Speeding up compiled code with SAT

[Ian Lance Taylor]

Brian Budge <brian.budge@gmail.com> writes:

>> It sounds like an idea which would be extremely expensive in compilation
>> time.  Nobody will use a compiler which takes too long to compile.
>>
>> We don't need to go as far as you suggest.  For example, we could get
>> better register allocation we if put more time into it, by trying out
>> several different possibilities.  That would be a fairly straightforward
>> change, almost certainly easier to implement than what you suggest.  But
>> we don't do it, because it would take too long, so nobody would use it.
>>
>> You can casually say that nobody cares if the final compilation of
>> firefox takes 1 CPU year, but in fact people do care, because they want
>> to test what they ship.
>>
>> I don't want to discourage you from exploring this idea if you find it
>> interesting, but I'm pretty skeptical that it would ever become part of
>> a gcc distribution.
>>
>
> On the other hand, I would put up with compile times that take twice
> or even 10 times as long (for our final test and final release build)
> for a 10% speedup of my code.  If this is very easy to do, maybe it
> makes sense to add an optimization flag that specifies how many
> permutations are allowed to be checked?

I wouldn't say it is very easy.  I would just say that it is easier than
a general solution solver.

Ian