[Bug target/60486] [4.8/4.9 Regression] [avr] superfluous or missing comparision after addition or subtraction

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

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60486

Georg-Johann Lay <gjl at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |wrong-code
          Component|other                       |target
            Summary|[avr] missed optimization   |[4.8/4.9 Regression] [avr]
                   |on detecting zero flag set  |superfluous or missing
                   |                            |comparision after addition
                   |                            |or subtraction
      Known to fail|                            |4.9.0

--- Comment #5 from Georg-Johann Lay <gjl at gcc dot gnu.org> ---
Target issue.

(In reply to Darryl Piper from comment #4)
> details also posted on avrfreaks.net

I am well aware of this post.

But GCC reports are supposed to be self contained. So here we go:


Root cause is swapped cc_plus and cc_minus in avr.c:avr_out_plus :

  /* Work out the shortest sequence.  */

  avr_out_plus_1 (op, &len_minus, MINUS, &cc_plus, code_sat, sign, out_label);
  avr_out_plus_1 (op, &len_plus, PLUS, &cc_minus, code_sat, sign, out_label);


Thus there are also cases where wrong code is generated like the following one:


extern void foo (unsigned);
char v;

void bar (unsigned long z)
{
  if (++z == 0)
     v = 0;

  foo (z);
}


Output is missing the comparison because addsi3 does not set cc0 in a usable
way when is uses the PLUS alternative:


bar:
    movw r26,r24     ;  20    *movsi/1    [length = 2]
    movw r24,r22
    adiw r24,1     ;  6    addsi3/2    [length = 3]
    adc r26,__zero_reg__
    adc r27,__zero_reg__
    brne .L5     ;  8    branch    [length = 1]
    sts v,__zero_reg__     ;  10    movqi_insn/3    [length = 2]
.L5:
    rjmp foo     ;  14    call_insn/4    [length = 1]