[clang] [ARM, AArch64] Fix passing of structures with aligned base classes (PR #135564)

[Eli Friedman via cfe-commits]

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@@ -1302,6 +1302,7 @@ ItaniumRecordLayoutBuilder::LayoutBase(const BaseSubobjectInfo *Base) {
     setSize(std::max(getSize(), Offset + Layout.getSize()));
 
   // Remember max struct/class alignment.
+  UnadjustedAlignment = std::max(UnadjustedAlignment, PreferredBaseAlign);
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efriedma-quic wrote:

If you pass arguments on the stack (use up all the registers), the alignment matters even on Apple... for non-HFA types, at least.  For HFAs, apparently it doesn't matter.  And MSVC has its own funny take on the rules for AArch64: it will pass a misaligned NEON vector on the stack for HVAs containing NEON vectors.

It would be nice to have testcases for all the combinations of Windows/OSX/Linux, register/stack, integer/HFA/HVA, and outer/member alignment... but I guess it doesn't need to be in this patch.  So I'm okay with this going in as-is.

https://github.com/llvm/llvm-project/pull/135564