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

[Duncan P. N. Exon Smith]

> 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.

>> (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!

> 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`?