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Propagate from borrowed locals in CopyProp #143624

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@tmiasko tmiasko commented Jul 8, 2025

r? cjgillot

@rustbot 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 Jul 8, 2025
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rustbot commented Jul 8, 2025

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

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tmiasko commented Jul 8, 2025

@bors2 try @rust-timer queue

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rust-bors bot added a commit that referenced this pull request Jul 8, 2025
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Auto merge of #143624 - tmiasko:copy-prop-borrowed, r=<try>
1e4b0b7 
Propagate from borrowed locals in CopyProp

r? cjgillot
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rust-bors bot commented Jul 8, 2025

⌛ Trying commit dd34b79 with merge 1e4b0b7…

To cancel the try build, run the command @bors2 try cancel.

@rustbot rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Jul 8, 2025
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☀️ Try build successful (CI)
Build commit: 1e4b0b7 (1e4b0b7e6afa40bec58198f6cdb7d7bbfdf3efd0, parent: 688ea65df6a47866d0f72a00f1e18b47a7edf83b)

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Finished benchmarking commit (1e4b0b7): comparison URL.

Overall result: ✅ 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)		 0.4% [0.4%, 0.4%] 1
Regressions ❌
(secondary)		 - - 0
Improvements ✅
(primary)		 -0.2% [-0.3%, -0.1%] 18
Improvements ✅
(secondary)		 -0.3% [-0.6%, -0.0%] 26
All ❌✅ (primary) -0.1% [-0.3%, 0.4%] 19

Max RSS (memory usage)

Results (primary 1.9%, secondary 2.2%)

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

mean range count
Regressions ❌
(primary)		 5.5% [5.5%, 5.5%] 1
Regressions ❌
(secondary)		 2.2% [2.2%, 2.2%] 1
Improvements ✅
(primary)		 -1.7% [-1.7%, -1.7%] 1
Improvements ✅
(secondary)		 - - 0
All ❌✅ (primary) 1.9% [-1.7%, 5.5%] 2

Cycles

Results (primary 2.4%, 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)		 2.4% [2.4%, 2.4%] 1
Regressions ❌
(secondary)		 - - 0
Improvements ✅
(primary)		 - - 0
Improvements ✅
(secondary)		 -3.9% [-5.7%, -2.8%] 3
All ❌✅ (primary) 2.4% [2.4%, 2.4%] 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)		 0.3% [0.0%, 1.3%] 11
Regressions ❌
(secondary)		 0.1% [0.0%, 0.3%] 6
Improvements ✅
(primary)		 -0.1% [-0.4%, -0.0%] 20
Improvements ✅
(secondary)		 -0.3% [-0.6%, -0.0%] 8
All ❌✅ (primary) 0.1% [-0.4%, 1.3%] 31

Bootstrap: 465.817s -> 464.2s (-0.35%)
Artifact size: 372.34 MiB -> 372.35 MiB (0.00%)

@rustbot rustbot removed the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Jul 8, 2025
cjgillot
cjgillot reviewed Jul 8, 2025
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compiler/rustc_mir_transform/src/ssa.rs
// the class is borrowed.
// We can propagate from a borrowed local into other locals that aren't borrowed, since we know
// that all uses of the other locals are dominated by the definition. This bitset is keyed by
// *class head* and contains whether any member of the class is borrowed.
let mut borrowed_classes = ssa.borrowed_locals().clone();
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Do we still need this separate bitset?

compiler/rustc_mir_transform/src/ssa.rs
// We must not unify two locals that are borrowed. But this is fine if one is borrowed and
// the other is not. This bitset is keyed by *class head* and contains whether any member of
// the class is borrowed.
// We can propagate from a borrowed local into other locals that aren't borrowed, since we know
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Could you elaborate why we must not propagate if the lhs is borrowed?

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