[Bug tree-optimization/61743] [4.10 Regression] Complete unroll is not happened for loops with short upper bound

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61743

--- Comment #4 from Richard Biener <rguenth at gcc dot gnu.org> ---
We fail to determine an upper bound on the number of iterations for those
loops.
With 4.9 I only see one loop completely unrolled, that at t.c:20.  4.8 doesn't
unroll any loop either.

4.9 seems to conclude that by

Statement *pretmp_81[j_51] = _39;
 is probably executed at most 7 (bounded by 7) + 1 times in loop 4.

which also is seen on trunk.

It would be interesting to know what makes us decide otherwise in the end
(can you bisect this?)