[PATCH] D10979: Update the branch weight metadata in JumpThreading pass.

Cong Hou congh at google.com
Mon Jul 13 14:39:27 PDT 2015


On Thu, Jul 9, 2015 at 4:16 PM, Xinliang David Li <davidxl at google.com> wrote:
> On Thu, Jul 9, 2015 at 2:54 PM, Duncan P. N. Exon Smith
> <dexonsmith at apple.com> wrote:
>>
>>> On 2015-Jul-09, at 14:28, Cong Hou <congh at google.com> wrote:
>>>
>>> On Tue, Jul 7, 2015 at 4:53 PM, Duncan P. N. Exon Smith
>>> <dexonsmith at apple.com> wrote:
>>>>
>>>>> On 2015-Jul-07, at 13:24, Xinliang David Li <davidxl at google.com> wrote:
>>>>>
>>>>> On Tue, Jul 7, 2015 at 10:39 AM, Duncan P. N. Exon Smith
>>>>> <dexonsmith at apple.com> wrote:
>>>>>>
>>>>>>> On 2015-Jul-06, at 17:22, Cong Hou <congh at google.com> wrote:
>>>>>>>
>>>>>>>
>>>>>>> congh added reviewers: chandlerc, davidxl.
>>>>>>> congh added a subscriber: llvm-commits.
>>>>>>>
>>>>>>> Currently in JumpThreading pass, the branch weight metadata is not updated after CFG modification. Consider the jump threading on PredBB, BB, and SuccBB. After jump threading, the weight on BB->SuccBB should be adjusted as some of it is contributed by the edge PredBB->BB, which doesn't exist anymore. This patch tries to update the edge weight in metadata on BB->SuccBB by scaling it by 1 - Freq(PredBB->BB) / Freq(BB->SuccBB). Two more analyses (BlockFrequencyInfo and BranchProbabilityInfo) are needed then.
>>>>>>
>>>>>> Thanks for working on this!
>>>>>>
>>>>>> Generally you don't need these analyses to keep branch weights
>>>>>> up-to-date.  The design premise is that you don't need global
>>>>>> information for local updates.
>>>>>>
>>>>>
>>>>> This is true in general, but probably not the case for jump-threading.
>>>>> When a new thread is formed (from NewBB to SuccBB), the  profile
>>>>> update delta to the original edge BB->SuccBB comes from a different
>>>>> edge (Pred->NewBB which is inherited from Pred->BB). Local update by
>>>>> only looking at BB seems impossible.
>>>>
>>>> Ah, I see.   Yes, you need a global view to know predecessor
>>>> probabilities.
>>>>
>>>>>
>>>>>> You should be able to calculate the new !prof attachment based on the
>>>>>> old ones, without running BFI.  (I'm skeptical of even running BPI -- if
>>>>>> there's no !prof attachment on the old block, then you have no real
>>>>>> information; what's the benefit in generating a new !prof attachment
>>>>>> based on heuristics?  The new CFG will come with its own new
>>>>>> heuristics.)
>>>>>>
>>>>>
>>>>> I believe Cong's fix is targeting PGO.
>>>>
>>>> Cong, I have a few concerns with the current approach.
>>>>
>>>> Firstly, this requires BFI even when there is no profile data (as David
>>>> also noted in one of the Phabricator emails).  I don't think it's
>>>> reasonable to require all users of jump threading to run an analysis
>>>> that will never be used.  One option would be to `addOptional<>`, but
>>>> I'm not sure that'll get you what you want.
>>>>
>>>> To prevent everyone paying for profile data, we should do the following:
>>>> if the basic block in question has branch weights, then (and only then)
>>>> retrieve the BFI analysis so they can be updated.  I think you'll either
>>>> have to port the analysis retrieval and caching logic from the new
>>>> `PassManager` over to `LegacyPassManager`, or wait for it to be used in
>>>> tree.
>>>
>>> It seems at this point there isn't a good way to conditionally add a
>>> pass dependency to another one, unless the new PassManager is
>>> launched. Right?
>>
>> Right.  AFAICT, you'll either have to port the logic over to the
>> `LegacyPassManager`, or wait for (or help with!) the new one.
>
> If we refactor BPI and BFI code  so that they are simply function pass
> wrappers to bpi/bfi utility class, then those utility analysis can be
> invoked on demand and conditionally. Is that something worth doing?

This is a good idea. Do we already have similar passes (or utility
classes) in LLVM?


Cong

>
>
>>
>>>> (On a related point, I doubt users of PGO + jump threading with partial
>>>> profiles (e.g., JITs) care enough about fidelity to justify running BFI
>>>> for this edge.  IMO, this update should be configurable.)
>>>>
>>>> Secondly, the current code requires BFI and then immediately invalidates
>>>> it, but it should be straightforward to incrementally update (preserve)
>>>> BFI here.
>>>
>>> BFI doesn't provide any interface to incrementally update itself.
>>> There isn't a way to directly access and modify the frequencies stored
>>> in BlockFrequencyInfoImpl.
>>
>> Fortunately, we have the source code for BFI ;).  I think this
>> should be trivial to add.  IIRC there's some sort of `DenseMap<>`
>> with the frequencies there.  Patches welcome!
>
> The incremental update interfaces will be useful in other contexts too.
>
> David
>
>
>>
>>> I think maybe we need to update it for the
>>> whole function (though it is expensive)?
>>
>> Wouldn't you just need to (1) modify `BB` and (2) create `NewBB`?
>>




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