[PATCH] D143287: [Clang][X86] Change X86 cast intrinsics to use __builtin_nondeterministic_value

[Manuel Brito via Phabricator via cfe-commits]

ManuelJBrito planned changes to this revision.
ManuelJBrito added a comment.

Implementing the 128 to 512 casts by filling the rest of the vector with the same definition of a nondeterministic_value is not correct because  :

a = freeze poison
v = <a, a>

is not the same as

v = freeze poison

The only solution I'm seeing ,using the shufflevector, is doing the conversion in two steps:

- build a 256 vector with the upper half being undefined( freeze poison)
- build a 512 vector where the lower half is the previous 256 vector and the upper half being undefined

I think this would require two shuffles which is unfortunate.

This would ensure no miscompilations due to multiple uses of the same freeze undef/poison but would probably require some backend work to ensure the pattern is recognized to emit efficient assembly.

Would this work ? @RKSimon @craig.topper


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  https://reviews.llvm.org/D143287/new/

https://reviews.llvm.org/D143287