[Patch,AVR]: Tweak division

[Denis Chertykov]

2011/11/3 Georg-Johann Lay <avr@gjlay.de>:
> This is a tweak for signed 16- and 32-bit division routines.
> The old code called subroutine __divmod{si|hi}4_neg1 and returned if T-flag is
> not set.  This is costly.  By shuffling the instructions the test can be moved
> up without increasing the code size but saving calls here and there.
>
> The speed gain is 1..17 ticks for ATmega88 which is a speed-up of up to 7% for
> 16-bit division (formerly about 230-240 ticks).  For 16-bit division the
> absolute speed gain is the same.
>
> Moreover, addqi3 can handle +/-2 now which saves reload if the constant in
> non-d register.  The new *negqihi2 insn is for code like
>
> int minus (char a)
> {
>    return -a;
> }
>
> that compiled to
>
> minus:
>        clr r25  ;  6   extendqihi2/1   [length = 3]
>        sbrc r24,7
>        com r25
>        com r25  ;  7   neghi2/1        [length = 3]
>        neg r24
>        sbci r25,lo8(-1)
>        ret      ;  25  return  [length = 1]
>
> and now is compiled to a shorter, faster sequence without need of d-register:
>
> minus:
>        clr r25  ;  7   *negqihi2       [length = 4]
>        neg r24
>        brge .+2
>        com r25
>        ret      ;  25  return  [length = 1]
>
> Tested without regressions. Moreover, the new sequences are tested individually
> against the old code.
>
> The patch is against the old infrastructure but the changelog is already for
> the new libgcc layout.
>
> Ok for trunk?
>
> Johann
>
> gcc/
>        * config/avr/constraints.md (Cm2): New constraint for int -2.
>        * config/avr/avr.md (addqi3): Use it. New alternatives for +/-2.
>        (*negqihi2): New insn.
> libgcc/
>        * config/avr/lib1funcs.S (__divmodhi4, __divmodsi4): Tweak speed.
>

Approved.

Denis.