[llvm-dev] Polly as an Analysis pass in LLVM

[Hongbin Zheng via llvm-dev]

In the design the LLVM passes always directly communicate with PolyhedralInfo,
this requires Polly tightly integrate in to LLVM.

If we do not want a tight integration, we can do the following:
1. Introduce an abstract memory dependency query interface, like
AliasAnalysis
2. I remember LLVM had already have dependency analysis, this can be the
default implementation of the memory dependency query interface. Or the
default implementation can be a "may depend" analysis.
3. PolyhedralInfo is yet another implementation of memory dependency
 analysis.

That is:

LLVM Passes --> DependencyAnalysis ---> Default implementation
                                                           \
                                                            \---->
Implementation in LLVM
                                                             \
                                                              ----> Polyhedral
dependency analysis in Polly



On Thu, Mar 24, 2016 at 5:05 PM, Johannes Doerfert <
doerfert at cs.uni-saarland.de> wrote:

> On 03/23, Hongbin Zheng wrote:
> > Hi Johannes,
> >
> > On Mon, Mar 21, 2016 at 6:35 PM, Johannes Doerfert <
> > doerfert at cs.uni-saarland.de> wrote:
> >
> > > Hey Utpal,
> > >
> > > First of, I think you made nice process here and have some very good
> > > ideas of what we could do in the future.
> > >
> > > [NOTE: I CC'ed some people that have shown interest in this topic but I
> > > might have forgotten some, therefor I also added the llvm-dev list.]
> > >
> > > For the upcoming GSoC proposal we should slow down a little bit and
> > > reevaluate our goals. After talking to a couple of LLVM and Polly folks
> > > at EuroLLVM last week, I hope to have a fairly good idea of how to
> > > proceed. To this end, I will give you my personal road map that might
> be
> > > a good start for the proposal too, though it is not the only way we
> > > could do this:
> > >
> > >   1) Make ScopInfo & DependenceInfo function passes to avoid the
> > >      RegionPassManager for these Polly analysis parts. [This doesn't
> > >      need to be the first step but it should be done at some point.]
> > >   2) Create a secondary ScopDetection & ScopInfo that is restricted to
> > >      analyze a single loop. We might just create a dummy region for
> this
> > >      loop and use the original ScopDetection & ScopInfo. The goal is to
> > >      make (this secondary) ScopDetection & ScopInfo demand driven and
> > >      non-dependent on the RegionInfo analysis. [Same as before, this
> > >      does not need to happen right away.]
> > >
> > I really like this direction. In general, we may want to decouple the
> > ScopDetection/ScopInfo construction logic from the pass logic, such that
> we
> > can run the logic ScopDetection and ScopInfo construction in a function
> > pass, call graph scc pass, or even a loop pass.
> Totally agreed. Some kind of ScopBuilder interface that can be queried
> would be perfect. Additionally a DependenceBuilder.
>
> I would imagine something along the lines of:
>
>
>    Passes:   ScopInfo <-----------[depends]----------- DependenceInfo
>                  |                                              |
>              [queries]                                      [queries]
>                  |                                              |
>                  V                                              V
> Interface:  ScopBuilder                          ------ DependenceBuilder
>               A  |                               |              |       A
>               |  |                               |          [creates]   |
>               |  |                               |              |       |
>               |  |                               |              V       |
>   Objects:    |  |--[creates]--> Scop <--[uses]--|         Dependences  |
>               |                    |                            |       |
>               |-----[queries]--|   |-----[uses]--| |---[uses]---|       |
>                                |                 V V                    |
> New Pass/Interface:            |----------- PolyhedralInfo --[queries]--|
>                                                   A
>                                                   |
>                                                   |
>                                       [non-polyhedral queries]
>                                                   |
>                                                   |
>                                            **LLVM Passes**
>
> > For 1), can we always construct a whole function Scop (excluding the
> entry
> > block which contains allocas)?
> We could even with allocas ;)
>
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