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Re: Tr : [redundency elimination, code motion, commun expression elimination] GCC optimizations
David Brown <david@westcontrol.com> writes:
>> It's the same for
>>
>> if (x == 1) { a=b; c=d; e=f; foo(); }
>> else if (x == 2) { a=b; c=d; e=f; foo(); }
>>
>> GCC don't factor out the common part
>>
>
> You are right - which is odd, since there is no reason why it could
> not (unlike the original case where there is no "else"). Surely this
> would count as a significant missed optimisation, especially for big
> switch() code.
I don't think it is a significant missed optimization, as people rarely
write code like that. They normally write
if (x == 1 || x == 2) { a=b; c=d; e=f; foo(); }
The proposed optimization only applies when somebody has laboriously
written out the exact same sequence of code twice.
I'm not opposed to such an optimization if it works reliably and is not
too expensive. I just don't think it will make much difference on
ordinary code.
Ian