Computed gotos on Reddit

[Dmitry Olshansky]

On 24-Jul-12 14:04, Walter Bright wrote:
> On 7/24/2012 12:58 AM, Dmitry Olshansky wrote:
>> Now if I use final switches there is still:
>>
>> A) jump table per switch, or maybe less but there is no guarantees
>>   (= depend on another optimization that's not here)
>> B) it's an ugly and a roundabout way to do this
>>
>> However I think that always requiring tail call optimization or
>> providing syntax
>> to enforce it would work:
>>
>> void op_1()
>> {
>>      ...//some code for instruction 1
>>      opcode = cast(function void ())code[pc++];
>>      goto opcode(); //opcode is pointer to function op_xx
>> }
>> //can do without goto fn() iff tail call is GUARANTEED
>
> I believe you can do this with:
>
>      switch (pc++)
>
> and there are the same number of indirections.

And how is pc is supposed to be an opcode? It's a counter after all...

The trick is that it must be switch(code[pc++])...
It's just if code contains function pointers (or goto jump pointers) 
then separate jump table table is not needed:

code = [ &op_1, &op_2, &op_1, ... ]; //generated somewhere
pc = 0;
code[pc]();
// op_1 increments ( or changes somehow) pc, decodes next op and jumps to it


So I still of opinion that enforced tail call is the cleanest way to 
support this idiom.

-- 
Dmitry Olshansky