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Re: [PATCH] decide edge's hotness when there is profile info


On Mon, Oct 14, 2013 at 2:50 PM, Jan Hubicka <hubicka@ucw.cz> wrote:
>> For my test case, the entire inline instance is optimized away, so
>> there is no info about it in the profile. I can do some fixup in the
>> rebuild_cgraph_edge though.
>
> Yep, I understand that.  In this case we should turn PROFILE_READ to PROFILE_GUESSED
> and guess the profile when we detect this (i.e. we have edges with non-0 counts into
> functions with 0 profile).  That should prvent these from getting UNLIKELY_EXECUTED
> and they will be inlined normal way.

Oh, actually in AutoFDO, only functions with samples will be marked as
PROFILE_READ. Others will all be marked as PROFILE_GUESSED.

Dehao

>
> Honza
>>
>> Dehao
>>
>> On Mon, Oct 14, 2013 at 2:27 PM, Xinliang David Li <davidxl@google.com> wrote:
>> > Is it possible to update the callee node summary after profile
>> > annotate (using information from inline instances which are not
>> > inlined in early inline)?
>> >
>> > David
>> >
>> > On Mon, Oct 14, 2013 at 2:18 PM, Dehao Chen <dehao@google.com> wrote:
>> >> On Mon, Oct 14, 2013 at 12:49 PM, Jan Hubicka <hubicka@ucw.cz> wrote:
>> >>>> Not for instrumented FDO (not as I know of). But for AutoFDO, this
>> >>>> could be a potential risk because some callee is marked unlikely
>> >>>> executed simply because they are inlined and eliminated in the O2
>> >>>> binary. But in ipa-inline it will not get inlined because the edge is
>> >>>> not hot from cgraph_maybe_hot_edge_p (because callee is
>> >>>> UNLIKELY_EXECUTED), while the edge->count is actually hot.
>> >>>
>> >>> Can't you prevent setting calle to UNLIKELY_EXECUTED in these cases instead?
>> >>> It seems that having profile set incorrectly will lead to other problems later, too.
>> >>> We discussed similar problem with Teresa about the missing profiles for comdat,
>> >>> basically one should detect these cases as profile being lost and go with guessed
>> >>> profile.  (I believe patch for that was posted, too, and so far it seems best approach
>> >>> to this issue)
>> >>
>> >> The current AutoFDO implementation will take all functions that do not
>> >> have have profile as normally executed, thus use guessed profile for
>> >> it. This is like using profile for truly hot functions, and using O2
>> >> for other functions. This works fine. However, it leads to larger code
>> >> size (approximately 10%~20% larger than FDO).
>> >>
>> >> I'd like to introduce another mode for users who care about both
>> >> performance and code size, and can be sure that profile is
>> >> representative. In this mode, we will mark all functions without
>> >> sample as "unlikely executed". However, because AutoFDO use debug info
>> >> (of optimized code) to represent profile, it's possible that some hot
>> >> functions (say foo) are inlined and fully eliminated into another hot
>> >> function (say bar). So in the profile, bar is cold, and because the
>> >> profile for foo::bar is eliminated, bar will not be inlined into foo
>> >> before the profile annotation. However, after profile annotate, we can
>> >> infer from the bb count that foo->bar is hot, thus it should be
>> >> inlined in ipa-inline phase. However, because bar itself is marked
>> >> UNLIKELY_EXECUTED, it will not be inlined.
>> >>
>> >> One possible workaround would be that during rebuild_cgraph_edges, if
>> >> we find an edge's callee is unlikely executed, add the edge count to
>> >> the callee's count and recalculate callee's frequency.
>> >>
>> >> Dehao
>> >>
>> >>>
>> >>> Honza


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