Fix atomic compare_exchange to return correct failure status #4542
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Fix codegen_atomic_cxchg to properly return (current_value, false) when the expected value doesn't match the current value. Previously, the function always returned true as the success indicator, which was incorrect. Resolves model-checking#4537
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Pull request overview
This PR fixes the lowering of atomic compare-and-exchange intrinsics in the Kani Goto-C backend so that failed compare_exchange/compare_exchange_weak operations correctly report failure (i.e., return the old value with a false success flag), matching Rust’s semantics and resolving #4537.
Changes:
- Fix
codegen_atomic_cxchgto return(previous_value, success)instead of always returning success. - Add regression tests ensuring
compare_exchangeandcompare_exchange_weakreturnErr(old)when the expected value does not match.
Reviewed changes
Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.
| File | Description |
|---|---|
kani-compiler/src/codegen_cprover_gotoc/codegen/intrinsic.rs |
Corrects the emitted compare-and-exchange code to compute and return the real success flag. |
tests/kani/Intrinsics/Atomic/Stable/CompareExchange/compare_exchange_failure.rs |
Adds proof harnesses covering failure behavior for compare_exchange and compare_exchange_weak. |
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Fix codegen_atomic_cxchg to properly return (current_value, false) when the expected value doesn't match the current value. Previously, the function always returned true as the success indicator, which was incorrect.
Resolves #4537
By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 and MIT licenses.