[Bug inline-asm/58818] New: parameters optimized out using -O2

[Bug inline-asm/58818] New: parameters optimized out using -O2

[gonwan at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58818

            Bug ID: 58818
           Summary: parameters optimized out 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 testasm2.c
#include <stdio.h>

int add3(int a, int b)
{
    int res;
    __asm__ __volatile__ ("movl 12(%%ebp), %%eax\n\t"
                          "addl 8(%%ebp), %%eax"
                          : "=a" (res)
    );
    return res;
}

int main()
{
    int ret;
    ret = add3(5, 6);
    printf("ret3=%d\n", ret);
    return 0;
}

# gcc testasm2.c -o testasm2
# ./testasm2
ret3=11
# gcc -O2 testasm2.c -o testasm2
# ./testasm2
ret3=-1078224411

Output of gcc -S -O2:
main:
.LFB23:
    .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        # <-- no parameter is passed after sub %esp.
#APP
# 6 "testasm2.c" 1
    movl 12(%ebp), %eax
    addl 8(%ebp), %eax
# 0 "" 2
#NO_APP
    movl    %eax, 8(%esp)
    movl    $.LC0, 4(%esp)
    movl    $1, (%esp)
    call    __printf_chk
    xorl    %eax, %eax
    leave
    .cfi_restore 5
    .cfi_def_cfa 4, 4
    ret
    .cfi_endproc

No parameter is passed after sub %esp. gcc 4.6/4.7/4.8 can reproduce this.

[Bug inline-asm/58818] parameters optimized out using -O2

[pinskia at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58818

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

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

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
You cannot depend on those being the correct locations in memory for the
arguments.  With -O2, the code could be inlined which is what is happening in
your case.

[Bug inline-asm/58818] parameters optimized out using -O2

[jakub at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58818

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jakub at gcc dot gnu.org

--- Comment #2 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
And it isn't just inlining that can change that, also cloning can do.