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

[jakub at gcc dot gnu.org]

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

--- Comment #8 from Jakub Jelinek <jakub at gcc dot gnu.org> 2013-03-11 15:49:00 UTC ---
Created attachment 29637
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=29637
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.