[clang] [Clang] Add __builtin_vectorelements to get number of elements in vector (PR #69010)

[Erich Keane via cfe-commits]

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@@ -13595,6 +13595,15 @@ bool IntExprEvaluator::VisitUnaryExprOrTypeTraitExpr(
                     Info.Ctx.getOpenMPDefaultSimdAlign(E->getArgumentType()))
             .getQuantity(),
         E);
+  case UETT_VectorElements: {
+    QualType Ty = E->getTypeOfArgument();
+    // If the vector has a fixed size, we can determine the number of elements
+    // at compile time.
+    if (Ty->isVectorType())
+      return Success(Ty->castAs<VectorType>()->getNumElements(), E);
+
+    return false;
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erichkeane wrote:

Your understanding of how this works is correct, I missed you were handling sizeless vectors that way.  So above 'return false' there 
could be an `assert(Ty->isSizelessVectorType())` ?  

Note that we're not guaranteed to come to ExprConstant.cpp in the case where it is a sized-vector, so you likely need to make sure the CGExprScalar code handles sized types as well.

https://github.com/llvm/llvm-project/pull/69010