[PATCH] D115441: [X86][MS] Add 80bit long double support for Windows

[Phoebe Wang via Phabricator via cfe-commits]

pengfei added a comment.

> I have a thought. Why do you need to change the LLVM data layout at all? Clang's record layout is distinct from LLVM's data layout. This is similar to how -malign-double works, which does not affect LLVM's data layout, it is entirely a frontend change.

We have to change LLVM data layout because it's required by the calling conversion.

1. We specified the alignment of `f80` on 32 bits to 0: https://github.com/llvm/llvm-project/blob/main/llvm/lib/Target/X86/X86CallingConv.td#L869
2. Which means the alignment is actually determined by the data layout: https://github.com/llvm/llvm-project/blob/main/llvm/utils/TableGen/CallingConvEmitter.cpp#L200

As far as I understand it, `-malign-double` only affects the alignment of local and global variables. It has nothing to do with calling conversion.


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  https://reviews.llvm.org/D115441/new/

https://reviews.llvm.org/D115441