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Tell LLVM about read-only captures #145259

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@nikic nikic commented Aug 11, 2025 •
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&Freeze parameters are not only readonly within the function, but any captures of the pointer can also only be used for reads. This can now be encoded using the captures(address, read_provenance) attribute.

@rustbot rustbot added A-LLVM Area: Code generation parts specific to LLVM. Both correctness bugs and optimization-related issues. 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 Aug 11, 2025
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nikic commented Aug 11, 2025

@bors try @rust-timer queue

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rust-bors bot added a commit that referenced this pull request Aug 11, 2025
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Auto merge of #145259 - nikic:read-only-capture, r=<try>
6bc3793 
Tell LLVM about read-only captures
@rustbot rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Aug 11, 2025
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☀️ Try build successful (CI)
Build commit: 6bc3793 (6bc37934c6eaa84bfd300c54189bec62e15d42ed, parent: 577166503aee7290e09374da21f4045c455acfd5)

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Finished benchmarking commit (6bc3793): 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% [0.3%, 0.4%] 5
Regressions ❌
(secondary)		 1.0% [0.9%, 1.6%] 7
Improvements ✅
(primary)		 - - 0
Improvements ✅
(secondary)		 -0.8% [-0.8%, -0.7%] 5
All ❌✅ (primary) 0.3% [0.3%, 0.4%] 5

Max RSS (memory usage)

Results (secondary 3.9%)

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)		 3.9% [3.9%, 3.9%] 1
Improvements ✅
(primary)		 - - 0
Improvements ✅
(secondary)		 - - 0
All ❌✅ (primary) - - 0

Cycles

Results (secondary -2.1%)

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)		 -2.1% [-2.1%, -2.1%] 1
All ❌✅ (primary) - - 0

Binary size

Results (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
Regressions ❌
(secondary)		 0.0% [0.0%, 0.0%] 23
Improvements ✅
(primary)		 - - 0
Improvements ✅
(secondary)		 - - 0
All ❌✅ (primary) - - 0

Bootstrap: 465.465s -> 464.036s (-0.31%)
Artifact size: 377.33 MiB -> 377.30 MiB (-0.01%)

@rustbot rustbot added perf-regression Performance regression. and removed S-waiting-on-perf Status: Waiting on a perf run to be completed. labels Aug 11, 2025
@nikic nikic force-pushed the read-only-capture branch from 2f266b1 to a68e8be Compare August 19, 2025 10:37
@nikic nikic marked this pull request as ready for review August 19, 2025 10:38
@rustbot 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 Aug 19, 2025
nikic
nikic commented Aug 19, 2025
View reviewed changes
tests/ui/codegen/equal-pointers-unequal/as-cast/segfault.rs
@@ -55,6 +55,7 @@ fn main() {
// The `Box` has been deallocated by now, so this is a dangling reference!
let r: &u8 = &*r;
println!("{:p}", r);
println!("{}", i);
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These adjustments are necessary so these tests still get miscompiled.

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nikic commented Aug 19, 2025

r? compiler

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nikic added 2 commits August 19, 2025 13:28
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Tell LLVM about read-only captures
b045bc8 
`&Freeze` parameters are not only `readonly` within the function,
but any captures of the pointer can also only be used for reads.
This can now be encoded using the `captures(address, read_provenance)`
attribute.
@nikic
Adjust test to still show miscompile
66fd5d8 
The capture of i in assert_ne!() is now known read-only, which
enables early SROA. Block this by passing i to println, where
we currently cannot recognize this.
@nikic nikic force-pushed the read-only-capture branch from a68e8be to 66fd5d8 Compare August 19, 2025 11:29
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A-LLVM Area: Code generation parts specific to LLVM. Both correctness bugs and optimization-related issues. perf-regression Performance regression. 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.
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