Dynamic Dispatch

[bitRAKE]

Sometimes we want to expose a complex interface with reduced communication bandwidth - similar to a demultiplexer in electronics. Compilers typically generate a vector table of functions and expose a this pointer for external use. We also see this in COM, where an object pointer leads to a vector table of functions and object data.

Another technique eliminates the vector table by spacing functions and just exporting function indices. This is particularly advantageous in assembly - given the control available. Here is a macro to simplify execution of this model.

CODE_ALIGNMENT := 32 ; bytes

struc(name) MakeFunctions
	match =?end?,name
		; note: last function doesn't need a tail
		; only one array of functions
		restruc MakeFunctions
	else
		if ~ definite FirstFunction
			FirstFunction := 1

			FunctionMax = 0
			postpone
				match =?end?,PreviousFunction
				else match any,PreviousFunction
					err 10,'"?End MakeFunctions" required to terminate function list (',\
					`any,' is last function)'
				end match
				FunctionSize := (FunctionMax + (CODE_ALIGNMENT-1)) and (0-CODE_ALIGNMENT)
			end postpone

			macro CallFunction value*
				imul eax,value,FunctionSize
				lea rcx,[name] ; RIP relative
				add rax,rcx
				call rax
			end macro
			align CODE_ALIGNMENT
		else
			.code_bytes := $ - PreviousFunction
			if FunctionMax < .code_bytes
				FunctionMax = .code_bytes
			end if
; TODO: dynamic code fill
			db FunctionSize - .code_bytes dup (90h)
		end if
		name:
	end match
	PreviousFunction reequ name
end struc

[bitRAKE]

And a typical use case ...

align CODE_ALIGNMENT

; just being silly, creating different sized functions

Grape	MakeFunctions
	mov eax,0
	retn

Molly	MakeFunctions
	xor qword [rdx*8+rbx+0x1234567],-0x01234568
	push 1
	pop rax
	retn

Jovial	MakeFunctions
	retn

Yellow	MakeFunctions
	mov ax,3
	retn

Bacon	MakeFunctions
	mov al,4
	retn

?End	MakeFunctions



; functions can be called directly from internal code:
	call Yellow

; or they can be called dynamically by index:
	YELLOW := 3
	fNum dd YELLOW

	CallFunction [fNum]

A nice additional feature would be to generate the indices/names file for external use. Support an index offset to allow [-128,127] values - for small PUSH 0xXX code. Imagine just creating functions for interface and refactoring them as needed, and just rebuilding would propagate to external use.

Of course, it's possible to wrap this STRUC in another macro for multiple distinct function lists.