[clang] [Headers][X86] Allow MMX/SSE/AVX MOVMSK intrinsics to be used in constexpr (PR #161914)

[Shafik Yaghmour via cfe-commits]

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@@ -2817,6 +2817,46 @@ static bool interp__builtin_ia32_pshuf(InterpState &S, CodePtr OpPC,
   return true;
 }
 
+static bool interp__builtin_ia32_movmsk_op(InterpState &S, CodePtr OpPC,
+                                           const CallExpr *Call) {
+  assert(Call->getNumArgs() == 1);
+
+  const Pointer &Source = S.Stk.pop<Pointer>();
+
+  unsigned SourceLen = Source.getNumElems();
+  const QualType ElemQT = getElemType(Source);
+  const OptPrimType ElemPT = S.getContext().classify(ElemQT);
+  unsigned LaneWidth = S.getASTContext().getTypeSize(ElemQT);
+
+  if (ElemQT->isIntegerType()) {
+    unsigned Byte = 8;
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shafik wrote:

So isn't `8` just `getCharBit()`? CC @tbaederr 

Also this feels like it should called `BitsInAByte`?

https://github.com/llvm/llvm-project/pull/161914