Re: Executing code at compilation time

"Fernando" <pronesto@gmail.com> wrote in message


> GCC does a pretty good job at optimizing a program like this one
> below:


> int i = 0;
> int sum = 0;
> for (; i < 10; i++) {
> sum += i;
> }
> printf("The sum is %d\n", sum);
> }
>
> GCC -O1 produces an assembly that simply prints the answer, 45. It
> completely resolves the loop.


(And which is a nuisance when comparing benchmarks with gcc...)


> Additionally, GCC does not resolve the loop in the program below:


> int i, j, k, t = 0;
> for(i = 0; i < 1000; i++)
> for(j = 0; j < 1000; j++)
> for(k = 0; k < 1000; k++)
> if(i * i + j * j + k * k % 7 == 0)
> t++;
> printf("%d\n", t);
> return 0;
> }
>
> But it is "solvable" at compilation time. Is there any sort of non-
> standard optimization that does the job, in this case?


I haven't tried 'folding' statements (only expressions).


But I would imagine that if the first loop was unrolled, you'd get 10
assignments, which, if you can fold one, you can fold them all (eg. sum+=0
is a no-op, sum+=1 sets sum to 1, and so on).


The second loop however, involving a billion compound statements, won't
yield so easily to this method.


--
Bartc

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