[Bug inline-asm/58819] New: return value ignored using -O2

[gonwan at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58819

            Bug ID: 58819
           Summary: return value ignored using -O2
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: inline-asm
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gonwan at gmail dot com

# cat testasm.c
#include <stdio.h>

int add1(int a, int b)
{
    __asm__ __volatile__ ("movl 12(%ebp), %eax\n\t"
                          "addl 8(%ebp), %eax"
    );
}

int add2(int a, int b)
{
    __asm__ __volatile__ ("movl 12(%%ebp), %%eax\n\t"
                          "addl 8(%%ebp), %%eax"
                          : : : "%eax"
    );
}

int main()
{
    int ret;
    ret = add1(1, 2);
    printf("ret1=%d\n", ret);
    ret = add2(3, 4);
    printf("ret2=%d\n", ret);
    return 0;
}

# gcc testasm.c -o testasm
# ./testasm
ret1=3
ret2=7
# gcc -O2 testasm.c -o testasm
# ./testasm
ret1=0
ret2=0

Output of gcc -S -O2:
main:
.LFB24:
    .cfi_startproc
    pushl   %ebp
    .cfi_def_cfa_offset 8
    .cfi_offset 5, -8
    movl    %esp, %ebp
    .cfi_def_cfa_register 5
    andl    $-16, %esp
    subl    $16, %esp
#APP
# 5 "testasm.c" 1
    movl 12(%ebp), %eax
    addl 8(%ebp), %eax
# 0 "" 2
#NO_APP
    movl    $0, 8(%esp)        # <-- 0 is passed to print
    movl    $.LC0, 4(%esp)
    movl    $1, (%esp)
    call    __printf_chk
#APP
# 12 "testasm.c" 1
    movl 12(%ebp), %eax
    addl 8(%ebp), %eax
# 0 "" 2
#NO_APP
    movl    $0, 8(%esp)        # <-- 0 is passed to print
    movl    $.LC1, 4(%esp)
    movl    $1, (%esp)
    call    __printf_chk
    xorl    %eax, %eax
    leave
    .cfi_restore 5
    .cfi_def_cfa 4, 4
    ret
    .cfi_endproc

0 is passed to print, even %eax is added to clobber list. gcc 4.6/4.7/4.8 can
reproduce this.