[llvm-dev] AutoFDO sample profiles v. SelectInst,

[Xinliang David Li via llvm-dev]

On Wed, Aug 17, 2016 at 10:06 AM, Sanjay Patel <spatel at rotateright.com>
wrote:

>
>
> On Wed, Aug 17, 2016 at 10:12 AM, Xinliang David Li <xinliangli at gmail.com>
> wrote:
>
>>
>>
>> On Wed, Aug 17, 2016 at 8:19 AM, Sanjay Patel <spatel at rotateright.com>
>> wrote:
>>
>>>
>>> On Fri, Aug 12, 2016 at 12:15 PM, Xinliang David Li via llvm-dev <
>>> llvm-dev at lists.llvm.org> wrote:
>>>
>>>> +dehao.
>>>>
>>>> There are two potential problems:
>>>>
>>>> 1) the branch gets eliminated in the binary that is being profiled, so
>>>> there is no profile data
>>>>
>>>
>>> This seems like a fundamental problem for PGO. Maybe it is also
>>> responsible for this bug: https://llvm.org/bugs/show_bug.cgi?id=27359 ?
>>>
>>> Should we limit select optimizations in IR for a PGO-training build? Or
>>> should there be a 'select smasher' pass later in the pipeline that turns
>>> selects into branches for a PGO-training build? (I don't have a good
>>> understanding of PGO, so I'm just throwing out ideas...maybe a better
>>> question is: how do other compilers handle this?)
>>>
>>>
>> For instrumentation based PGO (IR-based), this is a known problem. I have
>> a solution for it and will send out patches soon. Before that, there will
>> be more changes in LLVM to make sure profile data associated with
>> selectInst is well preserved.
>>
>
> Sounds great. Let me know if I can help without getting in your way. If
> there's more like https://reviews.llvm.org/D23590 , I can try to fix them
> up in parallel.
>


yes there might be more missing cases -- I only did manual audition so
there is no guarantee to be exhaustive. Other passes may be dropping
profile data too. What we need is to  introduce a verification pass (that
can be inserted after any given pass just like IR dumping) to do sanity
checking: if any branch/selectInst/switchInst has dropped branch profile
data (with PGO is on), emits some warning. For machine inst passes, we need
something similar.  More elaborate check can also be added in the future to
check the integrity of the profile data -- some pass such as
jump-threading, switch lowering needs complicated update and things can go
wrong there.

If you can help with that (verification), that will be great :)


>
> On a related note, I want to ask about profile-guided inlining. It does
> not seem to exist after https://reviews.llvm.org/D16381 was reverted. Is
> there a plan to bring it back independently of the new pass manager?
>
>
yes there is a plan. We will update the status early next month.



> Profile-guided inlining was the original motivation for the test case in
> https://llvm.org/bugs/show_bug.cgi?id=28964 . But I think we'll miss this
> case if we fix SimplifyCFG to produce a 'select' before fixing IR-based PGO
> (and making inlining work again)?
>
>

it should not. After the selectInst  is produced, the calll will be in the
block outside the original if-then-else so its profile data/hotness should
not be affected.

David
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