[HLSL] Allow truncation to scalar (#104844)

HLSL allows implicit conversions to truncate vectors to scalar
pr-values. These conversions are scored as vector truncations and should
warn appropriately.

This change allows forming a truncation cast to a pr-value, but not an
l-value. Truncating a vector to a scalar is performed by loading the
first element of the vector and disregarding the remaining elements.

Fixes #102964

1 files changed, 11 insertions, 6 deletions

diff --git a/clang/lib/CodeGen/CGExprScalar.cpp b/clang/lib/CodeGen/CGExprScalar.cpp
index 9027bab6b680..82caf65ac68d 100644
--- a/clang/lib/CodeGen/CGExprScalar.cpp
+++ b/clang/lib/CodeGen/CGExprScalar.cpp
@@ -2709,14 +2709,19 @@ Value *ScalarExprEmitter::VisitCastExpr(CastExpr *CE) {
     return CGF.CGM.createOpenCLIntToSamplerConversion(E, CGF);
 
   case CK_HLSLVectorTruncation: {
-    assert(DestTy->isVectorType() && "Expected dest type to be vector type");
+    assert((DestTy->isVectorType() || DestTy->isBuiltinType()) &&
+           "Destination type must be a vector or builtin type.");
     Value *Vec = Visit(const_cast<Expr *>(E));
-    SmallVector<int, 16> Mask;
-    unsigned NumElts = DestTy->castAs<VectorType>()->getNumElements();
-    for (unsigned I = 0; I != NumElts; ++I)
-      Mask.push_back(I);
+    if (auto *VecTy = DestTy->getAs<VectorType>()) {
+      SmallVector<int> Mask;
+      unsigned NumElts = VecTy->getNumElements();
+      for (unsigned I = 0; I != NumElts; ++I)
+        Mask.push_back(I);
 
-    return Builder.CreateShuffleVector(Vec, Mask, "trunc");
+      return Builder.CreateShuffleVector(Vec, Mask, "trunc");
+    }
+    llvm::Value *Zero = llvm::Constant::getNullValue(CGF.SizeTy);
+    return Builder.CreateExtractElement(Vec, Zero, "cast.vtrunc");
   }
 
   } // end of switch