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

[Xinliang David Li]

Many analysis passes in LLVM are implemented as 'XXXWrapperPass' where
XXX is the analysis info which is decoupled from FunctionPass. For
instance,

class DominatorTreeWrapperPass : public Function Pass {
   DominatorTree DT;                  // <<--- this is where the
implementation is sitting.
   ..
};


Before you go by this route, please run by Duncan to make sure he is
ok with this ..

David


On Mon, Jul 13, 2015 at 2:39 PM, Cong Hou <congh at google.com> wrote:
> 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`?
>>>