Labels as values and threaded-code interpretation

[Alex Rønne Petersen]

On 02-06-2013 19:43, Dmitry Olshansky wrote:
> 02-Jun-2013 20:48, Alex Rønne Petersen пишет:
>> On 02-06-2013 10:52, Dmitry Olshansky wrote:
>>> 01-Jun-2013 20:13, Timon Gehr пишет:
>>>> On 06/01/2013 07:29 AM, Alex Rønne Petersen wrote:
>>>>> Hi,
>>>>>
>>>>> I'm sure this has been brought up before, but I feel I need to
>>>>> bring it
>>>>> up again (because I'm going to be writing a threaded-code
>>>>> interpreter):
>>>>> http://gcc.gnu.org/onlinedocs/gcc/Labels-as-Values.html
>>>>>
>>>>> This is an incredibly important extension. The final switch
>>>>> statement is
>>>>> not a replacement because it doesn't allow the programmer to store a
>>>>> label address directly into a code stream, which is what's
>>>>> essential to
>>>>> write a threaded-code interpreter.
>>>>>
>>>>
>>>> I'd also like to see this.
>>>>
>>>
>>> Same here.
>>>
>>> Though I believe a way to force tail-call can support the same use case
>>> also helping functional programming.
>>>
>>> Say:
>>>
>>> goto Call-Expression; //forced tail call
>>>
>>> instead of:
>>>
>>> return Call-Expression;
>>>
>>
>> I'm not sure that can support threaded-code interpretation.
>
> Why not:
>
> alias OpCode = function void();
>
> OpCode[] opcodes = [ opcode_1, ... ];
>
> int pc;
> ...
>
> void opcode_1()
> {
>      ... //pick operands do whatever
>      pc = pc + num_operands; //skip over arguments
>      OpCode next = cast(OpCode)bytecode[pc];
>      goto next(); //this is baked-in threaded dispatch
> }
>
> void opcode_2(){ ... }
>
> //say bytecode contains operands and addresses of fucntions
> void execute(size_t[] bytecode, int pc)
> {
>      OpCode start  = cast(OpCode)bytecode[pc];
>      pc++;
>      goto start();
> }
>
> One can get away without casting if data is in a separate array.
> Then this solution is perfectly safe in this limited form. Call would
> only point to a function hence no problem with jumping who knows where
> and less problems for optimizer.
>

The problem here is assuming the interpreter state can be global. Once 
you make it non-global (and thus have to pass it in the goto start(...) 
call) you get all the overhead of a regular function call.

-- 
Alex Rønne Petersen
alex at alexrp.com / alex at lycus.org
http://alexrp.com / http://lycus.org