[clang][bytecode] Implement HLSLVectorTruncation casts (#108499)

Author: tbaederr

clang/lib/AST/ByteCode/Compiler.cpp

@@ -644,6 +644,31 @@ bool Compiler<Emitter>::VisitCastExpr(const CastExpr *CE) {
     return true;
   }
 
+  case CK_HLSLVectorTruncation: {
+    assert(SubExpr->getType()->isVectorType());
+    if (std::optional<PrimType> ResultT = classify(CE)) {
+      assert(!DiscardResult);
+      // Result must be either a float or integer. Take the first element.
+      if (!this->visit(SubExpr))
+        return false;
+      return this->emitArrayElemPop(*ResultT, 0, CE);
+    }
+    // Otherwise, this truncates from one vector type to another.
+    assert(CE->getType()->isVectorType());
+
+    if (!Initializing) {
+      unsigned LocalIndex = allocateTemporary(CE);
+      if (!this->emitGetPtrLocal(LocalIndex, CE))
+        return false;
+    }
+    unsigned ToSize = CE->getType()->getAs<VectorType>()->getNumElements();
+    assert(SubExpr->getType()->getAs<VectorType>()->getNumElements() > ToSize);
+    if (!this->visit(SubExpr))
+      return false;
+    return this->emitCopyArray(classifyVectorElementType(CE->getType()), 0, 0,
+                               ToSize, CE);
+  };
+
   case CK_ToVoid:
     return discard(SubExpr);
 

clang/test/AST/ByteCode/hlsl.hlsl

@@ -1,8 +1,8 @@
-// RUN: %clang_cc1 -std=hlsl2021 -finclude-default-header -x hlsl -triple \
+// RUN: %clang_cc1 -std=hlsl2021 -finclude-default-header -x hlsl -std=hlsl202x -triple \
 // RUN:   dxil-pc-shadermodel6.3-library %s -emit-llvm -disable-llvm-passes \
 // RUN:   -o - | FileCheck %s
 
-// RUN: %clang_cc1 -std=hlsl2021 -finclude-default-header -x hlsl -triple \
+// RUN: %clang_cc1 -std=hlsl2021 -finclude-default-header -x hlsl -std=hlsl202x -triple \
 // RUN:   dxil-pc-shadermodel6.3-library %s -emit-llvm -disable-llvm-passes \
 // RUN:   -o - -fexperimental-new-constant-interpreter | FileCheck %s
 
@@ -16,4 +16,16 @@ int2 ToTwoInts(int V){
   return V.xx;
 }
 
+export void fn() {
+  // This compiling successfully verifies that the vector constant expression
+  // gets truncated to an integer at compile time for instantiation.
+  _Static_assert(((int)1.xxxx) + 0 == 1, "Woo!");
 
+  // This compiling successfully verifies that the vector constant expression
+  // gets truncated to a float at compile time for instantiation.
+  _Static_assert(((float)1.0.xxxx) + 0.0 == 1.0, "Woo!");
+
+  // This compiling successfully verifies that a vector can be truncated to a
+  // smaller vector, then truncated to a float as a constant expression.
+  _Static_assert(((float2)float4(6, 5, 4, 3)).x == 6, "Woo!");
+}