[Bug tree-optimization/100499] Different results with -fpeel-loops -ftree-loop-vectorize options

[amker at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100499

--- Comment #19 from bin cheng <amker at gcc dot gnu.org> ---
(In reply to bin cheng from comment #18)
> Did some experiments, there are two fallouts after explicitly returning
> false for unsigned/wrapping types in MULT_EXPR/MINUS_EXPR/PLUS_EXPR.  One is
> the mentioned use of multiple_of_p in number_of_iterations_ne, the other is
> for alignment warning in stor-layout.c.  As pointed out, the latter case is
> known not overflow/wrap.  
> 
> So I am thinking to introduce an additional parameter indicating that caller
> knows "top" doesn't overfow/wrap, otherwise, try to get rid of the
> undocumented assumption.  we can always improve the accuracy using ranger or
> other tools.  Not sure if this is the right way to do.
> 
> As for MULT_NO_OVERFLOW/PLUS_NO_OVERFLOW, IMHO, it's not that simple?  For
> example, unsigned_num(multiple of 4, and larger than 0) + 0xfffffffc is
> multiple of 4, but it's overflow behavior on which we rely here.

Hmm, 4 is special and not a correct example.  Considering:
  n (unsigned, multiple of 3, and > 0) + 0xfffffffd
It's multiple of 3, but we need to rely on wrapping to get answer.