Re: [PATCH] Optimize divmod expansion (PR middle-end/79665)

[Georg-Johann Lay <avr@gjlay.de>]

On 31.05.2017 10:15, Jakub Jelinek wrote:
> On Wed, May 31, 2017 at 10:06:34AM +0200, Georg-Johann Lay wrote:
>> Hi, this causes a performance degradation for avr.
>>
>> When optimizing for speed, and with a known denominatior, then v6 uses
>> s/umulMM3_highpart insn to avoid division because no div instruction is
>> available.
>>
>> unsigned scale256 (unsigned val)
>> {
>>     return value / 255;
>> }
>>
>> With this patch, v7 now uses __divmodhi4 which is very expensive but
>> the costs are not computed because rtlanal.c:seq_cost assumes a cost of
>> ONE:
>>
>>   for (; seq; seq = NEXT_INSN (seq))
>>     {
>>       set = single_set (seq);
>>       if (set)
>>         cost += set_rtx_cost (set, speed);
>>       else
>>         cost++;
>>     }
>>
>> because divmod in not a single_set:
>> (gdb) p seq
>> $10 = (const rtx_insn *) 0x7ffff730d500
>> (gdb) pr
>> warning: Expression is not an assignment (and might have no effect)
>> (insn 14 13 0 (parallel [
>>             (set (reg:HI 52)
>>                 (div:HI (reg:HI 47)
>>                     (reg:HI 54)))
>>             (set (reg:HI 53)
>>                 (mod:HI (reg:HI 47)
>>                     (reg:HI 54)))
>>             (clobber (reg:QI 21 r21))
>>             (clobber (reg:HI 22 r22))
>>             (clobber (reg:HI 24 r24))
>>             (clobber (reg:HI 26 r26))
>>         ]) "scale.c":7 -1
>>      (nil))
>> (gdb)
>>
>> Hence the divmod appears to be much less expensive than the unsigned
>> variant that computed the costs for mult_highpart.
>
> Then you should fix the cost computation - be able to use a target hook
> on insns that are not a single set or something similar.
>
> 	Jakub
>

Are you saying that cost computation in GCC is fundamentally flawed
for anything that it not a single_set?

Johann