GVN: Only propagate borrows from SSA locals by dianqk · Pull Request #150485 · rust-lang/rust

dianqk · 2025-12-29T14:11:18Z

Fixes#141313. This is a more principled fix than #147886.

Using a reference that is not a borrowing of an SSA local at a new location may be UB.

The PR has two major changes.

The first one, when introducing a new dereference at a new location, is that the reference must point to an SSA local or be an immutable argument. dereference_address has handled SSA locals.

The second one, if we cannot guard to the reference point to an SSA local in visit_assign, we have to rewrite the value to opaque. This avoids unifying the following dereferences that also are references:

let b:&T = *a;// ... `a` is allowed to be modified. `c` and `b` have different borrowing lifetime.// Unifying them will extend the lifetime of `b`.let c:&T = *a;

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)	0.3%	[0.1%, 0.9%]	20
Regressions ❌ (secondary)	0.4%	[0.1%, 2.0%]	27
Improvements ✅ (primary)	-0.6%	[-1.2%, -0.2%]	4
Improvements ✅ (secondary)	-0.2%	[-0.4%, -0.1%]	7
All ❌✅ (primary)	0.1%	[-1.2%, 0.9%]	24

Max RSS (memory usage)

Results (primary -1.1%, secondary -4.7%)

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

	mean	range	count
Regressions ❌ (primary)	2.4%	[1.9%, 2.8%]	2
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-3.4%	[-6.8%, -0.6%]	3
Improvements ✅ (secondary)	-4.7%	[-6.5%, -3.7%]	3
All ❌✅ (primary)	-1.1%	[-6.8%, 2.8%]	5

Cycles

This benchmark run did not return any relevant results for this metric.

Binary size

Results (primary 0.1%, 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)	0.7%	[0.0%, 6.9%]	11
Regressions ❌ (secondary)	0.1%	[0.0%, 0.2%]	14
Improvements ✅ (primary)	-0.1%	[-1.0%, -0.0%]	27
Improvements ✅ (secondary)	-0.1%	[-0.1%, -0.0%]	5
All ❌✅ (primary)	0.1%	[-1.0%, 6.9%]	38

Bootstrap: 475.254s -> 472.456s (-0.59%)
Artifact size: 390.85 MiB -> 390.82 MiB (-0.01%)

dianqk · 2026-01-02T14:21:44Z

@bors try parent=fcd630976c460c819c4bbcaf107d0c94501205d8 @rust-timer queue

[DRAFT] GVN: Only propagate borrows from SSA-locals

rust-bors · 2026-01-02T16:45:18Z

☀️ Try build successful (CI)
Build commit: 2183340 (218334063dc5d70b7f9d69202bf22750167c6cbf, parent: fcd630976c460c819c4bbcaf107d0c94501205d8)

rust-timer · 2026-01-02T19:12:15Z

Finished benchmarking commit (2183340): 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)	0.3%	[0.1%, 0.9%]	17
Regressions ❌ (secondary)	0.3%	[0.1%, 0.8%]	23
Improvements ✅ (primary)	-0.6%	[-1.2%, -0.2%]	4
Improvements ✅ (secondary)	-0.2%	[-0.4%, -0.1%]	8
All ❌✅ (primary)	0.1%	[-1.2%, 0.9%]	21

Max RSS (memory usage)

Results (primary -1.9%)

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

	mean	range	count
Regressions ❌ (primary)	1.7%	[1.7%, 1.7%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-3.0%	[-4.6%, -0.6%]	3
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-1.9%	[-4.6%, 1.7%]	4

Cycles

Results (secondary -2.3%)

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)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.3%	[-2.3%, -2.3%]	1
All ❌✅ (primary)	-	-	0

Binary size

Results (primary 0.2%, 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)	0.7%	[0.0%, 6.9%]	11
Regressions ❌ (secondary)	0.1%	[0.0%, 0.2%]	14
Improvements ✅ (primary)	-0.2%	[-0.9%, -0.0%]	11
Improvements ✅ (secondary)	-0.1%	[-0.1%, -0.0%]	5
All ❌✅ (primary)	0.2%	[-0.9%, 6.9%]	22

Bootstrap: 475.254s -> 472.783s (-0.52%)
Artifact size: 390.85 MiB -> 390.84 MiB (-0.00%)

dianqk · 2026-01-17T21:00:25Z

@bors try @rust-timer queue

rust-bors · 2026-02-05T02:03:14Z

☀️ Try build successful (CI)
Build commit: f2def88 (f2def887c3d08e9845884a5677473348acc2e23f, parent: db3e99bbab28c6ca778b13222becdea54533d908)

rust-timer · 2026-02-05T02:43:42Z

Finished benchmarking commit (f2def88): 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)	1.5%	[0.9%, 2.1%]	2
Regressions ❌ (secondary)	0.7%	[0.3%, 2.0%]	5
Improvements ✅ (primary)	-1.5%	[-1.9%, -1.0%]	2
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.0%	[-1.9%, 2.1%]	4

Max RSS (memory usage)

Results (primary -0.6%, 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)	2.5%	[1.7%, 3.5%]	3
Regressions ❌ (secondary)	1.5%	[0.5%, 2.5%]	2
Improvements ✅ (primary)	-5.3%	[-7.6%, -3.1%]	2
Improvements ✅ (secondary)	-2.0%	[-2.0%, -2.0%]	1
All ❌✅ (primary)	-0.6%	[-7.6%, 3.5%]	5

Cycles

Results (primary 2.8%)

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

	mean	range	count
Regressions ❌ (primary)	2.8%	[2.8%, 2.8%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	2.8%	[2.8%, 2.8%]	1

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)	0.4%	[0.0%, 1.2%]	5
Regressions ❌ (secondary)	1.2%	[1.2%, 1.2%]	1
Improvements ✅ (primary)	-0.3%	[-1.4%, -0.0%]	6
Improvements ✅ (secondary)	-0.1%	[-0.2%, -0.0%]	8
All ❌✅ (primary)	0.0%	[-1.4%, 1.2%]	11

Bootstrap: 472.94s -> 474.175s (0.26%)
Artifact size: 398.12 MiB -> 398.12 MiB (-0.00%)

dianqk · 2026-02-05T12:36:41Z

The two latest perf results that were built with LLVM 22 are different. I am not sure why, but the latest result seems acceptable to me.

cjgillot · 2026-02-08T02:18:43Z

@bors r+

rust-bors · 2026-02-08T02:18:45Z

📌 Commit 9c029d2 has been approved by cjgillot

It is now in the queue for this repository.

rust-bors · 2026-02-08T08:12:07Z

☀️ Test successful - CI
Approved by: cjgillot
Duration: 3h 8m 13s
Pushing 13c3873 to main...

github-actions · 2026-02-08T08:14:44Z

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 08a4ce5 (parent) -> 13c3873 (this PR)

Test differences

Show 4 test diffs

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

Test dashboard

Run

cargo run --manifest-path src/ci/citool/Cargo.toml -- \
test-dashboard 13c38730d981289cc7ae4cc109fd7756bf83ee67 --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-apple-various: 1h 12m -> 1h 1m (-15.5%)
aarch64-gnu-llvm-20-1: 1h 3m -> 57m 6s (-10.6%)
x86_64-gnu-llvm-21-2: 1h 28m -> 1h 36m (+9.5%)
x86_64-msvc-ext3: 1h 42m -> 1h 52m (+9.4%)
dist-i586-gnu-i586-i686-musl: 1h 26m -> 1h 34m (+9.2%)
x86_64-msvc-2: 2h 17m -> 2h 29m (+8.9%)
x86_64-gnu-stable: 2h 28m -> 2h 15m (-8.9%)
x86_64-msvc-ext2: 1h 35m -> 1h 42m (+8.2%)
dist-x86_64-apple: 2h 9m -> 1h 59m (-7.4%)
pr-check-1: 34m 21s -> 31m 50s (-7.3%)

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 · 2026-02-08T09:22:00Z

Finished benchmarking commit (13c3873): comparison URL.

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

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)	1.0%	[0.3%, 2.0%]	3
Regressions ❌ (secondary)	0.7%	[0.3%, 1.9%]	4
Improvements ✅ (primary)	-1.0%	[-1.0%, -1.0%]	2
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.2%	[-1.0%, 2.0%]	5

Max RSS (memory usage)

Results (primary 1.1%)

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

	mean	range	count
Regressions ❌ (primary)	2.8%	[2.7%, 3.0%]	3
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-4.0%	[-4.0%, -4.0%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	1.1%	[-4.0%, 3.0%]	4

Cycles

Results (primary 2.1%, secondary 1.1%)

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

	mean	range	count
Regressions ❌ (primary)	2.1%	[2.1%, 2.1%]	1
Regressions ❌ (secondary)	2.6%	[2.1%, 3.2%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.0%	[-2.0%, -2.0%]	1
All ❌✅ (primary)	2.1%	[2.1%, 2.1%]	1

Binary size

Results (primary 0.1%, 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)	0.4%	[0.0%, 1.2%]	5
Regressions ❌ (secondary)	1.2%	[1.2%, 1.2%]	1
Improvements ✅ (primary)	-0.1%	[-0.4%, -0.0%]	6
Improvements ✅ (secondary)	-0.1%	[-0.2%, -0.0%]	8
All ❌✅ (primary)	0.1%	[-0.4%, 1.2%]	11

Bootstrap: 476.152s -> 471.737s (-0.93%)
Artifact size: 397.96 MiB -> 397.92 MiB (-0.01%)

panstromek · 2026-02-09T15:02:13Z

perf triage:

Results match pre merge results, which was accepted in #150485 (comment).

@rustbot label: +perf-regression-triaged

RalfJung · 2026-04-27T15:47:10Z

+ // ```
+ if projection_ty.ty.is_ref() {
+ return None;
+ }


I do not understand how this check is enough. What if we have a &(&T,) type? Then the projection_ty is a tuple, not a reference, but it contains a reference and that could still lead to the same problems, couldn't it?
View changes since the review

@RalfJung

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

@RalfJung

GVN: Don't assume nested shared references are read-only. Fixesrust-lang/rust#155884. rust-lang/rust#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

@RalfJung

GVN: Don't assume nested shared references are read-only. Fixesrust-lang/rust#155884. rust-lang/rust#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

@RalfJung

GVN: Don't assume nested shared references are read-only. Fixesrust-lang/rust#155884. rust-lang/rust#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

@RalfJung

GVN: Don't assume nested shared references are read-only. Fixesrust-lang/rust#155884. rust-lang/rust#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 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 Dec 29, 2025

dianqk mentioned this pull request Dec 29, 2025
GVN: Use the borrows only if they are always live #147886
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Auto merge of#150485- dianqk:gvn-ssa-borrow, r=<try>
f23e5de
[DRAFT] GVN: Only propagate borrows from SSA-locals

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rust-borsBot added a commit that referenced this pull request Jan 2, 2026
Auto merge of#150485- dianqk:gvn-ssa-borrow, r=<try>
2183340
[DRAFT] GVN: Only propagate borrows from SSA-locals

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dianqk mentioned this pull request Jan 3, 2026
GVN: Avoid merging borrows from the dereferenced value #150270
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rust-borsBot 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 Feb 8, 2026

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rust-borsBot 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 Feb 8, 2026

rust-borsBot merged commit 13c3873 into rust-lang:mainFeb 8, 2026
13 checks passed

rustbot added this to the 1.95.0 milestone Feb 8, 2026

dianqk deleted the gvn-ssa-borrow branch February 8, 2026 10:49

rustbot added the perf-regression-triaged The performance regression has been triaged. label Feb 9, 2026

dianqk mentioned this pull request Mar 24, 2026
fix/extend some mir-opt comments #154297
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RalfJung reviewed Apr 27, 2026
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RalfJung mentioned this pull request Apr 27, 2026
GVN assumes nested shared references are read-only #155884
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dianqk mentioned this pull request Jun 3, 2026
GVN: Don't assume nested shared references are read-only. #157346
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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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Overall result: ❌✅ regressions and improvements - please read the text below

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Test differences

Job duration changes

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

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