[clang] [Clang] VectorExprEvaluator::VisitCallExpr / InterpretBuiltin - Allow shufps/pd shuffles intrinsics to be used in constexpr (PR #164078)

[Timm Baeder via cfe-commits]

================
@@ -3128,6 +3128,52 @@ static bool interp__builtin_ia32_vpconflict(InterpState &S, CodePtr OpPC,
   return true;
 }
 
+static bool interp__builtin_ia32_shuf(InterpState &S, CodePtr OpPC,
+                                      const CallExpr *Call) {
+  assert(Call->getNumArgs() == 3);
+
+  unsigned ShuffleMask = popToAPSInt(S, Call->getArg(2)).getZExtValue();
+  QualType Arg0Type = Call->getArg(0)->getType();
+  const auto *VecT = Arg0Type->castAs<VectorType>();
+  PrimType ElemT = *S.getContext().classify(VecT->getElementType());
+  unsigned NumElems = VecT->getNumElements();
+  unsigned LaneWidth = S.getContext().getBitWidth(VecT->getElementType());
+  unsigned NumLanes = LaneWidth * NumElems / 128;
+  unsigned NumElemPerLane = 128 / LaneWidth;
+
+  const Pointer &B = S.Stk.pop<Pointer>();
+  const Pointer &A = S.Stk.pop<Pointer>();
+  const Pointer &Dst = S.Stk.peek<Pointer>();
+
+  unsigned NumSelectableElems = NumElemPerLane / 2;
+  unsigned BitsPerElem = NumSelectableElems == 1 ? 1 : 2;
+  unsigned IndexMask = BitsPerElem == 2 ? 0x3 : 0x1;
+  unsigned MaskBits = 8;
+
+  TYPE_SWITCH(ElemT, {
+    unsigned BitIndex = 0;
+    unsigned DstIdx = 0;
+
+    for (unsigned LaneId = 0; LaneId != NumLanes; ++LaneId) {
+      unsigned LaneOffset = LaneId * NumElemPerLane;
+
+      for (unsigned i = 0; i < NumSelectableElems; ++i) {
----------------
tbaederr wrote:

```suggestion
      for (unsigned I = 0; i != NumSelectableElems; ++i) {
```

https://github.com/llvm/llvm-project/pull/164078