[clang] [llvm] [HLSL][DirectX] Implement HLSL `mul` function and DXIL lowering of `llvm.matrix.multiply` (PR #184882)

[Farzon Lotfi via cfe-commits]

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@@ -1054,6 +1055,68 @@ Value *CodeGenFunction::EmitHLSLBuiltinExpr(unsigned BuiltinID,
     Value *Mul = Builder.CreateNUWMul(M, A);
     return Builder.CreateNUWAdd(Mul, B);
   }
+  case Builtin::BI__builtin_hlsl_mul: {
+    Value *Op0 = EmitScalarExpr(E->getArg(0));
+    Value *Op1 = EmitScalarExpr(E->getArg(1));
+    QualType QTy0 = E->getArg(0)->getType();
+    QualType QTy1 = E->getArg(1)->getType();
+
+    bool IsVec0 = QTy0->isVectorType();
+    bool IsVec1 = QTy1->isVectorType();
+    bool IsMat0 = QTy0->isConstantMatrixType();
+    bool IsMat1 = QTy1->isConstantMatrixType();
+
+    if (IsVec0 && IsVec1) {
+      // Case 5: vector * vector -> scalar (dot product)
----------------
farzonl wrote:

But yeah like Deric said the more intrinsics we can write in hlsl the less we have to do to handle custom semantic checking in SemaHLSL and the less custom codgen we have to do.

https://github.com/llvm/llvm-project/pull/184882