[Bug c/63450] New: Optimizing -O3 generates rep ret on an almost empty function

[Bug c/63450] New: Optimizing -O3 generates rep ret on an almost empty function

[lc at luiscoloradosistemas dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63450

            Bug ID: 63450
           Summary: Optimizing -O3 generates rep ret on an almost empty
                    function
           Product: gcc
           Version: 4.9.1
            Status: UNCONFIRMED
          Severity: minor
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: lc at luiscoloradosistemas dot com

When I try to compile this piece of code:
$ cat pru.c
void f()
{
    int b=0;
}

using
gcc -O3 -S -o pru.S pru.c

I get this assembler output:

$ gcc -O3 -S -o - pru.c
    .file    "pru.c"
    .text
    .p2align 4,,15
.globl f
    .type    f, @function
f:
.LFB0:
    .cfi_startproc
    rep                      <<<<<<<<< string prefix just before a ret ???
    ret
    .cfi_endproc
.LFE0:
    .size    f, .-f
    .ident    "GCC: (GNU) 4.4.7 20120313 (Red Hat 4.4.7-3)"
    .section    .note.GNU-stack,"",@progbits


In some other cases I have got even the `rep' and the `ret' in the same line,
so the assembler complained about it (in release 2.20 of the assembler, not in
later 2.23.2) with this piece of code:

$ cat program.c
#include <stdio.h>

void printer(void)
{
    int a=0;
    printf("%d\n", a);
}

void setter(void)
{
    int b = 42;
}

int main()
{
    setter();
    printer();
    return 0;
}


arch: Linux ***** 2.6.32-358.23.2.el6.x86_64 #1 SMP Sat Sep 14 05:32:37 EDT
2013 x86_64 x86_64 x86_64 GNU/Linux
compiler version:
gcc-4.8 (GCC) 4.8.1
Copyright (C) 2013 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

[Bug c/63450] Optimizing -O3 generates rep ret on an almost empty function

[lc at luiscoloradosistemas dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63450

--- Comment #1 from Luis Colorado <lc at luiscoloradosistemas dot com> ---
when i compile a completely empty function i get the rep ret in the same line.

[Bug c/63450] Optimizing -O3 generates rep ret on an almost empty function

[andi-gcc at firstfloor dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63450

Andi Kleen <andi-gcc at firstfloor dot org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |andi-gcc at firstfloor dot org

--- Comment #2 from Andi Kleen <andi-gcc at firstfloor dot org> ---
This is a feature in -mtune=generic because it helps branch prediction in some
older AMD CPUs. If you're optimizing for Atom you'll get even more padding due
to other reasons. Optimizing e.g. for nehalem should avoid it.

[Bug c/63450] Optimizing -O3 generates rep ret on an almost empty function

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63450

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |INVALID

--- Comment #4 from Richard Biener <rguenth at gcc dot gnu.org> ---
feature.