[clang] [X86][Clang] Support constexpr evaluation of cvtpd2ps intrinsics (PR #169980)

[Simon Pilgrim via cfe-commits]

================
@@ -207,9 +207,8 @@ _mm512_undefined(void)
   return (__m512)__builtin_ia32_undef512();
 }
 
-static __inline__ __m512 __DEFAULT_FN_ATTRS512
-_mm512_undefined_ps(void)
-{
+static __inline__ __m512 __DEFAULT_FN_ATTRS512_CONSTEXPR
+_mm512_undefined_ps(void) {
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RKSimon wrote:

No, the _mm*_undefined_ps intrinsics are there to indicate there are undefined elements and I'm very reluctant to allow them to canonicalize to a fixed constant value - I do not want to be in a position where we start getting a lot of reports about constexpr and runtime code giving different results. For now I think just focusing on the SSE/AVX builtins is the way to go - we have various other avx512 intrinsics that are still causing trouble so adding these to the pile isn't a problem.

But _mm512_cvtpd_ps can be addressed by replacing _mm256_undefined_ps with _mm256_setzero_ps which should help at least some cases.

https://github.com/llvm/llvm-project/pull/169980