[clang] [HLSL] Implement HLSL Flat casting (excluding splat cases) (PR #118842)

[Chris B via cfe-commits]

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@@ -491,6 +491,81 @@ static bool isTrivialFiller(Expr *E) {
   return false;
 }
 
+// emit a flat cast where the RHS is a scalar, including vector
+static void EmitHLSLScalarFlatCast(CodeGenFunction &CGF, Address DestVal,
+                                   QualType DestTy, llvm::Value *SrcVal,
+                                   QualType SrcTy, SourceLocation Loc) {
+  // Flatten our destination
+  SmallVector<QualType, 16> DestTypes; // Flattened type
+  SmallVector<std::pair<Address, llvm::Value *>, 16> StoreGEPList;
+  // ^^ Flattened accesses to DestVal we want to store into
+  CGF.FlattenAccessAndType(DestVal, DestTy, StoreGEPList, DestTypes);
+
+  if (const VectorType *VT = SrcTy->getAs<VectorType>()) {
+    SrcTy = VT->getElementType();
+    assert(StoreGEPList.size() <= VT->getNumElements() &&
+           "Cannot perform HLSL flat cast when vector source \
+           object has less elements than flattened destination \
+           object.");
+    for (unsigned i = 0; i < StoreGEPList.size(); i++) {
+      llvm::Value *Load =
+          CGF.Builder.CreateExtractElement(SrcVal, i, "vec.load");
+      llvm::Value *Cast =
+          CGF.EmitScalarConversion(Load, SrcTy, DestTypes[i], Loc);
+
+      // store back
+      llvm::Value *Idx = StoreGEPList[i].second;
+      if (Idx) {
+        llvm::Value *V =
+            CGF.Builder.CreateLoad(StoreGEPList[i].first, "load.for.insert");
+        Cast = CGF.Builder.CreateInsertElement(V, Cast, Idx);
+      }
+      CGF.Builder.CreateStore(Cast, StoreGEPList[i].first);
+    }
+    return;
+  }
+  llvm_unreachable("HLSL Flat cast doesn't handle splatting.");
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llvm-beanz wrote:

If this function is only ever supposed to be called where the source type is a vector, I'd rewrite this to have an assert on that precondition rather than a branch with an unreachable.

https://github.com/llvm/llvm-project/pull/118842