriding is possible but disadvised: it risks changing compiler perf.

Next, please: If you can, justify the regressions found in this try perf run in writing along with @rustbot label: +perf-regression-triaged. If not, fix the regressions and do another perf run. Neutral or positive results will clear the label automatically.

@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)	0.3%	[0.1%, 1.0%]	12
Regressions ❌ (secondary)	0.3%	[0.0%, 0.5%]	36
Improvements ✅ (primary)	-0.3%	[-0.3%, -0.2%]	12
Improvements ✅ (secondary)	-0.3%	[-1.0%, -0.0%]	12
All ❌✅ (primary)	0.0%	[-0.3%, 1.0%]	24

Max RSS (memory usage)

Results (primary 3.6%)

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

	mean	range	count
Regressions ❌ (primary)	4.9%	[1.8%, 10.4%]	6
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-3.9%	[-3.9%, -3.9%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	3.6%	[-3.9%, 10.4%]	7

Cycles

Results (secondary -0.4%)

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)	7.2%	[5.0%, 10.8%]	3
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.7%	[-6.2%, -2.4%]	7
All ❌✅ (primary)	-	-	0

Binary size

Results (primary -0.0%, secondary 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%	[0.0%, 0.6%]	19
Regressions ❌ (secondary)	0.1%	[0.0%, 0.6%]	16
Improvements ✅ (primary)	-0.4%	[-1.1%, -0.1%]	8
Improvements ✅ (secondary)	-0.1%	[-0.1%, -0.1%]	1
All ❌✅ (primary)	-0.0%	[-1.1%, 0.6%]	27

Bootstrap: 509.701s -> 512.229s (0.50%)
Artifact size: 400.67 MiB -> 398.49 MiB (-0.55%)

RalfJung · 2026-06-03T07:00:53Z

+                    // DO NOT reason the pointer value that has references.
                    // We cannot unify two pointers that dereference same local, because they may
                    // have different lifetimes.


This isn't quite grammatical. Maybe you mean "do not reason about ..."? But even then I'm a bit confused about what the comment is trying to say.

This should definitely mention that nested shared references are not read-only and have a link to the issue.

View changes since the review

RalfJung · 2026-06-03T15:01:28Z

                    // let c: &T = *a;
                    // ```
-                    if projection_ty.ty.is_ref() {
+                    // Unifying them can violate Stacked Borrows or Tree Borrows.


Suggested change

// Unifying them can violate Stacked Borrows or Tree Borrows.

// Furthermore, unifying them can also violate Stacked Borrows or Tree Borrows.

View changes since the review

cjgillot · 2026-06-21T16:59:41Z

@bors r+

rust-bors · 2026-06-21T16:59:43Z

📌 Commit 0abc695 has been approved by cjgillot

It is now in the queue for this repository.

rust-bors · 2026-06-21T22:59:02Z

☀️ Test successful - CI
Approved by: cjgillot
Duration: 3h 14m 8s
Pushing 91fe22d to main...

github-actions · 2026-06-21T23:02:24Z

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 d29dae8 (parent) -> 91fe22d (this PR)

Test differences

Show 2 test diffs

Stage 1

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

Stage 2

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

Job group index

Test dashboard

Run

cargo run --manifest-path src/ci/citool/Cargo.toml -- \
    test-dashboard 91fe22da8084a1c9e993d78d4a56f22ab8396236 --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-x86_64-msvc-alt: 1h 59m -> 2h 44m (+38.0%)
x86_64-msvc-ext3: 1h 37m -> 1h 3m (-35.0%)
x86_64-gnu-gcc: 48m 29s -> 1h 4m (+32.5%)
i686-gnu-1: 1h 42m -> 2h 11m (+27.9%)
x86_64-msvc-1: 1h 59m -> 2h 32m (+27.7%)
x86_64-gnu: 2h 24m -> 1h 45m (-27.2%)
x86_64-gnu-llvm-21-2: 1h 35m -> 1h 11m (-24.9%)
pr-check-2: 28m 58s -> 35m 42s (+23.2%)
dist-riscv64-linux: 1h 13m -> 1h 29m (+21.8%)
aarch64-apple-macos-26: 2h 27m -> 2h 57m (+20.3%)

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 · 2026-06-21T23:40:21Z

Finished benchmarking commit (91fe22d): comparison URL.

Overall result: ❌✅ regressions and improvements - please read:

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)	0.3%	[0.1%, 0.5%]	9
Regressions ❌ (secondary)	0.3%	[0.1%, 0.4%]	11
Improvements ✅ (primary)	-0.3%	[-0.4%, -0.2%]	10
Improvements ✅ (secondary)	-0.5%	[-1.1%, -0.2%]	5
All ❌✅ (primary)	0.0%	[-0.4%, 0.5%]	19

Max RSS (memory usage)

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)	2.3%	[2.3%, 2.3%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-2.1%	[-2.1%, -2.1%]	1
Improvements ✅ (secondary)	-0.6%	[-0.6%, -0.6%]	1
All ❌✅ (primary)	0.1%	[-2.1%, 2.3%]	2

Cycles

Results (primary -1.9%, secondary 3.2%)

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)	3.2%	[3.2%, 3.2%]	1
Improvements ✅ (primary)	-1.9%	[-1.9%, -1.9%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-1.9%	[-1.9%, -1.9%]	1

Binary size

Results (primary 0.2%, secondary 0.0%)

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%, 1.9%]	17
Regressions ❌ (secondary)	0.1%	[0.0%, 0.6%]	15
Improvements ✅ (primary)	-0.5%	[-0.6%, -0.3%]	2
Improvements ✅ (secondary)	-0.4%	[-0.4%, -0.4%]	3
All ❌✅ (primary)	0.2%	[-0.6%, 1.9%]	19

Bootstrap: 483.058s -> 480.522s (-0.52%)
Artifact size: 391.26 MiB -> 391.37 MiB (0.03%)

Add miscompiled nested shared references tests

7e286d0

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 3, 2026

rustbot assigned cjgillot Jun 3, 2026

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rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Jun 3, 2026

rust-bors Bot pushed a commit that referenced this pull request Jun 3, 2026

Auto merge of #157346 - dianqk:gvn-nest-ref, r=<try>

0eeeb18

GVN: Don't assume nested shared references are read-only.

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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 3, 2026

RalfJung reviewed Jun 3, 2026

View reviewed changes

dianqk force-pushed the gvn-nest-ref branch from fcfe33b to f90d027 Compare June 3, 2026 14:47

RalfJung reviewed Jun 3, 2026

View reviewed changes

GVN: Don't assume nested shared references are read-only.

0abc695

dianqk force-pushed the gvn-nest-ref branch from f90d027 to 0abc695 Compare June 3, 2026 21:44

rust-bors Bot 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 Jun 21, 2026

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rust-bors Bot added merged-by-bors This PR was explicitly merged by bors. and removed S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. labels Jun 21, 2026

rust-bors Bot merged commit 91fe22d into rust-lang:main Jun 21, 2026
13 checks passed

rustbot added this to the 1.98.0 milestone Jun 21, 2026

dianqk deleted the gvn-nest-ref branch June 22, 2026 09:10

	// Unifying them can violate Stacked Borrows or Tree Borrows.
	// Furthermore, unifying them can also violate Stacked Borrows or Tree Borrows.

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Overall result: ❌✅ regressions and improvements - please read:

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Test differences

Stage 1

Stage 2

Job duration changes

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rust-timer commented Jun 21, 2026

Overall result: ❌✅ regressions and improvements - please read:

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