[Bug tree-optimization/26939] PRE confuses loop number of iterations analysis

[rguenth at gcc dot gnu dot org]

------- Comment #7 from rguenth at gcc dot gnu dot org  2006-03-30 15:11 -------
You are probably right about improving SCEV - I hope Sebastian can make it work
for this and similar cases.  Wrt the loop header it is that we convert the loop
to a do-while style loop, which at least iterates once, but to make this
transformation valid, we need to copy the loop header.  The "bug" is of course
that we later try to prove again that this is a do-while style loop, which we
could better have remembered somehow.  I.e.

  for (i = start; i < end; ++i)
    ;

iterates end-start times, if start <= end.  So we transform it to

  if (start < end)
    for (i = start; i < end; ++i)
      ;

and later we "prove" that this new loop runs at least once by taking
the loop exit condition and trying to simplify that based on dominating
conditions from the loop header.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26939