[Bug middle-end/17549] [4.0 Regression] 25% increase in codesize (3.3.4 -> 4.0.0 20040917)

["roger at eyesopen dot com" <gcc-bugzilla at gcc dot gnu dot org>]

------- Additional Comments From roger at eyesopen dot com  2004-09-25 18:10 -------
A quick analysis of just the first doubly nested loop of susan_edges, comparing
gcc 3.3.3 to 3.5.0 20040828 (i.e. before ivopts), shows that even here the cause
of problems is induction variables (rather than unrolling or basic block
duplication).  The number of assembly lines in the basic block in the middle
of the loop nest increases in size by nearly 15% (197 lines -> 224 lines).
The size of the stack frame doubles from 24 bytes to 48 bytes.

Growth of the stack frame for the whole function susan_edges is even more
dramatic increasing from 28 bytes to an impressive 620 bytes.  That's a lot
of new pseudos!  The section of code in comment #3, shows us shuffling these
extra temporaries [I was shocked to discover that it wasn't an inadvertently
inlined memcpy]

The code that's freaking out the induction variable heuristics looks like:

  for (i=3;i<y_size-3;i++)
    for (j=3;j<x_size-3;j++)
    {
      n=100;
      p=in + (i-3)*x_size + j - 1;
      cp=bp + in[i*x_size+j];

      n+=*(cp-*p++);
      n+=*(cp-*p++);
      n+=*(cp-*p);
      p+=x_size-3;

      n+=*(cp-*p++);
      n+=*(cp-*p++);
      n+=*(cp-*p++);
      n+=*(cp-*p++);
      n+=*(cp-*p);
      p+=x_size-5;

      n+=*(cp-*p++);
      n+=*(cp-*p++);
      n+=*(cp-*p++);
      n+=*(cp-*p++);
      n+=*(cp-*p++);
      n+=*(cp-*p++);
      n+=*(cp-*p);
      p+=x_size-6;

      n+=*(cp-*p++);
      n+=*(cp-*p++);
      n+=*(cp-*p);
      p+=2;
      n+=*(cp-*p++);
      n+=*(cp-*p++);
      n+=*(cp-*p);
      p+=x_size-6;

      n+=*(cp-*p++);
      n+=*(cp-*p++);
      n+=*(cp-*p++);
      n+=*(cp-*p++);
      n+=*(cp-*p++);
      n+=*(cp-*p++);
      n+=*(cp-*p);
      p+=x_size-5;

      n+=*(cp-*p++);
      n+=*(cp-*p++);
      n+=*(cp-*p++);
      n+=*(cp-*p++);
      n+=*(cp-*p);
      p+=x_size-3;

      n+=*(cp-*p++);
      n+=*(cp-*p++);
      n+=*(cp-*p);

      if (n<=max_no)
        r[i*x_size+j] = max_no - n;
    }


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17549