[clang] [HLSL] add support for HLSLAggregateSplatCast and HLSLElementwiseCast to constant expression evaluator (PR #164700)

[Joshua Batista via cfe-commits]

================
@@ -10851,6 +11194,67 @@ bool RecordExprEvaluator::VisitCastExpr(const CastExpr *E) {
     Result = *Value;
     return true;
   }
+  case CK_HLSLAggregateSplatCast: {
+    APValue Val;
+    const Expr *SE = E->getSubExpr();
+
+    if (!Evaluate(Val, Info, SE))
+      return Error(E);
+
+    unsigned NEls = elementwiseSize(Info, E->getType());
+    // flatten the source
+    SmallVector<APValue, 1> SrcEls;
+    SmallVector<QualType, 1> SrcTypes;
+    if (!flattenAPValue(Info.Ctx, Val, SE->getType(), SrcEls, SrcTypes, NEls))
+      return Error(E);
+
+    // check there is only one and splat it
+    assert(SrcEls.size() == 1);
+    SmallVector<APValue> SplatEls(NEls, SrcEls[0]);
+    SmallVector<QualType> SplatType(NEls, SrcTypes[0]);
+
+    APValue Tmp;
+    handleDefaultInitValue(E->getType(), Tmp);
+
+    // cast the elements and construct our struct result
+    const FPOptions FPO = E->getFPFeaturesInEffect(Info.Ctx.getLangOpts());
+    if (!constructAggregate(Info, FPO, E, Result, E->getType(), SplatEls,
+                            SplatType))
+      return Error(E);
+
+    return true;
+  }
+  case CK_HLSLElementwiseCast: {
+    APValue Val;
+    const Expr *SE = E->getSubExpr();
+
+    if (!Evaluate(Val, Info, SE))
+      return Error(E);
+
+    // must be dealing with a record;
+    if (Val.isLValue()) {
+      LValue LVal;
+      LVal.setFrom(Info.Ctx, Val);
+      if (!handleLValueToRValueConversion(Info, SE, SE->getType(), LVal, Val))
+        return false;
+    }
+
+    // flatten the source
+    SmallVector<APValue> SrcEls;
+    SmallVector<QualType> SrcTypes;
+    if (!flattenAPValue(Info.Ctx, Val, SE->getType(), SrcEls, SrcTypes,
----------------
bob80905 wrote:

Why not get NEls from elementwiseSize here again, instead of using UINT_MAX?

https://github.com/llvm/llvm-project/pull/164700