[clang] [llvm] Clang: convert `__m64` intrinsics to unconditionally use SSE2 instead of MMX. (PR #96540)

[James Y Knight via cfe-commits]

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+USE_XMM=
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jyknight wrote:

These are the tests I used to validate that the re-implementation works properly: I found it extremely important to have validation that my implementation works properly, because i certainly messed up a few times.

I did upload it intentionally, but I don't propose to commit as-is -- especially not in this location.

I definitely would like to commit something along these lines _somewhere_, however, so that future maintainers have the benefit of being able to test this functionality. Probably it belongs in the llvm-test-suite repository?

I'd love help on getting this into a submittable form.

https://github.com/llvm/llvm-project/pull/96540