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

[Timm Baeder via cfe-commits]

================
@@ -2817,6 +2817,82 @@ 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 &Arg1Type = Call->getArg(0)->getType();
+  const QualType &Arg2Type = Call->getArg(1)->getType();
+  const QualType &Arg3Type = Call->getArg(2)->getType();
+
+  // Non-vector integer types.
+  if (!Arg1Type->isVectorType()) {
+    assert(!Arg2Type->isVectorType());
+    assert(!Arg3Type->isVectorType());
+    const APSInt &Shift = popToAPSInt(
+        S.Stk, *S.getContext().classify(Call->getArg(2)->getType()));
+    const APSInt &Lo = popToAPSInt(
+        S.Stk, *S.getContext().classify(Call->getArg(1)->getType()));
+    const APSInt &Hi = popToAPSInt(
+        S.Stk, *S.getContext().classify(Call->getArg(0)->getType()));
+    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(Arg1Type->isVectorType() && Arg2Type->isVectorType() &&
+         Arg3Type->isVectorType());
+
+  const VectorType *VecT = Arg1Type->castAs<VectorType>();
+  const PrimType &ElemT = *S.getContext().classify(VecT->getElementType());
+  unsigned NumElems = VecT->getNumElements();
+
+  assert(VecT->getElementType() ==
+             Arg2Type->castAs<VectorType>()->getElementType() &&
+         VecT->getElementType() ==
+             Arg3Type->castAs<VectorType>()->getElementType());
----------------
tbaederr wrote:

Yes, I think the assertions still make sense in both cases.

I _think_ we should also use `ASTContext::hasSameUnqualifiedType()` in `Sema::BuiltinElementwiseTernaryMath()` like we do in `BuiltinVectorMath()`.

See also https://github.com/llvm/llvm-project/commit/4d650ef4b3d7a0f21f8681e73586d0319fc3953b, but that can be a follow-up commit (which will have to adjust the assertions as well).

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