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

[Erich Keane via cfe-commits]

================
@@ -3083,6 +3083,19 @@ ScalarExprEmitter::VisitUnaryExprOrTypeTraitExpr(
                 E->getTypeOfArgument()->getPointeeType()))
             .getQuantity();
     return llvm::ConstantInt::get(CGF.SizeTy, Alignment);
+  } else if (E->getKind() == UETT_VectorElements) {
+    // For scalable vectors, we don't know the size at compile time. We can use
+    // @llvm.vscale to calculate it at runtime.
+    if (E->getTypeOfArgument()->isSizelessVectorType()) {
+      auto *VecTy = dyn_cast<llvm::ScalableVectorType>(
+          ConvertType(E->getTypeOfArgument()));
+      uint64_t NumUnscaledElements = VecTy->getMinNumElements();
----------------
erichkeane wrote:

I'm not particularly familiar with the sizeless vector type here, I would have expected the argument here to be an unknown value/expression, right?  Or is VScale doing something I'm unfamiliar with?  But it too doesn't seem to take the vector type as an argument.

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