Re: [PATCH] Look through widening type conversions for possible edge assertions

[Patrick Palka <patrick at parcs dot ath dot cx>]

On Tue, Nov 11, 2014 at 8:48 AM, Richard Biener
<richard.guenther@gmail.com> wrote:
> On Tue, Nov 11, 2014 at 1:10 PM, Patrick Palka <patrick@parcs.ath.cx> wrote:
>> This patch is a replacement for the 2nd VRP refactoring patch.  It
>> simply teaches VRP to look through widening type conversions when
>> finding suitable edge assertions, e.g.
>>
>> bool p = x != y;
>> int q = (int) p;
>> if (q == 0) // new edge assert: p == 0 and therefore x == y
>
> I think the proper fix is to forward x != y to q == 0 instead of this one.
> That said - the tree-ssa-forwprop.c restriction on only forwarding
> single-uses into conditions is clearly bogus here.  I suggest to
> relax it for conversions and compares.  Like with
>
> Index: tree-ssa-forwprop.c
> ===================================================================
> --- tree-ssa-forwprop.c (revision 217349)
> +++ tree-ssa-forwprop.c (working copy)
> @@ -476,7 +476,7 @@ forward_propagate_into_comparison_1 (gim
>         {
>           rhs0 = rhs_to_tree (TREE_TYPE (op1), def_stmt);
>           tmp = combine_cond_expr_cond (stmt, code, type,
> -                                       rhs0, op1, !single_use0_p);
> +                                       rhs0, op1, false);
>           if (tmp)
>             return tmp;
>         }
>
>
> Thanks,
> Richard.

That makes sense.  Attached is what I have so far.  I relaxed the
forwprop restriction in the case of comparing an integer constant with
a comparison or with a conversion from a boolean value.  (If I allow
all conversions, not just those from a boolean value, then a couple of
-Wstrict-overflow faillures trigger..)  Does the change look sensible?
 Should the logic be duplicated for the case when TREE_CODE (op1) ==
SSA_NAME? Thanks for your help so far!

Attachment: tmp.diff
Description: Text document