Computed gotos on Reddit

[Dmitry Olshansky]

On 25-Jul-12 23:58, Walter Bright wrote:
> On 7/25/2012 12:52 PM, Dmitry Olshansky wrote:
>> On 25-Jul-12 21:47, Walter Bright wrote:
>>> On 7/25/2012 10:29 AM, Dmitry Olshansky wrote:
>>>>> Is it possible you could code it up and test it using inline asm?
>> ...
>>
>>
>>>> Any tips on which spare registers to use (I guess ecx is no go, as
>>>> there is
>>>> 'this' pointer present) ?
>>>
>>> I wouldn't worry about it. EAX is good.
>>>
>> OK. I'm almost there, here is what I have:
>>      //my byte code sets 8-bit to discern code/data
>>       uint c = re.ir[t.pc].code - 128;
>>       //no idea how to code the above in asm
>>       //.code is { return x & mask; } property
>>       asm{
>>                mov EAX, c;
>>                lea EAX, L_jumptable[EAX][EAX*4];
>>                jmp EAX;
>>              }
>>          L_jumptable:
>>           mixin(`asm{`
>>                ~ genJumpTable()
>>                ~ `} `);
>>
>> So I have proper table generated and it all goes fine untill I get:
>>
>> std\regex.d(5118): Error: undefined identifier 'L_jumptable'
>>
>
> I was afraid of that. You may have to approximate it by loading the
> address of L_jumptable into a register and adding it in instead of using
> the addressing mode.
like this ?
	mov EDX, L_jumpable
	move EAX, EDX[EAX][EAX*4]
doesn't work. Seems like label is nonexistent anywhere but jump instruction.

Will this one do it:
	lea EAX, $[EAX+5][EAX*4];
         jmp EAX;

Compiles. Maybe I miscalculated though. Indirect jump has size of ?

>Then, add those extra instructions as dummies into
> the other path.

Ehm? Other path like what? You mean compare it with jump table that uses 
plain addresses? Please expand a bit.


-- 
Dmitry Olshansky