[Bug tree-optimization/53265] Warn when undefined behavior implies smaller iteration count

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53265

--- Comment #9 from Richard Biener <rguenth at gcc dot gnu.org> 2013-03-11 15:58:41 UTC ---
(In reply to comment #8)
> Created attachment 29637 [details]
> gcc48-pr53265.patch
> 
> Untested patch.  Not sure about the warning wording, plus no idea how to call
> the warning option (-Wnum-loop-iterations, -Wundefined-behavior-in-loop,
> something else?), whether to enable it by default, or just for -Wall.
> A bigger issue is that I see multiple warnings for the same stmts, despite the
> guard in loop structure, because apparently the same loop is represented by
> different loop structures during the optimizations.

Yeah, before the tree loop optimization pipeline loops are not preserved ...
(easy to change, apart from code missing to inline a loop tree).

Eventually use gimple_no_warning on the stmt?  Supposedly not very reliable
either.

I think with your patch you also fail to warn for bounds discovered by
discover_iteration_bound_by_body_walk or maybe_lower_iteration_bound.
That said, I'd expected you warn from within record_niter_bound.  Btw,
after number_of_latch_executions () its return value is cached in
loop->nb_iterations, no need to pass around another value.