------- Comment #10 from rakdver at gcc dot gnu dot org 2007-02-05 21:45
-------
Hello,
(In reply to comment #8)
> (In reply to comment #1)
> > With -funsafe-loop-optimizations we remove them.
> > Using -Wunsafe-loop-optimizations, we get:
> > t.c:2: warning: cannot optimize loop, the loop counter may overflow
>
> If the loop counter will overflow for signed types, that is undefined and
> we should do it no matter what unless -fwrapv is turned on.
actually, the warning is missleading here. The problem is that
the number of iterations of this loop is
((unsigned) b - (unsigned) a + 3) / 4
but this formula does not work if (unsigned) b - (unsigned) a + 3 overflows,
which is what we fail to verify.
The obvious-looking solution is to use formula 1 + ((unsigned) b - (unsigned) a
-1) / 4 that does not suffer from this problem. I tried that, there are some
problems with this as well (but I forgot what the problems are, so I will need
to check again).
--
rakdver at gcc dot gnu dot org changed:
What |Removed |Added
----------------------------------------------------------------------------
AssignedTo|unassigned at gcc dot gnu |rakdver at gcc dot gnu dot
|dot org |org
Status|NEW |ASSIGNED
Last reconfirmed|2005-08-12 19:10:31 |2007-02-05 21:45:12
date| |
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23361
