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

Joe Groff arcata at gmail.com
Thu Mar 28 08:59:42 PDT 2013

On Thu, Mar 28, 2013 at 8:55 AM, Reid Kleckner <rnk at google.com> wrote:

> On Thu, Mar 28, 2013 at 8:22 AM, Joe Groff <arcata at gmail.com> wrote:
>> On Wed, Mar 27, 2013 at 8:15 PM, Chandler Carruth <chandlerc at google.com>wrote:
>>> This is why I suggest that the Windows ABI is whatever MSVC happens to
>>> do. The mingw ABI happens to consist of a conjunction of what MSVC does for
>>> C and what GCC on Linux does for C++. That is a unique and different ABI,
>>> and I don't think you can reasonably call it *just* "windows" at any point.
>>> This isn't quite true. Mingw and MSVC's ABIs on i386 also differ at the
>> C level—for instance, alignment of pass-by-value structs sometimes differs,
>> and an sret pointer is callee-cleanup in MSVC but caller-cleanup in Mingw.
>> There are other differences too.
> Can you clarify what you mean by the "sret pointer is callee-cleanup" vs
> caller-cleanup?

In the epilog for a function with an indirect struct return argument, MSVC
emits a 'ret 4' to pop the return pointer argument. Mingw emits a 'ret'.

> What we were specifically hitting yesterday was that MSVC expects the sret
> pointer passed into also be returned in eax.  LLVM already does this for
> x86_64 everywhere.  Mingw doesn't do this, but I think it was just a
> coincidence.  This behavior was undocumented, and that there was probably
> no OS API takes a callback which returns a struct larger than 8 bytes.
> This makes me think that it would be better if we started returning the
> sret pointer from such functions under Mingw for improved compatibility at
> the C ABI level.  This shouldn't break any mingw code so far as I can tell.
>  It seems reasonable for a user to expect that the Mingw and Microsoft C
> ABIs be compatible, but maybe there are enough other corner cases that this
> is just naive.

Good catch. Returning the sret pointer in eax shouldn't affect Mingw
compatibility since EAX isn't preserved across calls in either the
Microsoft or Mingw ABIs (IIRC).

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