[Bug rtl-optimization/31799] Failed to optimize out test instruction

[ubizjak at gmail dot com]

------- Comment #1 from ubizjak at gmail dot com  2007-05-03 17:08 -------
(In reply to comment #0)

> foo:
>         addl    $1, %edi
>         testl   %edi, %edi
>         movl    %edi, (%rdx)
>         je      .L3
>         movl    $1, (%rsi)
> .L3:
>         rep ; ret

This is just an unfortunate corner case. During combine phase we get:

(insn 10 6 11 2 (parallel [
            (set (reg/v:SI 58 [ x.27 ])
                (plus:SI (reg/v:SI 59 [ x ])
                    (const_int 1 [0x1])))
            (clobber (reg:CC 17 flags))
        ]) 210 {*addsi_1} (insn_list:REG_DEP_TRUE 3 (nil))
    (expr_list:REG_DEAD (reg/v:SI 59 [ x ])
        (expr_list:REG_UNUSED (reg:CC 17 flags)
            (nil))))

(insn 11 10 13 2 (set (mem:SI (reg/v/f:DI 61 [ z ]) [2 S4 A32])
        (reg/v:SI 58 [ x.27 ])) 40 {*movsi_1} (insn_list:REG_DEP_TRUE 5
(insn_list:REG_DEP_TRUE 10 (nil)))
    (expr_list:REG_DEAD (reg/v/f:DI 61 [ z ])
        (nil)))

(insn 13 11 14 2 (set (reg:CCZ 17 flags)
        (compare:CCZ (reg/v:SI 58 [ x.27 ])
            (const_int -1 [0xffffffffffffffff]))) 5 {*cmpsi_1_insn} (nil)
    (expr_list:REG_DEAD (reg/v:SI 58 [ x.27 ])
        (nil)))


It looks that insn 10 and insn 13 don't combine into flags setting insn due to
insn 11 interfering in some way. If the code is changed to:

void
foo (int x, int *y, int *z)
{
  if (++x != 0)
    *y = 1;
  *z = x;
}

then gcc generates expected asm without test insn.


-- 

ubizjak at gmail dot com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|normal                      |enhancement
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|0                           |1
   Last reconfirmed|0000-00-00 00:00:00         |2007-05-03 17:08:39
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31799