[llvm-dev] RFC: Synthetic function entry counts

Fri Dec 15 11:13:20 PST 2017

On Fri, Dec 15, 2017 at 10:22 AM, Easwaran Raman <eraman at google.com> wrote:

>
>
> On Fri, Dec 15, 2017 at 12:22 AM, Sean Silva <chisophugis at gmail.com>
> wrote:
>
>> IIUC, this proposal is just saying that we should infer a static profile
>> for entry counts just like we do for branch probabilities. In the case of
>> entry counts, we do not hide that information behind an analysis like BPI,
>> so currently just annotating synthetic PGO entry counts is a simple
>> solution that piggybacks on the PGO mechanism and Just Works.
>>
>> If that is correct then this makes perfect sense to me.
>>
> Yes, that is the proposal.
>
>>
>> It could be argued that we ought to refactor things so that the raw PGO
>> metadata is only ever accessed via a wrapper CGSCC analysis that falls back
>> to interprocedural analysis (i.e. static profile heuristics) when the entry
>> count metadata is missing, just like BPI does with static intraprocedural
>> analysis and branch weight metadata. However, we probably don't want to do
>> that while folks are still depending on the old PM in production since
>> CGSCC analyses don't exist there which would force us to maintain an old
>> and new way of doing it.
>>
>> (Also, it sounds like you want to compute this with a top-down CGSCC
>> traversal, so it might not actually be computable incrementally as a bottom
>> up CGSCC analysis which is what CGSCC analyses currently do; an auxiliary
>> module analysis for the top-down part might work around this though)
>>
>
>>
> Wrapping function entry counts behind an analysis might be doable but
> requires more careful thought.
>

Definitely.

> Right now getEntryCount is called from various passes: inliner, loop
> passes (unroll, loop sink), codegenprepare (for function layout), machine
> block placement, module summary analysis ... This might be worth attempting
> when we fully migrate to the new pass manager but not now.
>

It seems like many of those places that directly reference getEntryCount
are using it to check "HasProfileData". So adding synthetic values would
cause us to take a lot of code paths in the compiler that were intended to
use real profile data.

As a simple example, MachineBlockPlacement::collectLoopBlockSet uses MBFI
and MBPI guarded by a check of getEntryCount to see if PGO data is present
(the comment even says "This needs precise profile data and we only do this
when profile data is available."). Do we need to refactor things first to
use a more semantically meaningful check, like "hasRealProfileData" so that
synthetic counts don't fool this code into thinking it has real profile
data? At least, comments like the one I quoted need to be updated to
indicate that a static profile is considered precise enough.

Also, one other question: in the case of FullLTO, how do we ensure that the
synthetic profile data from static heuristics is consistent across TU's? Do
we need to recalculate the synthetic profile data? If we need to
recalculate it, presumably we need to know which profile data is synthetic
or not. E.g. in a scenario where most TU's are compiled with profile data,
but some are not (e.g. vendor or runtime lib delivered as bitcode),
recalculated synthetic counts at FullLTO time should honor real profile
counts but ignore synthetic counts from per-TU compilation.

The PGO+FullLTO case with a runtime lib delivered as bitcode is not
hypothetical; this was actually a common configuration used in production
while I was at PlayStation. CC'ing Davide, River to confirm that they still
care about this case.

Also, more generally, can you clarify how your static entry count
calculation algorithm is expected to work in the presence of partial
profile data?

-- Sean Silva

>
> Also, the need to run this logic (or similar logic) as a "ThinLTO
>> analysis" suggests not wedding it too much with the intricacies of the
>> IR-level pass management (although admittedly we already do that with the
>> inliner and then ThinLTO has to approximate those inlining decisions, so it
>> might not be the end of the world to have some divergence).
>>
>> -- Sean Silva
>>
>> On Dec 12, 2017 5:02 PM, "Easwaran Raman via llvm-dev" <
>> llvm-dev at lists.llvm.org> wrote:
>>
>>> Functions in LLVM IR have a function_entry_count metadata that is
>>> attached in PGO compilation. By using the entry count together with the
>>> block frequency info, the compiler computes the profile count of call
>>> instructions based on which the hotness/coldness of callsites can be
>>> determined. Experiments have shown that using a higher threshold for hot
>>> callsites results in improved runtime performance of the generated code
>>> without significant code size increases. We propose to generate synthetic
>>> function counts for non-PGO compilation and use the counts for boosting hot
>>> callsites during inlining.
>>>
>>> Synthetic function entry counts of functions are initialized based on
>>> properties of the function such as whether it is visible outside the
>>> module, whether it has an inline keyword and so on. Then, the callgraph SCC
>>> is traversed in reverse post-order. Counts of callsites are determined
>>> based on the entry count and the block frequency of the callsite. The
>>> callsite count gets added to the entry count of the callee. For targets of
>>> indirect calls, we will use the !callees metadata to find the possible
>>> targets and distribute the count equally among them. For functions in a
>>> non-trivial SCC, the algorithm has to ensure that the counts are stable and
>>> deterministic.
>>>
>>> In ThinLTO mode, the function summary contains the list of call edges
>>> from the function. We propose to add the relative block frequency on these
>>> edges. During the thinlink phase, we propagate the function counts on the
>>> entire call graph and update the function summary with the synthetic
>>> counts. Additionally, we plan to use the computed counts to drive the
>>> importing decisions.
>>>
>>> Alternative approach
>>>
>>> -----------------------------
>>>
>>>
>>> An alternative to generating synthetic counts is to make block frequency
>>> info an inter-procedural analysis. Such an analysis would allow comparing
>>> BFI of callsites in two different functions. This has several downsides:
>>>
>>>    -
>>>
>>>    The inter-procedural BFI computation is likely to be more expensive
>>>    in terms of compile-time.
>>>    -
>>>
>>>    Many function passes invalidate the BFI. This will require selective
>>>    invalidation of function BFIs.
>>>    -
>>>
>>>    Inliner correctly updates function counts of a callee after a
>>>    callsite is inlined. We can piggyback on this mechanism by using synthetic
>>>    counts.
>>>
>>>
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> llvm-dev at lists.llvm.org
>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>
>>>
>
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