jyknight added inline comments.
================
Comment at: clang/lib/CodeGen/TargetInfo.cpp:4539
CharUnits CCAlign = getParamTypeAlignment(Ty);
CharUnits TyAlign = getContext().getTypeAlignInChars(Ty);
----------------
Xiangling_L wrote:
> jasonliu wrote:
> > Question:
> > It looks like getNaturalAlignIndirect and getTypeAlignInChars here are all
> > returning ABI alignment.
> > But according to the comments, we should use a preferred alignment when
> > it's a complete object. Isn't this complete object? Or I'm missing
> > something?
> @jyknight Could you shine a light on this? Personally, I would agree that we
> have complete objects here, so preferred alignment should be used. And if
> that is true, changes should be applied on all other target within this file?
This code specifies that aggregate parameters are passed on the stack aligned
to CCAlign (4 or 8), even when the aggregate _requires_ higher alignment. E.g.
`struct X { int a, b; } __attribute__((aligned(8)));` would only be passed on
the stack with 4-byte alignment for powerpc-aix. The callee needs the object to
have the correct 8-byte alignment, so it must copy the parameter to an 8-byte
aligned temporary object (this is what "Realign" asks for).
We don't want that overhead for optional (preferred) alignment. It wouldn't be
wrong, but would degrade performance and isn't needed. So, no, this shouldn't
use preferred alignment.
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D86790/new/
https://reviews.llvm.org/D86790
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