[Bug tree-optimization/49234] [4.5/4.6/4.7/4.8 Regression] -Wstrict-overflow gives obviously unwarranted warning

[aldyh at redhat dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49234

--- Comment #12 from Aldy Hernandez <aldyh at redhat dot com> 2013-03-01 19:17:32 UTC ---
> --- Comment #11 from Ian Lance Taylor <ian at airs dot com> 2013-03-01 14:52:53 UTC ---
> I suspect we can handle this case by observing that all the incoming values to
> the PHI node are constants.

Ian.

They're not all constants.  In this particular case we have phis like this:

state_2 = PHI <0(6), state_3(12), 2(5)>

I suppose we could chase the SSA def chain and if all the phi arguments 
are either constants, or known to point to an SSA that has been 
previously analyzed as constant, then we can avoid generating an 
overflow.  But we'd have to keep track of states across calls to 
vrp_visit_phi_node.  Is this what you had in mind, or am I 
misunderstanding something?

Obviously, Richi's idea is much simpler :).  With his suggestion we 
could probably do with:

@@ -8111,11 +8109,9 @@ vrp_visit_phi_node (gimple phi)
        if (cmp_max < 0 || cmp_max > 0)
         {
           if (!needs_overflow_infinity (TREE_TYPE (vr_result.max))
-             || !vrp_var_may_overflow (lhs, phi))
+             || !vrp_var_may_overflow (lhs, phi)
+             || supports_overflow_infinity (TREE_TYPE (vr_result.max)))
             vr_result.max = TYPE_MAX_VALUE (TREE_TYPE (vr_result.max));
-         else if (supports_overflow_infinity (TREE_TYPE (vr_result.max)))
-           vr_result.max =
-               positive_overflow_infinity (TREE_TYPE (vr_result.max));
         }

And similarly for MIN.  In the above, supports_overflow_infinity is just 
there to make sure we have a CONSTANT_CLASS_P.  If that's not needed, we 
could even do:

    if (cmp_max < 0 || cmp_max > 0)
      vr_result.max = TYPE_MAX_VALUE (TREE_TYPE (vr_result.max));

and be done with it.

Let me know.  I am willing to entertain any approach y'all suggest.