[LLVMdev] [PATCH] Re: Pluggable Register Coalescers

[Evan Cheng]

On Jul 17, 2007, at 8:49 PM, David A. Greene wrote:

> On Tuesday 17 July 2007 14:21, David Greene wrote:
>
>>> I don't care for a MachineFunctionPass that can be directly  
>>> called. I
>>> think it's a very good idea to keep the coalescers independent from
>>> the allocators. If that's desired, we should enhance passmanager so
>>> each allocator can run some sub-passes as part of the allocation
>>> pass.
>>
>> I agree that this is a good long-term goal.  Unfortunately, I  
>> don't have
>> the time resources to implement it right now.  If someone else can  
>> take up
>> this as a sub-project, that would be great!  I agree that several  
>> other
>> passes could make use of it.
>
> I've got something in the works that provides an interface for the  
> coalescer
> to query an interference object.  So the RegisterAllocator class no  
> longer
> exists and coalescers don't directly depend on register  
> allocators.  As a
> bonus, the interference interface doesn't make any assumptions about
> the underlying representation but has the interfaces necessary to make
> equivalent queries to what a coalescer might ask an interference  
> graph.

Great!

> As a proof of concept, I have an example of what the linear scan  
> algorithm
> might provide.  It's very inefficient and in practice linear scan  
> probably
> won't ever interact with a coalescer that partners with the  
> allocator but
> it serves to show that the interface can be met by something that  
> doesn't
> use a graph.

Is this something you are going to contribute back to the repository?

>
> I've also implemented a graph-based realization using what I  
> currently have,
> but as I said in another message, I'm not at liberty to release  
> that publicly.
> It's pretty straightforward code, though.

Ok.


> The RegisterCoalescer interface still exists because register  
> allocators need
> to be able to invoke the coalescer, sometimes repeatedly.   
> PassManager can't
> do that.  The current SimpleRegisterCoalescer (which should really  
> be named
> AggressiveRegisterCoalescer -- another patch on the way) inherits  
> from both
> MachineFunctionPass and RegisterCoalescer.  The former because it  
> is run and
> managed by PassManager and the latter because it needs to belong to  
> the
> RegisterAllocator AnalysisGroup, similar to how alias analysis  
> works.  We need
> a way for register allocators to ask PassManager for the coalescer  
> object
> (even if the coalescer doesn't inherit from MachineFunctionPass)  
> and this is
> the only way I know how to do it until someone takes on revamping how
> PassManager works and providing the capability of constructing  
> hierarchies
> of PassManagers.

Ok. I am willing to be flexible if it helps moving things along. Just  
make sure you pick decorative names for the entry points. For  
example, rather than invoke(), use something like CoalesceFunction().  
And please try to limit the number of such entries.

>
> I'm still working out getting things to build, etc. and will post a  
> patch when
> I've got through all that.  But does this sound like something  
> closer to what
> we want?

Ok. Thanks!

Evan

>
>                                                 -Dave
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