[Bug tree-optimization/18704] [4.0 Regression] Inlining limits cause 340% performance regression

["rguenth at tat dot physik dot uni-tuebingen dot de" <gcc-bugzilla at gcc dot gnu dot org>]

------- Additional Comments From rguenth at tat dot physik dot uni-tuebingen dot de  2004-11-29 11:04 -------
Looking at the 3.4 branch the defaults for the relevant inlining parameters are
the same.  So the difference in performance has to be accounted to different
tree-node counting (or to differences in the accounting during inlining).

As we throttle inlining params if -Os is specified in opts.c:

  if (optimize_size)
    {
      /* Inlining of very small functions usually reduces total size.  */
      set_param_value ("max-inline-insns-single", 5);
      set_param_value ("max-inline-insns-auto", 5);
      flag_inline_functions = 1;

may I suggest to throttle inline-unit-growth there, too (though it
shouldn't have an effect with so small max-inline-insns-single).  And
then provide the documented limit (150) for inline-unit-growth?

One may even argue that limiting overall unit growth is not important,
as it is already limited by max-inline-insns-* and large-function-*.
Also both inline-unit-growth and large-function-growth cause inlining
to stop at the threshold leaving one with an unbalanced inlining decision.

Why were these (growth) limits invented?  Were there some particular testcases
that broke down otherwise?

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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=18704