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

[Xinliang David Li <davidxl at google dot com>]

On Mon, Oct 14, 2013 at 2:34 PM, Dehao Chen <dehao@google.com> wrote:
> For my test case, the entire inline instance is optimized away,

do you mean there is no out of line instance for the target function
in the profile binary?

David

> so
> there is no info about it in the profile. I can do some fixup in the
> rebuild_cgraph_edge though.
>
> 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