[clang] [clang][bytecode][HLSL] Complete the HLSL aggregate splat and elementwise cast implementations, and enable the new constant interpreter on all HLSL tests with static asserts (PR #189126)

[Finn Plummer via cfe-commits]

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@@ -423,6 +423,29 @@ class Compiler : public ConstStmtVisitor<Compiler<Emitter>, bool>,
                              const QualType DerivedType);
   bool emitLambdaStaticInvokerBody(const CXXMethodDecl *MD);
   bool emitBuiltinBitCast(const CastExpr *E);
+
+  bool emitHLSLAggregateSplat(PrimType SrcT, unsigned SrcOffset,
+                              QualType DestType, const Expr *E);
+
+  /// A scalar element extracted during HLSL aggregate flattening.
+  struct HLSLFlatElement {
+    unsigned LocalOffset;
+    PrimType Type;
+  };
+  unsigned countHLSLFlatElements(QualType Ty);
+  bool emitHLSLFlattenAggregate(QualType SrcType, unsigned SrcPtrOffset,
+                                SmallVectorImpl<HLSLFlatElement> &Elements,
+                                unsigned MaxElements, const Expr *E);
+  bool emitHLSLConstructAggregate(QualType DestType,
+                                  ArrayRef<HLSLFlatElement> Elements,
+                                  unsigned &ElemIdx, const Expr *E);
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inbelic wrote:

nit: I think the use of a worklist over a recursive approach might avoid the need for this `unsigned &ElemIdx`, which I am not a big fan of. But I won't block on it.

https://github.com/llvm/llvm-project/pull/189126