-
Notifications
You must be signed in to change notification settings - Fork 13.4k
Convert moves of references to copies in ReferencePropagation #142185
New issue
Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.
By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.
Already on GitHub? Sign in to your account
base: master
Are you sure you want to change the base?
Conversation
@bors try @rust-timer queue |
This comment has been minimized.
This comment has been minimized.
Convert moves of references to copies in ReferencePropagation This is a fix for #141101. The root cause of this miscompile is that the SsaLocals analysis that MIR transforms use is supposed to detect locals that are only written to once, in their single assignment. But that analysis is subtly wrong; it does not consider `Operand::Move` to be a write even though the meaning ascribed to `Operand::Move` (at least as a function parameter) by Miri is that the callee may have done arbitrary writes to the operand in the caller (because `Move` is pass-by-pointer). So Miri conwiders `Operand::Move` to be a write but both the MIR visitor system considers it a read, and so does SsaLocals. I have tried fixing this by changing the `PlaceContext` that is ascribed to an `Operand::Move` to a `MutatingUseContext` but that seems to have borrow checker implications, and changing SsaLocals seems to have wide-ranging regressions in MIR optimizations. So instead of doing those, this PR adds a new kludge to ReferencePropagation, which follows the same line of thinking as the kludge in CopyProp that solves this same problem inside that pass: https://github.com/rust-lang/rust/blob/a5584a8fe16037dc01782064fa41424a6dbe9987/compiler/rustc_mir_transform/src/copy_prop.rs#L65-L98
☀️ Try build successful - checks-actions |
This comment has been minimized.
This comment has been minimized.
Finished benchmarking commit (920f4b7): comparison URL. Overall result: ❌ regressions - no action neededBenchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. While you can manually mark this PR as fit for rollup, we strongly recommend not doing so since this PR may lead to changes in compiler perf. @bors rollup=never Instruction countThis is the most reliable metric that we have; it was used to determine the overall result at the top of this comment. However, even this metric can sometimes exhibit noise.
Max RSS (memory usage)Results (primary 1.1%, secondary 9.7%)This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.
CyclesResults (secondary 0.7%)This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.
Binary sizeThis benchmark run did not return any relevant results for this metric. Bootstrap: 752.018s -> 753.039s (0.14%) |
This is a fix for #141101.
The root cause of this miscompile is that the SsaLocals analysis that MIR transforms use is supposed to detect locals that are only written to once, in their single assignment. But that analysis is subtly wrong; it does not consider
Operand::Move
to be a write even though the meaning ascribed toOperand::Move
(at least as a function parameter) by Miri is that the callee may have done arbitrary writes to the caller's Local that the Operand wraps (becauseMove
is pass-by-pointer). So Miri conwidersOperand::Move
to be a write but both the MIR visitor system considers it a read, and so does SsaLocals.I have tried fixing this by changing the
PlaceContext
that is ascribed to anOperand::Move
to aMutatingUseContext
but that seems to have borrow checker implications, and changing SsaLocals seems to have wide-ranging regressions in MIR optimizations.So instead of doing those, this PR adds a new kludge to ReferencePropagation, which follows the same line of thinking as the kludge in CopyProp that solves this same problem inside that pass:
rust/compiler/rustc_mir_transform/src/copy_prop.rs
Lines 65 to 98 in a5584a8