[llvm] [clang] [clang-tools-extra] [HLSL] Vector standard conversions (PR #71098)

[John McCall via cfe-commits]

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@@ -2466,6 +2504,15 @@ Value *ScalarExprEmitter::VisitCastExpr(CastExpr *CE) {
   case CK_IntToOCLSampler:
     return CGF.CGM.createOpenCLIntToSamplerConversion(E, CGF);
 
+  case CK_HLSLVectorTruncation: {
+    assert(DestTy->isVectorType() && "Expected dest type to be vector type");
+    Value *Vec = Visit(const_cast<Expr *>(E));
+    SmallVector<int, 16> Mask;
+    Mask.insert(Mask.begin(), DestTy->castAs<VectorType>()->getNumElements(),
+                0);
+    return Builder.CreateShuffleVector(Vec, Mask, "trunc");
+  }
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rjmccall wrote:

Okay, *please* tell me this is wrong.  The result of truncating `<A,B,C,D>` to a 3-vector is `<A,A,A>`?  I'm not misunderstanding something fundamental about `shufflevector` here, right?

https://github.com/llvm/llvm-project/pull/71098