[clang] [Clang] Enable constexpr handling for builtin elementwise fshl/fshr (PR #153572)

[Timm Baeder via cfe-commits]

================
@@ -2817,6 +2817,81 @@ static bool interp__builtin_select(InterpState &S, CodePtr OpPC,
   return true;
 }
 
+static bool interp__builtin_elementwise_fsh(InterpState &S, CodePtr OpPC,
+                                            const CallExpr *Call,
+                                            unsigned BuiltinID) {
+  assert(Call->getNumArgs() == 3);
+
+  const QualType Arg0Type = Call->getArg(0)->getType();
+  const QualType Arg1Type = Call->getArg(1)->getType();
+  const QualType Arg2Type = Call->getArg(2)->getType();
+
+  // Non-vector integer types.
+  if (!Arg0Type->isVectorType()) {
+    assert(!Arg1Type->isVectorType());
+    assert(!Arg2Type->isVectorType());
+    const APSInt &Shift = popToAPSInt(
+        S.Stk, *S.getContext().classify(Arg2Type));
+    const APSInt &Lo = popToAPSInt(
+        S.Stk, *S.getContext().classify(Arg1Type));
+    const APSInt &Hi = popToAPSInt(
+        S.Stk, *S.getContext().classify(Arg0Type));
+    APSInt Result;
+    if (BuiltinID == Builtin::BI__builtin_elementwise_fshl)
+      Result = APSInt(llvm::APIntOps::fshl(Hi, Lo, Shift), Hi.isUnsigned());
+    else if (BuiltinID == Builtin::BI__builtin_elementwise_fshr)
+      Result = APSInt(llvm::APIntOps::fshr(Hi, Lo, Shift), Hi.isUnsigned());
+    else
+      llvm_unreachable("Wrong builtin ID");
+    pushInteger(S, Result, Call->getType());
+    return true;
+  }
+
+  // Vector type.
+  assert(Arg0Type->isVectorType() && Arg1Type->isVectorType() &&
+         Arg2Type->isVectorType());
+
+  const auto *VecT = Arg0Type->castAs<VectorType>();
+  const PrimType &ElemT = *S.getContext().classify(VecT->getElementType());
+  unsigned NumElems = VecT->getNumElements();
+
+  assert(VecT->getElementType() ==
+             Arg1Type->castAs<VectorType>()->getElementType() &&
+         VecT->getElementType() ==
+             Arg2Type->castAs<VectorType>()->getElementType());
+  assert(NumElems == Arg1Type->castAs<VectorType>()->getNumElements() &&
+         NumElems == Arg2Type->castAs<VectorType>()->getNumElements());
+  assert(VecT->getElementType()->isIntegralOrEnumerationType());
+
+  const Pointer &VecShift = S.Stk.pop<Pointer>();
+  const Pointer &VecLo = S.Stk.pop<Pointer>();
+  const Pointer &VecHi = S.Stk.pop<Pointer>();
+  const Pointer &Dst = S.Stk.peek<Pointer>();
+  for (unsigned I = 0; I != NumElems; ++I) {
+    APSInt Hi;
+    APSInt Lo;
+    APSInt Shift;
+    INT_TYPE_SWITCH_NO_BOOL(ElemT, {
+      Hi = VecHi.elem<T>(I).toAPSInt();
+      Lo = VecLo.elem<T>(I).toAPSInt();
+      Shift = VecShift.elem<T>(I).toAPSInt();
+    });
+    APSInt Result;
+    if (BuiltinID == Builtin::BI__builtin_elementwise_fshl) {
----------------
tbaederr wrote:

```suggestion
    if (BuiltinID == Builtin::BI__builtin_elementwise_fshl)
```

https://github.com/llvm/llvm-project/pull/153572