[Bug target/82041] New: Windows i686 should not return float aggregates in ST0

["jistone at redhat dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82041

            Bug ID: 82041
           Summary: Windows i686 should not return float aggregates in ST0
           Product: gcc
           Version: 7.2.0
            Status: UNCONFIRMED
          Keywords: ABI, wrong-code
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jistone at redhat dot com
  Target Milestone: ---
            Target: i686-w64-mingw32

This is nearly a clone of bug 82028, but it's less clear to me if it's actually
a real problem.  It may just be that gnu cdecl returns values differently than
msvc cdecl, especially since I see that clang alters behavior for the two
targets.

Given this input foo.c:

#include <math.h>
typedef struct { double x; } Foo;
Foo foo(Foo f) {
        f.x = fabs(f.x);
        return f;
}

mingw-gcc produces code that returns in ST0:

00000000 <_foo>:
   0:   55                      push   %ebp
   1:   89 e5                   mov    %esp,%ebp
   3:   83 ec 08                sub    $0x8,%esp
   6:   8b 45 08                mov    0x8(%ebp),%eax
   9:   89 45 f8                mov    %eax,-0x8(%ebp)
   c:   8b 45 0c                mov    0xc(%ebp),%eax
   f:   89 45 fc                mov    %eax,-0x4(%ebp)
  12:   dd 45 f8                fldl   -0x8(%ebp)
  15:   d9 e1                   fabs
  17:   dd 5d f8                fstpl  -0x8(%ebp)
  1a:   dd 45 f8                fldl   -0x8(%ebp)
  1d:   c9                      leave
  1e:   c3                      ret

MSVC returns in EDX:EAX:

00000000 <_foo>:
   0:   55                      push   %ebp
   1:   8b ec                   mov    %esp,%ebp
   3:   83 ec 08                sub    $0x8,%esp
   6:   f2 0f 10 45 08          movsd  0x8(%ebp),%xmm0
   b:   f2 0f 11 04 24          movsd  %xmm0,(%esp)
  10:   e8 00 00 00 00          call   15 <_foo+0x15>
  15:   83 c4 08                add    $0x8,%esp
  18:   dd 5d 08                fstpl  0x8(%ebp)
  1b:   8b 45 08                mov    0x8(%ebp),%eax
  1e:   8b 55 0c                mov    0xc(%ebp),%edx
  21:   5d                      pop    %ebp
  22:   c3                      ret

Clang for i686-w64-windows-gnu returns in ST0 like GCC:

00000000 <_foo>:
   0:   55                      push   %ebp
   1:   89 e5                   mov    %esp,%ebp
   3:   83 e4 f8                and    $0xfffffff8,%esp
   6:   83 ec 18                sub    $0x18,%esp
   9:   f2 0f 10 45 08          movsd  0x8(%ebp),%xmm0
   e:   f2 0f 11 44 24 08       movsd  %xmm0,0x8(%esp)
  14:   f2 0f 10 44 24 08       movsd  0x8(%esp),%xmm0
  1a:   0f 28 0d 00 00 00 00    movaps 0x0,%xmm1
  21:   66 0f db c1             pand   %xmm1,%xmm0
  25:   66 0f 13 44 24 08       movlpd %xmm0,0x8(%esp)
  2b:   f2 0f 10 44 24 08       movsd  0x8(%esp),%xmm0
  31:   f2 0f 11 44 24 10       movsd  %xmm0,0x10(%esp)
  37:   f2 0f 10 44 24 10       movsd  0x10(%esp),%xmm0
  3d:   f2 0f 11 04 24          movsd  %xmm0,(%esp)
  42:   dd 04 24                fldl   (%esp)
  45:   89 ec                   mov    %ebp,%esp
  47:   5d                      pop    %ebp
  48:   c3                      ret

Clang for i686-w64-windows-msvc returns in EDX:EAX like MSVC:

00000000 <_foo>:
   0:   55                      push   %ebp
   1:   89 e5                   mov    %esp,%ebp
   3:   83 e4 f8                and    $0xfffffff8,%esp
   6:   83 ec 10                sub    $0x10,%esp
   9:   f2 0f 10 45 08          movsd  0x8(%ebp),%xmm0
   e:   f2 0f 11 04 24          movsd  %xmm0,(%esp)
  13:   f2 0f 10 04 24          movsd  (%esp),%xmm0
  18:   0f 28 0d 00 00 00 00    movaps 0x0,%xmm1
  1f:   66 0f db c1             pand   %xmm1,%xmm0
  23:   66 0f 13 04 24          movlpd %xmm0,(%esp)
  28:   f2 0f 10 04 24          movsd  (%esp),%xmm0
  2d:   f2 0f 11 44 24 08       movsd  %xmm0,0x8(%esp)
  33:   8b 44 24 08             mov    0x8(%esp),%eax
  37:   8b 54 24 0c             mov    0xc(%esp),%edx
  3b:   89 ec                   mov    %ebp,%esp
  3d:   5d                      pop    %ebp
  3e:   c3                      ret


$ gcc -v
Using built-in specs.
COLLECT_GCC=C:\msys64\mingw32\bin\gcc.exe
COLLECT_LTO_WRAPPER=C:/msys64/mingw32/bin/../lib/gcc/i686-w64-mingw32/7.2.0/lto-wrapper.exe
Target: i686-w64-mingw32
Configured with: ../gcc-7.2.0/configure --prefix=/mingw32
--with-local-prefix=/mingw32/local --build=i686-w64-mingw32
--host=i686-w64-mingw32 --target=i686-w64-mingw32
--with-native-system-header-dir=/mingw32/i686-w64-mingw32/include
--libexecdir=/mingw32/lib --enable-bootstrap --with-arch=i686
--with-tune=generic --enable-languages=c,lto,c++,objc,obj-c++,fortran,ada
--enable-shared --enable-static --enable-libatomic --enable-threads=posix
--enable-graphite --enable-fully-dynamic-string --enable-libstdcxx-time=yes
--disable-libstdcxx-pch --disable-libstdcxx-debug --disable-isl-version-check
--enable-lto --enable-libgomp --disable-multilib --enable-checking=release
--disable-rpath --disable-win32-registry --disable-nls --disable-werror
--disable-symvers --with-libiconv --with-system-zlib --with-gmp=/mingw32
--with-mpfr=/mingw32 --with-mpc=/mingw32 --with-isl=/mingw32
--with-pkgversion='Rev1, Built by MSYS2 project'
--with-bugurl=https://sourceforge.net/projects/msys2 --with-gnu-as
--with-gnu-ld --disable-sjlj-exceptions --with-dwarf2
Thread model: posix
gcc version 7.2.0 (Rev1, Built by MSYS2 project)

$ clang -v
clang version 4.0.0 (tags/RELEASE_400/final)
Target: i686-w64-windows-gnu
Thread model: posix
InstalledDir: C:\msys64\mingw32\bin

>cl
Microsoft (R) C/C++ Optimizing Compiler Version 19.00.24210 for x86
Copyright (C) Microsoft Corporation.  All rights reserved.