4 tests from spec/testsuite/conversions.wast and spec/testsuite/simd_conversions.wast fail on ppc64le #8626

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1. rathann commented on Apr 20, 2026

   

   rathann

   on Apr 20, 2026

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   Looks like #8528 and #8472 are related to this.
2. kripken commented on Apr 20, 2026

   

   kripken

   on Apr 20, 2026

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   cc @stevenfontanella based on the linked issues
3. stevenfontanella

   self-assigned this

   on Apr 21, 2026
4. stevenfontanella

   added a parent issue on Apr 21, 2026

   • Improve interpreter spec test coverage #8261
5. stevenfontanella commented on Apr 23, 2026

   

   stevenfontanella

   on Apr 23, 2026

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   Sent #8645 for this. Copying over my explanation:

   The bug is due to the fact that C++ doesn't guarantee a bitwise representation for NaNs that are casted from double -> float or vice versa. From https://en.cppreference.com/cpp/language/implicit_conversion:

   Floating-point promotion
   A prvalue of type float can be converted to a prvalue of type double. The value does not change.

   In the case of NaNs, the value (NaN) is preserved regardless of the particular NaN that is picked, so we have no guarantee of the bitwise representation. And for double -> float:

   If the conversion is listed under floating-point promotions, it is a promotion and not a conversion.

   • If the source value can be represented exactly in the destination type, it does not change.
   • If the source value is between two representable values of the destination type, the result is one of those two values (it is implementation-defined which one, although if IEEE arithmetic is supported, rounding defaults to nearest).
   • Otherwise, the behavior is undefined.

   I assume NaN conversions again fall under case 1 meaning that the "value" (NaN) is preserved but not the bitwise representation. Or if not, it must be case 3 in which case converting a double NaN to a float NaN is UB (!).
6. stevenfontanella

   closed this as completedin #8645on Apr 27, 2026
7. walkingeyerobot

   added a commit that references this issue on Apr 27, 2026

   Canonicalize NaNs when casting between float and double (WebAssembly#…

   ...

   This issue will close once commit a7fd052 is merged into the 'main' branch.

   a7fd052