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Subject: [Bug tree-optimization/53265] Warn when undefined behavior implies smaller iteration count
Date: Wed, 13 Mar 2013 11:01:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53265

--- Comment #24 from Richard Biener <rguenth at gcc dot gnu.org> 2013-03-13 11:00:11 UTC ---
(In reply to comment #23)
> Created attachment 29661 [details]
> gcc48-pr53265.patch
> 
> Updated patch as per IRC discussions.  Still need to look at longbranch2.C and
> do_1.f90, then test it.

Looks good.  Few comments:

+  number_of_latch_executions (loop);

add a comment what side-effect you are interested in.

+
+  /* If we know the exact number of iterations of this loop avoid all the
+     work below and most importantly do not break code with undefined
+     behavior by recording smaller maximum number of iterations.  */
+  if (loop->nb_iterations
+      && TREE_CODE (loop->nb_iterations) == INTEGER_CST
+      && loop->any_upper_bound
+      && loop->nb_iterations_upper_bound.ucmp
+          (tree_to_double_int (loop->nb_iterations)) < 0)
+    loop->nb_iterations_upper_bound = tree_to_double_int
(loop->nb_iterations);

We don't avoid any work, so adjust the comment.  I'd also simply do:

   /* If we know the exact number of iterations record that as the
      upper bound as well.  This avoids breaking code with undefined
      behavior by eventually recording a smaller maximum.  */
   if (loop->nb_iterations
       && TREE_CODE (loop->nb_iterations) == INTEGER_CST)
     {
       loop->any_upper_bound = true;
       loop->nb_iterations_upper_bound = tree_to_double_int
(loop->nb_iterations);
     }

that's always correct.