Re: [Bug target/42553] New: wrong code with -O1

[Andrew Pinski <pinskia at gmail dot com>]

Sent from my iPhone

On Dec 30, 2009, at 7:56 AM, "debian-gcc at lists dot debian dot org" <gcc-bugzilla@gcc.gnu.org 
> wrote:

> current trunk/branches on x86_64-linux-gnu. return values with  
> different
> compilers and different optimizations:
>
>         -O0  -O1  -O2  -O3
> gcc-4.1   0    2    2    2
> gcc-4.3   0    2    2    2
> gcc-4.4   2    0    0    0
> gcc-4.5   0    1    0    0
>
>  Matthias
>
> #include <stdbool.h>
> #include <stdio.h>
>
> static bool atomic_test_and_reset_bit(unsigned long *v,unsigned long  
> bit) {
>  bool res;
>
>  __asm__ __volatile__( "btr %2,%1\n"
>                        "adc $0,%0\n"
>                        :"=r"(res), "=m"(*v)
>                        :"r"(bit), "r"(0)) ;
>  return res ? 1 : 0;

This inline-asm looks wrong. Because it says *v is always overriden.  
So if that function is inlined, flags down below does not have to be  
set before it.

> }
>
> int main(void)
> {
>  unsigned long flags = 0;
>  int j;
>
>  j = atomic_test_and_reset_bit(&flags, 2);
>
>  printf("%d\n", j);
>
>  return 0;
> }
>
>
> --
>           Summary: wrong code with -O1
>           Product: gcc
>           Version: 4.4.3
>            Status: UNCONFIRMED
>          Severity: normal
>          Priority: P3
>         Component: target
>        AssignedTo: unassigned at gcc dot gnu dot org
>        ReportedBy: debian-gcc at lists dot debian dot org
> GCC target triplet: x86_64-linux-gnu
>
>
> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42553