Re: Optimization around mul and restoring r1

David Brown schrieb:
> On 01/10/13 09:41, Simon Kirby wrote:
>> Hello!
>>
>> How difficult would it be to convince avr-gcc to not re-zero r1 instantly
>> after mul, but delay it until a zero is actually needed? For example,
>> see this actual avr-gcc output:
>>
>>         Vneutral = (uint16_t)Vbus * t >> 8;
>> ->
>>         lds     r25, 0x20FD
>>         mul     r24, r25
>>         movw    r24, r0
>>         eor     r1, r1
>>         sts     0x205C, r25
>>
>> Could be just:
>>
>>         lds     r25, 0x20FD
>>         mul     r24, r25
>>         sts     0x205C, r1
>>         eor     r1, r1
>>
>> Saving one whole cycle! ;)
>>
> In general, it would be a good thing to delay the zeroing of r1, and it
> is worth searching the issue database for previous issues and filing a
> new "missed optimisation" issue if it is not there already:

IMHO everyone working on the avr back-end is aware of this problem.

Johann