[Bug middle-end/30201] gcc doesn't unroll nested loops

[bangerth at dealii dot org]

------- Comment #7 from bangerth at dealii dot org  2006-12-14 15:16 -------
I've reduced the code a bit by stripping all C++isms and collapsing
everything into a single function:
-----------------------------------
double data[9];

int main()
{
  for( int factor = 0; factor < 1000000000; factor++ )
    {
#ifdef UNROLL
      data[ 0 + 3 * 0 ] = factor;
      data[ 1 + 3 * 0 ] = 0;
      data[ 2 + 3 * 0 ] = 0;
      data[ 0 + 3 * 1 ] = 0;
      data[ 1 + 3 * 1 ] = factor;
      data[ 2 + 3 * 1 ] = 0;
      data[ 0 + 3 * 2 ] = 0;
      data[ 1 + 3 * 2 ] = 0;
      data[ 2 + 3 * 2 ] = factor;
#else
      for( int i = 0; i < 3; i++ )
        for( int j = 0; j < 3; j++ )
          data[i + 3 * j] = (i == j) * factor;
#endif
    }
}
-----------------------------

For this, we get these results:
g/x> gcc x.c -O3 -funroll-loops -std=c99
g/x> time ./a.out 

real    0m9.380s
user    0m9.381s
sys     0m0.000s

g/x> gcc x.c -O3 -funroll-loops -std=c99 -DUNROLL
g/x> time ./a.out 

real    0m0.420s
user    0m0.420s
sys     0m0.000s

W.


-- 

bangerth at dealii dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |bangerth at dealii dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=30201