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

[Zhenqiang Chen <zhenqiang dot chen at linaro dot org>]

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.

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)

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.

4) Another good thing of Andrew P.'s change is that it reuses some
existing functions. So it looks much simple.

>>
>> 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!
-Zhenqiang