Re: Executing code at compilation time

On Mon, Mar 15, 2010 at 6:53 PM, Fernando <pronesto@gmail.com> wrote:


> int main(int argc, char** argv) {
> 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.


Typically, such a loop is unwound to 10 invariant increment
instructions and then an iterative peephole optimization pass is used
to determine that final constant value. Loop unwinding and peephole
optimization are covered in various texts on compiler construction.


You then asked:


> int main(int argc, char **argv) {
> 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?


While the above could be theoretically achieved using those same above
optimization techniques, compilers do not unwind beyond a reasonable
limit (for some value of "reasonable").


Quinn

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