On 22 October 2007 17:16, Michael Matz wrote:
>> I'd rather wish the optimization would be done differently. Currently we
>> have:
>>
>> mem -> reg;
>> loop loop
>> if (condition) => optimize => if (condition)
>> val -> mem; val -> reg;
>> reg -> mem;
>>
>>
>> But it could use additional register and be:
>>
>> 0 -> flag_reg;
>> loop
>> if (condition)
>> val -> reg;
>> 1 -> flag_reg;
>> if (flag_reg == 1)
>> reg -> mem;
>
> That could be done but would be besides the point. You traded one
> conditional store with another one, so you've gained nothing in that
> transformation.
Not quite: he's hoisted it (lowered it? sunk it?) out of the bottom of the
loop, so the test/branch/store only occurs once, and inside the loop there's
no memory access at all (which should be faster even than a load-cmove-store
with hot caches and no branches...)
cheers,
DaveK
--
Can't think of a witty .sigline today....
