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

[manu at gcc dot gnu.org]

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

Manuel López-Ibáñez <manu at gcc dot gnu.org> changed:

           What    |Removed                     |Added
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                 CC|                            |manu at gcc dot gnu.org

--- Comment #9 from Manuel López-Ibáñez <manu at gcc dot gnu.org> 2013-02-28 20:40:55 UTC ---
(In reply to comment #3)
> We hit:
> 163724    rguenth       /* Similarly, if the new maximum is smaller or larger
> than
> 163724    rguenth        the previous one, go all the way to +INF.  */
> 163724    rguenth       if (cmp_max < 0 || cmp_max > 0)
> 163724    rguenth       {
> 163724    rguenth         if (!needs_overflow_infinity (TREE_TYPE
> (vr_result.max))
> 163724    rguenth             || !vrp_var_may_overflow (lhs, phi))
> 163724    rguenth           vr_result.max = TYPE_MAX_VALUE (TREE_TYPE
> (vr_result.max));
> 163724    rguenth         else if (supports_overflow_infinity (TREE_TYPE
> (vr_result.max)))
> 163724    rguenth           vr_result.max =
> 163724    rguenth               positive_overflow_infinity (TREE_TYPE
> (vr_result.max));
> 163724    rguenth       }
> 

(In reply to comment #8)
> 
> Does this seem like an approach worth exploring (this silences the warning), or
> does anyone have a better suggestion?

Isn't the problem that vrp_var_may_overflow returns true even though 'state'
cannot overflow? Jakub says:

> As the IV (i) might overflow, vrp_var_may_overflow returns true.
> In particular, chrec is SCEV_NOT_KNOWN.  Thus it just in case sets
> vr_result.max to +INF(OVF) and later on we warn about it.
> Before hitting this code, vr_result contains the right range [0, 2], but it
> doesn't know it can safely use that.

Couldn't be possible to detect this by the fact that 'state' does not depend on
anything variable?

Also, in such a case, the algorithm cannot iterate more than the number of phi
nodes in the loop (if I understand the VRP correctly, which I most likely
don't).

But I looked around and I honestly don't know how to implement this idea.

In any case, your patch would need to adjust the code for the minimum also, no?
Because the same behaviour can be triggered just by using negative numbers to
trigger a negative overflow infinity.