[Bug c++/52792] New: this pointer and return pointer are passed in wrong order when ms_abi is used (x86_64)

[Bug c++/52792] New: this pointer and return pointer are passed in wrong order when ms_abi is used (x86_64)

[piotr at codeweavers dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52792

             Bug #: 52792
           Summary: this pointer and return pointer are passed in wrong
                    order when ms_abi is used (x86_64)
    Classification: Unclassified
           Product: gcc
           Version: 4.7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: piotr@codeweavers.com


Created attachment 27045
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=27045
program that demonstrates the problem

When class function returns structure this pointer should be passed as first
argument. Pointer to memory used by returned value should be passed as second
argument. Currently gcc swaps these to parameters.

[Bug c++/52792] this pointer and return pointer are passed in wrong order when ms_abi is used (x86_64)

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52792

Paolo Carlini <paolo.carlini at oracle dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |cgf at gcc dot gnu.org,
                   |                            |ktietz at gcc dot gnu.org

--- Comment #1 from Paolo Carlini <paolo.carlini at oracle dot com> 2012-10-05 19:36:33 UTC ---
Not sure if the Christopher and Kai spend time also on the C++ front-end bits,
but CC-ing them anyway.

[Bug c++/52792] this pointer and return pointer are passed in wrong order when ms_abi is used (x86_64)

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52792

--- Comment #2 from Paolo Carlini <paolo.carlini at oracle dot com> 2012-12-05 12:52:00 UTC ---
Likewise, Kai, please.

[Bug c++/52792] this pointer and return pointer are passed in wrong order when ms_abi is used (x86_64)

[p_hampson at wargaming dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=52792

Paul "TBBle" Hampson <p_hampson at wargaming dot net> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |p_hampson at wargaming dot net

--- Comment #6 from Paul "TBBle" Hampson <p_hampson at wargaming dot net> ---
Since this came up on Stack Overflow, I thought I'd note the outcomes here.

https://stackoverflow.com/a/62880979/166389

The original report is accurate, as the ordering of "first argument hidden
this" and "first argument return-value pointer" differs in gcc compared to msvc
and clang.

It's ambiguous in the MSVC ABI documentation as the two behaviours are
specified in different document sets and both claim to become "the first
argument". I have raised an issue with Microsoft about this:
https://github.com/MicrosoftDocs/windows-driver-docs/issues/2586

There's also another related C++ issue (the actual SO question that triggered
my investigation) as gcc will return a user-defined type (struct) by value in
RAX for power-of-two-sized C++03 POD types, but the the MSVC ABI documentation
(and observed behaviour in msvc and clang) specify that
return-struct-in-register is only supported
> from global functions and static member functions.

https://docs.microsoft.com/en-us/cpp/build/x64-calling-convention#return-values

None of the examples here trigger this bug, as they all use sufficiently-large
aggregates that they would not be candidates for return-struct-in-register
anyway.

I was only looking at the x64 documentation, but minimal testing bears out the
same results for 32-bit MSVC C++ ABI.