[Bug target/57293] New: [4.8/4.9 Regression] not needed frame pointers on IA-32 (performance regression?)

[fanael4 at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57293

            Bug ID: 57293
           Summary: [4.8/4.9 Regression] not needed frame pointers on
                    IA-32 (performance regression?)
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: fanael4 at gmail dot com

About a week ago GCC started to emit frame pointers even in presence of
-fomit-frame-pointer whenever there's a call to a function with a "callee
cleans up the arguments" calling convention. Normally it wouldn't be a big
deal, but IA-32 is seriously register-starved, and since one of the secondary
platforms (i.e. Windows) uses __thiscall__ for C++ member functions by default,
for some code the availability of ebp can make a noticeable difference. I can
provide a benchmark if one is needed.

The culprit seems to be revisions 198140 and 198555.

Testcase:

/* compile with -O2 -fomit-frame-pointer */
__attribute__((__noinline__, __noclone__, __stdcall__)) void g(int a)
{
  __builtin_printf("in g(): %d\n", a);
}

__attribute__((__noinline__, __noclone__, __thiscall__)) void h(int a, int b)
{
  __builtin_printf("in h(): %d %d\n", a, b);
}

void f()
{
  g(0);
  h(0, 1);
  __builtin_puts("in f()");
}

What GCC 4.7.0 and 4.8.1 20130430 produce for f :

_f:
LFB2:
        .cfi_startproc
        subl    $28, %esp
        .cfi_def_cfa_offset 32
        movl    $0, (%esp)
        call    _g@4
        .cfi_def_cfa_offset 28
        xorl    %ecx, %ecx
        subl    $4, %esp
        .cfi_def_cfa_offset 32
        movl    $1, (%esp)
        call    _h
        .cfi_def_cfa_offset 28
        subl    $4, %esp
        .cfi_def_cfa_offset 32
        movl    $LC2, (%esp)
        call    _puts
        addl    $28, %esp
        .cfi_def_cfa_offset 4
        ret
        .cfi_endproc

What 4.8.1 20130510 produces:

_f:
LFB2:
        .cfi_startproc
        pushl   %ebp
        .cfi_def_cfa_offset 8
        .cfi_offset 5, -8
        movl    %esp, %ebp
        .cfi_def_cfa_register 5
        subl    $24, %esp
        movl    $0, (%esp)
        call    _g@4
        xorl    %ecx, %ecx
        subl    $4, %esp
        movl    $1, (%esp)
        call    _h
        subl    $4, %esp
        movl    $LC2, (%esp)
        call    _puts
        leave
        .cfi_restore 5
        .cfi_def_cfa 4, 4
        ret
        .cfi_endproc

Target: i686-w64-mingw32
Configured with: ../../src/gcc-svn/configure --build=x86_64-unknown-linux-gnu
--host=i686-w64-mingw32 --target=i686-w64-mingw32 --disable-multilib
--disable-multiarch --disable-nls --enable-languages=c,c++,lto
--disable-win32-registry --enable-openmp --enable-libgomp
--enable-threads=posix --enable-plugins --enable-static --enable-shared
--disable-symvers --enable-fully-dynamic-string --disable-sjlj-exceptions
--disable-libstdcxx-pch --enable-libstdcxx-time --with-arch=i686
--enable-checking=release --disable-werror --with-gnu-as --with-gnu-ld
--disable-rpath --with-gmp=/mingw/i686-final/libs-out-dir
--with-mpfr=/mingw/i686-final/libs-out-dir
--with-mpc=/mingw/i686-final/libs-out-dir
--with-isl=/mingw/i686-final/libs-out-dir
--with-cloog=/mingw/i686-final/libs-out-dir
--with-libiconv-prefix=/mingw/i686-final/libs-out-dir --with-system-zlib
--prefix=/mingw/i686-final/out-dir
Thread model: posix
gcc version 4.8.1 20130510 (prerelease) (GCC)