llvm dev - Oct 2009 - [LLVMdev] MinGW/MSVC++ uses different ABI for sret

Hello Eli.

Eli Friedman <eli.friedman at gmail.com> writes:
> On Fri, Sep 25, 2009 at 2:41 PM, Óscar Fuentes <ofv at wanadoo.es>
wrote:
>> I filed a bug yesterday ( http://llvm.org/bugs/show_bug.cgi?id=5046 )
>> and Anton kindly explained that LLVM is doing the right thing as per
the
>> ABI (the GCC ABI, I'll add).
>>
>>  1. Is there a LLVM way of dealing with this without using separate
code
>>  for VC++ and GCC?
>
> I'm not sure what you mean... LLVM can distinguish between MinGW and
> MSVC targets.   If we want to, it shouldn't be too hard to make
> X86TargetLowering::LowerFormalArguments and
> X86TargetLowering::LowerCall account for the difference.
Yes, automatically switching to the MSVC ABI when the target triple is
*-pc-win32 seems the Right Thing.
>>  3. Is a bug that LLVM does not distinguish among GCC and VC++ sret
>>  handling?
>
> Probably... see http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36834 .
Great! Googling two hours and didn't found that report nor the
documents linked from it :-/

I'll file a LLVM bug report about this issue.

BTW, it's even worse, as aggregates passed by value are, well... passed
by value, contrary to the 386 unix ABI which uses pointers. I'm afraid
that this has no so easy solution as the sret issue. Is there any LLVM
target where aggregates are "really" passed by value?

Thanks Eli.

-- 
Óscar

Óscar Fuentes <ofv at wanadoo.es> writes:
> BTW, it's even worse, as aggregates passed by value are, well... passed
> by value, contrary to the 386 unix ABI which uses pointers. I'm afraid
> that this has no so easy solution as the sret issue. Is there any LLVM
> target where aggregates are "really" passed by value?
This is not entirely true. MSVC never uses pointers for passing a
class/struct by value; GCC uses a pointer if the class/struct is not a
POD.

This is listed on http://www.agner.org/optimize/calling_conventions.pdf
page 19.

-- 
Óscar

Hi,

is there any news about this problem?
How hart it will be to resolve this compatibility problem with Visual Studio
functions ?

This is really great problem for my project with should call may different
function compiled by Visual Studio!!!

Here is also problem that is looking similar.
http://llvm.org/bugs/show_bug.cgi?id=5046

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