Re: Performace problems with gcc 3.0.4

[Joe Buck <Joe dot Buck at synopsys dot com>]

> According to [a friend], the following code is about
> 50% slower than the version without structs.
> 
> He was tried the ICC (Intel compiler) and it
> generates the same code for the two cases but
> gcc does not. He played with all optimization
> flags that affect numeric code but to no avail.
> 
> Does anyone know what might be the problem ?

Yes, it's a well-known problem with gcc.  Structs get committed to memory
too early.  Some compilers are able to split apart structs and treat their
members as independent scalar variables; gcc can only do this for structs
or classes with a single member.

> /* Code with structs */
> #include <stdio.h>
> int main() 
> {
>   typedef struct {
>     double a;
>     double b;
>   } twodoubles;
> 
>   twodoubles x;
>   int i;
> 
>   x.a = 0.0;
>   x.b = 0.0;
>   for (i = 0; i < 100000000; i++) {
>     x.a = x.b + 1.0;
>     x.b = x.a - x.b + 1.0;
>   }
> 
>   printf("a: %g\nb: %g\n", x.a, x.b);
>   return 0;
> }
> 
> -------------------------------------------------------
> /* Code without structs */
> #include <stdio.h>
> 
> int main() 
> {
>   double a, b;
>   int i;
> 
>   a = 0.0;
>   b = 0.0;
>   for (i = 0; i < 100000000; i++) {
>     a = b + 1.0;
>     b = a - b + 1.0;
>   }
> 
>    printf("a: %g\nb: %g\n", a, b);
>    return 0;
> }
> 
>