Re: Executing code at compilation time

On Mon, Mar 15, 2010 at 21:53, Fernando <pronesto@gmail.com> wrote:


> #include <stdio.h>
>
> 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.


In GCC, this is computed by scalar evolutions, which is used by
optimizations like constant propagation (in this case) to determine
the value of reduction variables in loops. Later, the loop is found
empty and removed by other optimizations.


To see the changes the compiler makes to the code as it optimizes, you
can use the flag -fdump-tree-all in GCC 4.x. IIRC, you should find
the beginnings of this optimization in the .sccp dump file.




Diego.

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