[LLVMdev] TableGen Enhancement Feasibility

David Greene dag at cray.com
Mon Apr 6 15:11:36 PDT 2009

I've got another idea for a tblgen extension but I don't have a good feel for 
how feasible it is.  Hopefully someone can provide guidance.

What I want to do is something like this:

class C1<int A, string B> {
  int foo = A;
  string bar = B;

class Bb<int A> : C1<A, "foo">;
class Cb<int A> : C1<A, "bar">;

class C2<C1 Base, int A> : Base<A>;

def I1 : C2<Bb, 1>;
def I2 : C2<Cb, 2>;

and this:

class Z<int A> {
  int moo = A;

class Z1 : Z<1>;
class Z2 : Z<2>;

multiclass C2<int A, string B, Z ZZ> {
  def D1 : Bb<A>, ZZ;
  def D2 : Cb<A>, ZZ;

defm D3 : C2<1, Z1>;
defm D4 : C2<2, Z2>;

Right now tblgen doesn't understand class name arguments like this as it 
expects to resolve subclasses immediately.  Being able to resolve subclasses 
lazily would open up a whole lot of opportunity to reduce redundancy in .td 

My sense is to implement this we'd need something like a ClassInit (analogous 
to a VarInit) and a call to resolve references at the right time.  But I don't 
know how deeply coded subclasses are in tblgen.  Are things going to break 
spectacularly if subclass information isn't available immediately?

Anyone have a sense of the amount of work to add a feature like this?


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