Saturday, July 25, 2015

Runtime Generic Dispatch

As promised, doing some of the basic compile time generic specialization I showed last time during runtime was next on the list. It is now done. The following test code will now properly dispatch at runtime based on the type of the parameters passed into bar:

foo (x: (T)) => void {
  print "in inference";

foo (x: int) => void {
  print "in int.";

bar (x: (T)) => void {
  foo x;

entrypoint => void {
  bar "bar";
  bar 42;

This though had a bit of a sticking point. Visual Studio compiles the basic type check as something like this:

  IL_0001:  ldtoken    [mscorlib]System.Int32
  IL_0006:  call       class [mscorlib]System.Type [mscorlib]System.Type::GetTypeFromHandle(valuetype [mscorlib]System.RuntimeTypeHandle)
  IL_000b:  ldarga.s   'value'
  IL_000d:  constrained. !!fooT
  IL_0013:  callvirt   instance class [mscorlib]System.Type [mscorlib]System.Object::GetType()
  IL_0018:  call       bool [mscorlib]System.Type::op_Equality(class [mscorlib]System.Type, class [mscorlib]System.Type)

Note this weird constrained. prefix. It is some magic that lets CIL call generic parameters without boxing them. The problem comes with the ldarga.s instruction above it. Instead of pulling out the parameter value like everything else under the sun, this is getting the address of that parameter. If you don't get the address and make that call, you get some nondescript ExecutionEngineException during runtime. Awesome.

The problem for me was that at that point of the compilation process, I don't know the parameter's index since it was abstracted a few layers above. So for now, Tangent just boxes the thing and goes about its business.

I'm not sure what is next on the docket. Perhaps improving this to allow partial specialization, perhaps local variables, perhaps .NET imports. We'll see what strikes my fancy.

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