[Bug tree-optimization/42027] New: Performance regression in convolution loop optimization

[nbenoit at tuxfamily dot org]

GCC trunk rev. 154141 seems to handle less efficiently a convolution code than
previous stable releases, it was also spotted in revision 153048.

Here are some average timings on an Intel E5320 clocked at 1.86 GHz with 4 MB
of L2 cache, Debian GNU/Linux with a 2.6.26 kernel.

* with -O2 -march=native
GCC 4.3.2               8239 ms
GCC-4.4.2               8102 ms
GCC-snapshot-20091105   9347 ms
GCC-trunk-r154141       9343 ms

* with -O2
GCC 4.3.2               8128 ms
GCC-4.4.2               8158 ms
GCC-snapshot-20091105   9824 ms
GCC-trunk-r154141       9828 ms

* with -O1
GCC 4.3.2              20926 ms
GCC-4.4.2               8277 ms
GCC-snapshot-20091105   9369 ms
GCC-trunk-r154141       9375 ms

* with -O0
GCC 4.3.2              34061 ms
GCC-4.4.2              34241 ms
GCC-snapshot-20091105  34903 ms
GCC-trunk-r154141      34910 ms


GCC compiled with : configure --prefix=/export/home/nicolas/gcc/trunk-install
--enable-languages=c --disable-multilib --disable-bootstrap
--enable-checking=release

I haven't been able to track down the origin of the performance difference.


Note that data are not initialized in the attached code, as the slowdown is
observed wether they are or not.

---BEGIN code---
#define N  1024*512
#define M  512
#define ITER 16

double in[N];
double H[M];
double vH[N];

int main ( int argc,
           char **argv )
{
  int i, j, k;

  for ( i=0; i<ITER; ++i )
    for ( j=0; j<N; ++j )
      for ( k=0; (k<M)&&(k<=j); ++k )
        vH[j] += H[k]*in[j-k];

  return (int) vH[argc];
}
---END code---


-- 
           Summary: Performance regression in convolution loop optimization
           Product: gcc
           Version: 4.5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: nbenoit at tuxfamily dot org
 GCC build triplet: i686-pc-linux-gnu
  GCC host triplet: i686-pc-linux-gnu
GCC target triplet: i686-pc-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42027