[Bug c/38209] New: [avr] branch optimisation generates worse code

[Bug c/38209] New: [avr] branch optimisation generates worse code

[k dot kosciuszkiewicz+gcc at gmail dot com]

Test code:

register unsigned char val asm("r4");

void negate(void)
{
    if (val)
        val = ~val;
    else
        val = ~val;
}

Code generated with -Os

.global negate
        .type   negate, @function
negate:
/* prologue: function */
/* frame size = 0 */
        tst r4
        breq .L2
        com r4
        ret
.L2:
        clr r4
        dec r4
        ret
        .size   negate, .-negate

In the "else" branch gcc knows that r4 is zero, and that it should change to
0xff - taking this approach generates longer code than in the first branch.

The same applies when operations like !, ++, -- are used.


-- 
           Summary: [avr] branch optimisation generates worse code
           Product: gcc
           Version: 4.3.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: k dot kosciuszkiewicz+gcc at gmail dot com
 GCC build triplet: i386-linux-gnu
  GCC host triplet: i386-linux-gnu
GCC target triplet: avr-*-*


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38209

[Bug c/38209] [avr] branch optimisation generates worse code

[k dot kosciuszkiewicz+gcc at gmail dot com]

------- Comment #1 from k dot kosciuszkiewicz+gcc at gmail dot com  2008-11-21 06:05 -------
Created an attachment (id=16730)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=16730&action=view)
Test case.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38209

[Bug middle-end/38209] branch optimisation generates worse code

[pinskia at gcc dot gnu dot org]

------- Comment #2 from pinskia at gcc dot gnu dot org  2008-11-23 18:55 -------
This happens on i386-darwin also (-m64 -O2 -fomit-frame-pointer):
        movl    %edi, %edx
        testb   %dil, %dil
        movl    $-1, %eax
        notl    %edx
        cmovne  %edx, %eax

Using the following source:
unsigned char negate(unsigned char val)
{
    if (val)
        val = ~val;
    else
        val = ~val;
  return val;
}

We should be able to just do ~val but this comes down to hoisting before
applying VRP/DOM.


-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|0                           |1
  GCC build triplet|i386-linux-gnu              |
   GCC host triplet|i386-linux-gnu              |
 GCC target triplet|avr-*-*                     |
   Last reconfirmed|0000-00-00 00:00:00         |2008-11-23 18:55:59
               date|                            |
            Summary|[avr] branch optimisation   |branch optimisation
                   |generates worse code        |generates worse code


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38209