[LLVMdev] HazardRecognizer and RegisterAllocation

[David Greene]

On Monday 19 January 2009 18:21, Dan Gohman wrote:

> > Dan, how does the scheduler handle memory dependence?  I'm working on
> > something that requires memory dependence information for
> > MachineInstructions.
>
> At the moment, it knows simple things, like constant pool loads
> don't have dependencies, and references to distinct stack slots are
> independent, and so on.

Ok.

> I have a few ideas for how more precise memory dependencies might be
> achieved.
>
> We have MachineMemOperands, which can be used to make AliasAnalysis
> or other LLVM-IR-level analysis queries. They need some work though;
> the main issue is that there are some places in codegen that don't
> preserve them.

Where are those places?  Can they be used in conjunction with 
MemoryDependenceAnalysis?  e.g. can we write a MachineInstructions-based 
memory dependence analysis that uses MachineMemoryOperands?

> Another possibility is to record dependence information from the
> SelectionDAG in MachineInstrs somehow. We don't yet have precise
> memory dependencies in the SelectionDAG, but it would be good to
> fix that too :-). 

Agreed.

> This would probably also involve AliasAnalysis  queries from codegen, 
> possibly going though the   MemoryDependenceAnalysis interface.

Do you have a vision for how this might work?  Wouldn't we need a new 
MachineFunctionPass to essentially do the same thing as 
MemoryDependenceAnalysis?

I don't think it's sufficient to just preserve the information we had from 
Instructions.  Codegen might introduce new memory operations after lowering 
(spilling, for example).  Some of these might be easily analyzable (spills) 
but others might not be.

But maybe we don't need to worry about that right now.  As I think about the 
problem I'm working on, "merely" preserving dependence information from 
Instructions would help.  It seems like if we can preserve that information in 
SelectionDAG we ought to be able to record it in MachineInstructions (or 
MachineMemOperands?) when lowering.

Hmm...then again looking at the MachineMemOperand documentation, fixing the 
places that invalidate those might work well too.  I'm a little wary of having 
the same information in two different places.

                                           -Dave