[Bug target/53138] New: [4.7/4.8 Regression] spaceship operator miscompiled

[bonzini at gnu dot org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53138

             Bug #: 53138
           Summary: [4.7/4.8 Regression] spaceship operator miscompiled
    Classification: Unclassified
           Product: gcc
           Version: 4.7.0
            Status: UNCONFIRMED
          Keywords: wrong-code
          Severity: normal
          Priority: P3
         Component: target
        AssignedTo: bonz...@gnu.org
        ReportedBy: bonz...@gnu.org
            Target: x86_64-pc-linux-gnu


This program is miscompiled at -O2:

typedef struct sreal
{
  unsigned sig;        /* Significant.  */
  int exp;        /* Exponent.  */
} sreal;

sreal_compare (sreal *a, sreal *b)
{
  if (a->exp > b->exp)
    return 1;
  if (a->exp < b->exp)
    return -1;
  if (a->sig > b->sig)
    return 1;
  return -(a->sig < b->sig);
}

sreal a[] = {
   { 0, 0 },
   { 1, 0 },
   { 0, 1 },
   { 1, 1 }
};

int main()
{
  int i, j;
  for (i = 0; i <= 3; i++) {
    for (j = 0; j < 3; j++) {
      if (i < j && sreal_compare(&a[i], &a[j]) != -1) abort();
      if (i == j && sreal_compare(&a[i], &a[j]) != 0) abort();
      if (i > j && sreal_compare(&a[i], &a[j]) != 1) abort();
    }
  }
}

The problem is that the comparison on sig is compiled to this:

    movl    (%rsi), %ecx
    cmpl    %ecx, (%rdi)
    sbbl    %edx, %edx
    cmovbe    %edx, %eax
    ret

but sbbl only preserves the carry flag, not the zero flag.