Check that fast math intrinsic results are finite - #4730
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Previously, Kani only verified that the inputs to fast math intrinsics (fadd_fast, fsub_fast, fmul_fast, fdiv_fast) were finite. However, per the Rust documentation, producing a non-finite result (infinity or NaN) from these intrinsics is also undefined behavior, even when both inputs are finite (e.g., f32::MAX + f32::MAX overflows to infinity). This commit adds a result finiteness check to all four fast math intrinsics. After the operation is performed and assigned to the destination, the result is asserted (and assumed) to be finite. Regression tests are added covering: - All four fast math intrinsics with finite inputs that overflow to infinity (kani-verify-fail). - Pointer wrapping_add with extreme offsets confirming these are correctly NOT flagged as UB under default settings. Signed-off-by: Felipe Monteiro <felisous@amazon.com>
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Copilot reviewed 3 out of 3 changed files in this pull request and generated no new comments.
Suppressed comments (3)
tests/kani/Intrinsics/FastMath/fast_math_result_overflow.rs:32
- This is overflow, not underflow: the magnitude grows beyond
f32::MAX. Underflow refers to results whose magnitude is too small to represent normally.
/// Check that fsub_fast detects underflow to negative infinity as UB.
/// (-f32::MAX) - f32::MAX underflows to negative infinity.
tests/kani/Intrinsics/FastMath/fast_math_result_overflow.rs:10
- This file-level
kani-verify-failonly checks that the overall Kani process exits unsuccessfully. Since one failing harness makes that true, the test still passes if result-finiteness checking regresses for any three of these four intrinsics. Please give each intrinsic its ownkani-verify-failtest (as the existing FastMath overflow tests do), so every implementation path is independently required to fail.
This issue also appears on line 31 of the same file.
// kani-verify-fail
kani-compiler/src/codegen_cprover_gotoc/codegen/intrinsic.rs:704
- Now that this helper also validates the destination result,
add_finite_args_checksno longer describes its behavior and suggests it only enforces argument preconditions. Rename it and its call sites to something that covers both checks, such asadd_fast_math_finiteness_checks.
place: &Place,
- Split the combined fast_math_result_overflow.rs into one file per intrinsic (add/mul/sub/div), so each intrinsic's result-finiteness check is independently required to fail. A single kani-verify-fail over four harnesses passed as long as any one harness failed, masking a regression in the other three. - Fix the fsub_fast test wording: the result grows in magnitude beyond f32::MAX, so it overflows to negative infinity (not underflow). - Rename add_finite_args_checks to add_fast_math_finiteness_checks to reflect that it now validates the result as well as the arguments. Signed-off-by: Felipe Monteiro <felisous@amazon.com>
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Previously, Kani only verified that the inputs to fast math intrinsics (
fadd_fast,fsub_fast,fmul_fast,fdiv_fast) were finite. However, per the Rust documentation, producing a non-finite result (infinity or NaN) from these intrinsics is also undefined behavior, even when both inputs are finite (e.g.,f32::MAX+f32::MAXoverflows to infinity).This commit adds a result finiteness check to all four fast math intrinsics. After the operation is performed and assigned to the destination, the result is asserted (and assumed) to be finite.
Regression tests are added covering:
kani-verify-fail).wrapping_addwith extreme offsets confirming these are correctly NOT flagged as UB under default settings.By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 and MIT licenses.