[PATCH] D34474: [RFC] [AArch64] Add a win64 specific aarch64 calling convention, for va_list handling

[Martin Storsjö via Phabricator via llvm-commits]

mstorsjo added a comment.

In https://reviews.llvm.org/D34474#792899, @t.p.northover wrote:

> I think MSVC has to have changed the ABI from AAPCS if they've managed to get a "void *" va_list to work; the extra complexity in AAPCS isn't there for the giggles. x86 appears to mirror floating args into integer registers.


Indeed. Since I don't have access to the actual MSVC compiler for arm64, I can't easily verify what it does with float arguments to vararg functions (unless I can find a easily triggered printf case in any of the arm64 exes included in the win10 sdk), but the va_list is a plain void pointer at least.

> If code isn't expected to interoperate, this becomes analogous to the DarwinPCS calling convention (it exists within lib/Target/AArch64 and affects CodeGen but doesn't need to be exposed in IR). If the Wine use-case does involve interoperation (and is supported) then a separate IR-level calling convention is probably unavoidable.

Indeed. I can try to update the patch and clarify the comments that it is mostly-AAPCS compliant, but floats are passed in integer registers (which is a sensible hypothesis until someone can confirm or deny this).

Mandeep, do you have access to proper MSVC tools for arm64? Could you verify how floats are passed as function arguments (in float registers or in integer registers) for both normal and variadic functions?


https://reviews.llvm.org/D34474