[Bug tree-optimization/68529] scev failed for while(i--)

[amker.cheng at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68529

--- Comment #5 from bin.cheng <amker.cheng at gmail dot com> ---
The dump before ldist is as below:

foo1 ()
{
  unsigned int nchar;
  char c[10000];

  <bb 2>:
  c = {};

  <bb 3>:
  # nchar_14 = PHI <nchar_5(4), 9998(2)>
  c[nchar_14] = 65;
  nchar_5 = nchar_14 + 4294967295;
  if (nchar_14 != 0)
    goto <bb 4>;
  else
    goto <bb 5>;

  <bb 4>:
  goto <bb 3>;

  <bb 5>:
  bar (&c);
  c ={v} {CLOBBER};
  return 0;

}

Apparently, nchar_14 iterates from 9998 to 0 in the loop and it doesn't
overflow, while nchar_5 does overflow for the last iterations.  It's true
&c[n_char_14] is an affine since the index doesn't overflow.
When I introduced the change computing non-overlfow control-iv in loop niter
analysis, I only did it for the simplest case, ie, loop exit condition with LT
comp code.  This case falls in another case, ie, NE as the comp code.  I think
this can be fixed in the same framework.  Actually it's easy to enable
non-overflow control-iv computation in this case, then we can use the recorded
control-iv to prove that (sizetype)nchar_14 doesn't overflow and &c[nchar_14]
is an affine.

I will dig deeper and test a patch for this.

Thanks,
bin