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Re: [PATCH] Look through widening type conversions for possible edge assertions
- From: Patrick Palka <patrick at parcs dot ath dot cx>
- To: Richard Biener <richard dot guenther at gmail dot com>
- Cc: GCC Patches <gcc-patches at gcc dot gnu dot org>
- Date: Sat, 15 Nov 2014 23:22:26 -0500
- Subject: Re: [PATCH] Look through widening type conversions for possible edge assertions
- Authentication-results: sourceware.org; auth=none
- References: <CA+C-WL-fbKDbNmMr0ah-72QQmYw7O_eY3uKpf5Z7UtMw8-7D4g at mail dot gmail dot com> <1415707831-652-1-git-send-email-patrick at parcs dot ath dot cx> <CAFiYyc0Ved1EyHPA4HXzkrrHeo=JhYJg4d6JZ7AasktoVn=VxA at mail dot gmail dot com> <CA+C-WL928gZXwjR0mXiDOsUWis8AVQ3UNi85TjGieJ8p39HsFw at mail dot gmail dot com> <CAFiYyc2WdT79QZqwpR=fceARqCiYEouBrKf_Lm_dFmCwWpCCsw at mail dot gmail dot com>
On Wed, Nov 12, 2014 at 3:38 AM, Richard Biener
<richard.guenther@gmail.com> wrote:
> On Wed, Nov 12, 2014 at 5:17 AM, Patrick Palka <patrick@parcs.ath.cx> wrote:
>> 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!
>
> It looks good though I'd have allowed all kinds of conversions, not only
> those from booleans.
>
> If the patch tests ok with that change it is ok.
Sadly changing the patch to propagate all kinds of conversions, not
only just those from booleans, introduces regressions that I don't
know how to adequately fix.