Forth like interpreter

[Christian Tismer]

Darrell wrote:
> 
> "Samuel A. Falvo II wrote:
> >  For a detailed reason as to why this is/was, feel free to ask the
> > question in comp.lang.forth.
> >
> Went looking and found my self linked back to Python.
> http://starship.skyport.net/crew/vlad/archive/threaded_code/

Well, I looked into this technique a while before. It may help
for some compilers. On Windows with MSVC, the main switch
translates to

; 1115 : 		f->f_next_instr = next_instr;
; 1116 : 		f->f_stackpointer = stack_pointer;
; 1117 : 
; 1118 : 		/* Main switch on opcode */
; 1119 : 		
; 1120 : 		switch (opcode) {

	lea	eax, DWORD PTR [ebx+7]
	mov	DWORD PTR [ebp+320], edi
	cmp	eax, 140				; 0000008cH
	mov	DWORD PTR [ebp+324], esi
	ja	$L4460
	xor	edx, edx
	mov	dl, BYTE PTR $L6154[eax]
	jmp	DWORD PTR $L6155[edx*4]

The first two mov's are necessary for stackless and just
pipelined in.
The real "overhead" is the check on default (cmp eax, 140)
and in the double lookup. The "mov dl" maps to an index
into the address table.
It may count for Forth, but for Python this is very fast.
Note the "ebx+7" in the beginning. That's because I
added a few extra opcodes for error handling, which
are negative. The savings were great: Error variables
have so short lifetime that they never occupy registers.

ciao - chris    (well this is nothing for C.L.P. I guess)

-- 
Christian Tismer             :^)   <mailto:tismer at appliedbiometrics.com>
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