[clang] [llvm] Add length HLSL function to DirectX Backend (PR #101256)

[Justin Bogner via cfe-commits]

================
@@ -157,6 +158,35 @@ static bool expandAnyIntrinsic(CallInst *Orig) {
   return true;
 }
 
+static bool expandLengthIntrinsic(CallInst *Orig) {
+  Value *X = Orig->getOperand(0);
+  IRBuilder<> Builder(Orig->getParent());
+  Builder.SetInsertPoint(Orig);
+  Type *Ty = X->getType();
+  Type *EltTy = Ty->getScalarType();
+
+  // Though dx.length does work on scalar type, we can optimize it to just emit
+  // fabs, in CGBuiltin.cpp. We shouldn't see a scalar type here because
+  // CGBuiltin.cpp should have emitted a fabs call.
+  Value *Elt = Builder.CreateExtractElement(X, (uint64_t)0);
+  auto *XVec = dyn_cast<FixedVectorType>(Ty);
+  unsigned size = XVec->getNumElements();
+  assert(Ty->isVectorTy() && size > 1 && "dx.length only works on vector type");
+
+  Value *Sum = Builder.CreateFMul(Elt, Elt);
+  for (unsigned i = 1; i < size; i++) {
+    Elt = Builder.CreateExtractElement(X, i);
----------------
bogner wrote:

Style nit, "i" should be capitalized
```suggestion
  for (unsigned I = 1; I < size; I++) {
    Elt = Builder.CreateExtractElement(X, I);
```

https://github.com/llvm/llvm-project/pull/101256