[LLVMdev] Regalloc Refactoring

[David Greene]

Evan Cheng wrote:

> While I agree spill cost computation does not belong in coalescer, I  
> am not sure if it should go into a separate pass. To me, spill cost  
> formulas should be register allocator specific. Perhaps it ought to  
> belong to a generic register allocator class. Each derivative  
> allocator is responsible for overriding it and calling it if it deems  
> necessary.

I'm actually fairly nervous about using inheritance this way.  I know
it's done like this all over LLVM but the Template Method pattern is
often better.  You don't _really_ need dynamic polymorphism for things
like this.  Static polymorphism is sufficient and it has the advantage
of more flexibility for reuse (doesn't depend on a specific base class).

For example, if I'm writing a register allocator, one way to do what
you're saying without inheritance is to parameterize the allocator
with a traits class and call into that for the custom routines:

template<class RegallocTraits>
class GCRegAlloc {
    void doAlloc() {
       ...
       RegallocTraits::computeSpillCost(someValue);
       ...
    };
};

An alternative to think about during these discussions.

                              -Dave