[LLVMdev] [cfe-dev] Handling SRet on Windows x86

[Timur Iskhodzhanov]

2013/3/20 Timur Iskhodzhanov <timurrrr at google.com>:
> Anton,
>
> [+Eric, Nick,
> the e-mail thread context has been broken a few times, so you should
> probably look at the llvmdev archives.
> It all starts here:
> http://lists.cs.uiuc.edu/pipermail/llvmdev/2012-October/053961.html ]
>
> So I've decided to make a new attempt to fix this issue.
>
> We've discussed my patches with Eric and came up with the following idea:
> a) IsTargetWindows should be true for all the targets that execute on
> the Windows kernel, including Cygwin, MinGW and MSVC ABIs.
>
> However, "Windows 32-bit ABI" usually implies "MSVC 32-bit ABI", so
> it's Cygwin and MinGW (that are similar more similar to Itanium ABI
> than to MSVC ABI) that should be exceptions from the general Win32
> handling code.
>
> That is,
> b) We should add a IsTargetWin32 which is true if the MSVC 32-bit ABI
> is used and false for Cygwin+MinGW.
FTR, there's already such a method but it is (mis?)used only in
lib/Target/X86/X86FrameLowering.cpp for segmented stacks.
Are segmented stacks actually used with MSVC or is it just the
conditions there should use isTargetMingw() && !is64Bit() instead?

> c) We should add CC_X86_Win32_C and CC_X86_Win32_ThisCall and use them
> only for MSVC 32-bit ABI.
> d) Cygwin and MinGW should use the CC_X86_32_C
>
> This way, Clang takes care of setting the SRet attribute wherever appropriate
> and LLVM takes care of putting such return values onto stack in the
> Win32/MSVC32 ABI.
>
> What do you think about such a proposal?
>
> One more data point: the ABI is wrong even in C!
> http://llvm.org/bugs/show_bug.cgi?id=15556
>
> --
> Timur