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

["lc at luiscoloradosistemas dot com" <gcc-bugzilla@gcc.gnu.org>]

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
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