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


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?

> 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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