[Bug c/43772] Errant -Wlogical-op warning when testing limits

["marc.glisse at normalesup dot org" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43772

--- Comment #17 from Marc Glisse <marc.glisse at normalesup dot org> 2012-04-28 18:49:49 UTC ---
(In reply to comment #16)
> I understand now, and I think you are right. We don't have a warning for
> "((int)x) < INT_MIN" or ((int)x) > INT_MAX but I think it should go to
> Wtype-limits.

Interestingly, for an int i, we don't warn for x<=INT_MAX, but we do warn for
x<=(long)INT_MAX (adapt if your platform has int and long of the same size).

> Do you think we could test this situation just before the Wlogical-op warning?

It is easy to re-check inside warn_logical_operator if one of the tests is
always true. I have no idea how to pass the information from Wtype-limits that
warn_logical_operator shouldn't be called.

> I can see that some macros may generate x >= INT_MIN but the x < INT_MIN case
> seems less likely to be intented and we should warn (and then return and avoid
> warning with Wlogical-op).

I think < INT_MIN and >= INT_MIN should either both warn of both be quiet. It
is a matter of style whether people write:
if (x in range) do the work;
or
if (x out of range) abort;
do the work;

(In reply to comment #12)
> Do you mean:
> 
> if (or_op && integer_onep(tem)) { warn();}
> else if (!or_op && integer_zerop(tem)) { warn();}

Even smaller would be to replace the current (TREE_CODE (tem) != INTEGER_CST)
with integer_zerop(tem) and pass build_range_check in_p^or_op (or in_p==or_op,
don't know which) instead of just in_p. It would already be an improvement over
the current situation, and I expect the remaining false positives to be very
rare. i>=INT_MIN&&i<something or i<INT_MIN||i>something are common, but
i<INT_MIN&&i>something seems less likely.