[Bug c/52802] New: Equality rewrites pessimizes code

[gcc at magfr dot user.lysator.liu.se]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52802

             Bug #: 52802
           Summary: Equality rewrites pessimizes code
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: gcc@magfr.user.lysator.liu.se


I have a function (in foo.c)

unsigned f(unsigned a, unsigned b)
{
        if (a < 8)
                return a;
        else if (a == 8)
                return b;
        else
                return 4711;
}

that GCC compiles to (x86_64, -Os)

        cmpl    $7, %edi        #, a
        movl    %edi, %eax      # a, a
        jbe     .L2     #,
        cmpl    $8, %edi        #, a
        movl    $4711, %eax     #, tmp63
        cmove   %esi, %eax      # b,, a
.L2:
        ret

Here it is confusing to see that it starts by doing a "cmp $7, %edi" and then
goes on to do a "cmp $8, %edi" since I never said anything but 8, so the big
question is why there are two compares.

The reason seems to be premature optimization. The C parser tries to minimize
the value of all comparision values so that -fdump-tree-original shows

;; Function f (null)
;; enabled by -tree-original


{
  if (a <= 7)
    {
      return a;
    }
  else
    {
      if (a == 8)
        {
          return b;
        }
      else
        {
          return 4711;
        }
    }
}

and there the 7 have magically appeared.