[Bug middle-end/35344] New: Loop unswitching to produce perfect loop nest

[xinliangli at gmail dot com]

GCC loop unswitching is very good -- handles most of the cases I tried. The
only only thing I notice is that the nested for loops in C in the example below
is not converted into a perfect loop nest:

int** p;

int** q;

void foo(int m, int n)
{
   int i, j;

   for (i = 0;  i < m; i++)
   {
      for (j = 0; j < n ; j++)
     {
        p[i][j] += q[i][j];
        q[i][j] ++;
     }
   }

}

It is generated into (roughly) the following code by gcc (-O3
-fno-tree-vectorize)

 if (m > 0)
 {
      do
      {
          if (n > 0 )
          {
              do
              {
                 p[i][j] += q[i][j];
                 q[i][j] += 1;
                 j += 1;
              } while ( j < n);
          }
          i+=1;
     } while (i < m);
 }

Ideally-- it should be:

 if (m > 0)
 {
    if (n >0)
    {
        do
        {
            do
            {
              ....
              j++;
            } while(...);
            i++;
        }while (...);
     }
    // empty loop deleted
}


-- 
           Summary: Loop unswitching to produce perfect loop nest
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: xinliangli at gmail dot com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35344