[clang] [X86][ByteCode] Allow PSHUFB intrinsics to be used in constexpr #156612 (PR #163148)

[Simon Pilgrim via cfe-commits]

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@@ -2790,6 +2790,34 @@ static bool interp__builtin_blend(InterpState &S, CodePtr OpPC,
   return true;
 }
 
+static bool interp__builtin_ia32_pshufb(InterpState &S, CodePtr OpPC,
+                                        const CallExpr *Call) {
+  assert(Call->getNumArgs() == 2 && "masked forms handled via select*");
+  const Pointer &Control = S.Stk.pop<Pointer>();
+  const Pointer &Src = S.Stk.pop<Pointer>();
+  const Pointer &Dst = S.Stk.peek<Pointer>();
+
+  unsigned NumElems = Dst.getNumElems();
+  assert(NumElems == Control.getNumElems());
+  assert(NumElems == Dst.getNumElems());
+
+  for (unsigned Idx = 0; Idx != NumElems; ++Idx) {
+    uint8_t Ctlb = static_cast<uint8_t>(Control.elem<int8_t>(Idx));
+
+    if (Ctlb & 0x80) {
+      Dst.elem<int8_t>(Idx) = 0;
+    } else {
+      unsigned LaneBase = (Idx / 16) * 16;
----------------
RKSimon wrote:

I'm not sure if MSBMask/LowNibbleMask obfuscation is a great idea - keeping reasonably close to the psuedocode in the Intel Intrinsics Guide is probably a better guideline.

https://github.com/llvm/llvm-project/pull/163148