[Bug rtl-optimization/63375] New: reordering of reads across fences

["bobby.prani at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63375

            Bug ID: 63375
           Summary: reordering of reads across fences
           Product: gcc
           Version: 4.9.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: bobby.prani at gmail dot com

A read within the region of code enclosed by barriers is being moved out of the
regions. Test case follows:

#include<stdio.h>

typedef struct {
    int counter;
} atomic_t;

static inline int atomic_read(atomic_t *v)
{
    return (*(volatile int *)&(v)->counter);
}

#define smp_mb() asm volatile ("mfence":::"memory")

int main()
{
    atomic_t val = {1};
    int p;

    smp_mb();
    p = atomic_read(&val);
    smp_mb();

    printf("%d\n", p);

    return 0;
}

Generated assembly:

  15:    mfence
  18:    mfence
  1b:    mov    $0x1,%esi
  20:    mov    $0x0,%rdi
  27:    xor    %eax,%eax
  29:    callq  2e <init_module+0x2e>