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Re: [PATCH] Enhance ifcombine to recover non short circuit branches


On 18 October 2013 18:15, Richard Biener <richard.guenther@gmail.com> wrote:
> On Fri, Oct 18, 2013 at 12:06 PM, Zhenqiang Chen
> <zhenqiang.chen@linaro.org> wrote:
>> On 18 October 2013 17:53, Richard Biener <richard.guenther@gmail.com> wrote:
>>> On Fri, Oct 18, 2013 at 11:21 AM, Zhenqiang Chen
>>> <zhenqiang.chen@linaro.org> wrote:
>>>> On 18 October 2013 00:58, Jeff Law <law@redhat.com> wrote:
>>>>> On 10/17/13 05:03, Richard Biener wrote:
>>>>>>>>
>>>>>>>> Is it OK for trunk?
>>>>>>>
>>>>>>>
>>>>>>> I had a much simpler change which did basically the same from 4.7 (I
>>>>>>> can update it if people think this is a better approach).
>>>>>>
>>>>>>
>>>>>> I like that more (note you can now use is_gimple_condexpr as predicate
>>>>>> for force_gimple_operand).
>>>>>
>>>>> The obvious question is whether or not Andrew's simpler change picks up as
>>>>> many transformations as Zhenqiang's change.  If not are the things missed
>>>>> important.
>>>>>
>>>>> Zhenqiang, can you do some testing of your change vs Andrew P.'s change?
>>>>
>>>> Here is a rough compare:
>>>>
>>>> 1) Andrew P.'s change can not handle ssa-ifcombine-ccmp-3.c (included
>>>> in my patch). Root cause is that it does not skip "LABEL". The guard
>>>> to do this opt should be the same the bb_has_overhead_p in my patch.
>>>
>>> I think we want a "proper" predicate in tree-cfg.c for this, like maybe
>>> a subset of tree_forwarder_block_p or whatever it will end up looking
>>> like (we need "effectively empty BB" elsewhere, for example in vectorization,
>>> add a flag to allow a condition ending the BB and the predicate is done).
>>>
>>>> 2) Andrew P.'s change always generate TRUTH_AND_EXPR, which is not
>>>> efficient for "||". e.g. For ssa-ifcombine-ccmp-6.c, it will generate
>>>>
>>>>   _3 = a_2(D) > 0;
>>>>   _5 = b_4(D) > 0;
>>>>   _6 = _3 | _5;
>>>>   _9 = c_7(D) <= 0;
>>>>   _10 = ~_6;
>>>>   _11 = _9 & _10;
>>>>   if (_11 == 0)
>>>>
>>>> With my patch, it will generate
>>>>
>>>>   _3 = a_2(D) > 0;
>>>>   _5 = b_4(D) > 0;
>>>>   _6 = _3 | _5;
>>>>   _9 = c_7(D) > 0;
>>>>   _10 = _6 | _9;
>>>>   if (_10 != 0)
>>>
>>> But that seems like a missed simplification in predicate combining
>>> which should be fixed more generally.
>>>
>>>> 3) The good thing of Andrew P.'s change is that "Inverse the order of
>>>> the basic block walk" so it can do combine recursively.
>>>>
>>>> But I think we need some heuristic to control the number of ifs. Move
>>>> too much compares from
>>>> the inner_bb to outer_bb is not good.
>>>
>>> True, but that's what fold-const.c does, no?
>>
>> Based on current fold-const, we can not generate more than "two"
>> compares in a basic block.
>
> I think you are wrong as fold is invoked recursively on a && b && c && d.

Please try to compile it with -fdump-tree-gimple. It will be broken into

 if (a && b)
   if (c && d)

>> But if ifs can be combined recursively in ifcombine, we might generate
>> many compares in a basic block.
>
> Yes, we can end up with that for the other simplifications done in
> ifcombine as well.  Eventually there needs to be a cost model for this
> (use edge probabilities for example, so that if you have profile data
> you never combine cold blocks).
>
> Any takers?

I will take it.

Thanks!
-Zhenqiang

> Richard.
>
>>>> 4) Another good thing of Andrew P.'s change is that it reuses some
>>>> existing functions. So it looks much simple.
>>>
>>> Indeed - that's what I like about it.
>>>
>>>>>>
>>>>>> With that we should be able to kill the fold-const.c transform?
>>>>>
>>>>> That would certainly be nice and an excellent follow-up for Zhenqiang.
>>>>
>>>> That's my final goal to "kill the fold-const.c transform". I think we
>>>> may combine the two changes to make a "simple" and "good" patch.
>>>
>>> Thanks,
>>> Richard.
>>>
>>>> Thanks!
>>>> -Zhenqiang


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