[PATCH] Add a flag to experiment with outlining optional branches.

[Xinliang David Li]

As I mentioned in the previous comment, LLVM's current block placement does
support outlining as in the following case:

START_A
if (__builtin_expect(cond,0)) {
        UNLIKELY_B
}
JOIN_C

The layout order will be START_A JOIN_C UNLIKELY_B

It is just in order to break the topo-order, it requires a very strongly
biased branch (the cold target < 20% )

The alternatives include:
1) tune the threshold to break topo order
2) improve the static prediction in llvm.

David


On Wed, Mar 4, 2015 at 12:17 AM, Daniel Jasper <djasper at google.com> wrote:

> Ah right.
>
> Well, it might be too limiting, but it might be a good incremental step.
> Any further suggestions are definitely welcome.
>
> On Wed, Mar 4, 2015 at 9:04 AM, Xinliang David Li <xinliangli at gmail.com>
> wrote:
>
>> The example mentioned has a diamond shape. Besides within the 'then' arm
>> of cond1 branch, there could be multiple consecutive 'if (cond_x) B_x'
>> where B_x blocks are cold. The heuristic in your patch seems to be too
>> limiting.
>>
>> David
>>
>> On Tue, Mar 3, 2015 at 11:19 PM, Daniel Jasper <djasper at google.com>
>> wrote:
>>
>>> Yes. The reasoning is that in this case the entire branch (containing A,
>>> B and C) will already be outlined and we don't want to fragment this
>>> further. In initial benchmarks, this seemed to have a slight benefit over
>>> additionally outlining B. Lots of stuff we can investigate, though.
>>>
>>> On Wed, Mar 4, 2015 at 8:03 AM, Xinliang David Li <xinliangli at gmail.com>
>>> wrote:
>>>
>>>> If I read the change correctly,  'B' block in the following case can
>>>> not be outlined. Is it intended?
>>>>
>>>> David
>>>>
>>>> if (cond1) {
>>>>     A
>>>>     if (cond2) {
>>>>
>>>>          B
>>>>      }
>>>>     C
>>>>     ...
>>>> }
>>>> else {
>>>>    D
>>>> }
>>>>
>>>> On Tue, Mar 3, 2015 at 10:46 PM, Daniel Jasper <djasper at google.com>
>>>> wrote:
>>>>
>>>>> Re-add the test.
>>>>>
>>>>>
>>>>> http://reviews.llvm.org/D7719
>>>>>
>>>>> Files:
>>>>>   lib/CodeGen/MachineBlockPlacement.cpp
>>>>>   test/CodeGen/X86/code_placement_outline_optional_branches.ll
>>>>>
>>>>> EMAIL PREFERENCES
>>>>>   http://reviews.llvm.org/settings/panel/emailpreferences/
>>>>>
>>>>> _______________________________________________
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>>>>>
>>>>>
>>>>
>>>
>>
>
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