[Bug tree-optimization/37810] Bad store sinking job

["carlo at gcc dot gnu dot org" <gcc-bugzilla@gcc.gnu.org>]

------- Comment #4 from carlo at gcc dot gnu dot org  2008-10-12 15:32 -------
Note that the original code was:

  A& operator++(void)
  {
    ++n;
    if (__builtin_expect(n == m, false))
      g();
    return *this;
  }

but g++ fails to optimize that by decrementing m outside
the loop (so I'm decrementing m myself now and use the
former code). The former code has as advantage, namely,
that the result of the addl $1,%eax can be used for the
conditional jump. However, gcc ALSO doesn't do that: in
the above assembly it follows the add with a redundant
testl %eax,%eax.

Anyway, using the operator++ given in this comment,
the assembly code is:

        movl    (%rdi), %eax
        jmp     .L3

.L4:
        addl    $1, %eax
        cmpl    4(%rdi), %eax
        movl    %eax, (%rdi)
        je      .L8
.L3:
        testl   %eax, %eax
        jne     .L4

which is essentially the same, except now the
testl %eax,%eax is indeed "needed" ...


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37810