Fix PR15556 - Make Win32 put the SRet address into EAX

[Timur Iskhodzhanov]

2013/3/28 Timur Iskhodzhanov <timurrrr at google.com>:
> [just realized I've forgotten to CC llvm-commits last time, so re-sending]
>
> Hi Eli, Nick,
>
> Please see llvm.org/PR15556 - it turns out we don't put the SRet
> address into EAX in the function prolog, though we should (similar to
> Win64).
> The attached patch fixes this for i686-pc-win32.
>
> Please note that for some reason if I don't do the
>   lib/Target/X86/X86ISelLowering.cpp:[1670-1690]
> part of the patch, the test still passes.
> I'd appreciate if you could suggest me a test case for those lines in win32.
> I feel like this part of the change should be there (based on
> comments), but something weird prevents me from checking that using
> llc | FileCheck.
>
> FTR, I ran
>   llc -mtriple=i686-pc-win32 -O0 < test/CodeGen/X86/x86-64-sret-return.ll
> and
>   llc -mtriple=i686-pc-win32 -O0 < test/CodeGen/X86/x86-64-ptr-arg-simple.ll
> with and without the [1670-1690] changes and the result is identical.
A-ha, I came up with a good test which does require the [1670-1690]
part of the change.
See an updated patch! (the test is the new sret3 function)

> Anton,
> Please tell me if you feel this should also be applied to i686-pc-mingw.
> (e.g., isTargetWindows() should be replaced with isTargetWin32() and
> the MinGW part of the test should be expanded).
>
> --
> Timur
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