[Bug tree-optimization/34114] Missed optimization: cannot determine loop termination

["lloyd at randombit dot net" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #2 from lloyd at randombit dot net  2007-11-16 00:50 -------
Is there be any way to modify the code such that GCC would have an easier time
seeing this? I tried using 'assert(rnd_to_2 % 2 == 0)' (since glibc's
__assert_fail is marked with noreturn I thought it might help), but that didn't
seem to have an effect.

Short background that might be relevant: the code this is derived from is doing
partial loop unrolling (8 iterations in the actual code) with a block of inline
asm inserted, and then another loop following that handles any slop. Would
rewriting the loop as

   while(in >= 2)
      {
      in -= 2;
      i += 2;
      }

be likely to help? I see that it does with one particular version (4.1.2), but
I have no intuition if that is because the optimizer understands such loops
better or if it is just random luck.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=34114