goto [variable], and address of code labels

[bcs]

On 10/28/2011 09:30 AM, Manu wrote:
> Hi people.
>
> I'd like to propose support for taking the address of code labels, and
> supporting variable goto statements.
> This is a feature I have found extremely useful, implemented as a GCC
> specific extension.
>
> I've used this to get great speedups and simplify code while writing
> emulators/vm's. Perhaps also useful in efficient state machine type code
> too...
>
>
> Simple example:
>
> void *opcodes[] = { &OP_ADDA, &OP_SUBA, &OP_MOVA, &OP_JMPA, &OP_etc... };
>
> void exec()
> {
>    // begin execution
>    goto opcodes[ pProgram[regs.PC++] ];
>
> OP_ADDA:
>    regs.A += pProgram[regs.PC++];
>    goto opcodes[ pProgram[regs.PC++] ];
>
> OP_SUBA:
>    regs.A -= pProgram[regs.PC++];
>    goto opcodes[  ];
>
> OP_MOVA:
>    regs.A = pProgram[regs.PC++];
>    goto opcodes[ pProgram[regs.PC++] ];
>
> OP_JMPA:
>    regs.PC = regs.A;
>    goto opcodes[ pProgram[regs.PC++] ];
>
> OP_etc:
>    ...
>    goto opcodes[ pProgram[PC++] ];
> }
>
> Notice how this structure completely eliminates branch logic, control
> statements, etc.
>
> Note, GCC code labels are void*, but in D, perhaps some special code
> label pointer type should be invented for type safety...
> One may also expect that function pointers may also be implicitly cast
> to this generalised code pointer type, which might be useful in some
> code where a naked function pointer is used to implement some custom
> calling convention.


For the given example, this could be re-cases via a switch statement 
that ends each case with a "goto case pProgram[regs.PC++];". That is 
assuming non-const expression are allowed as the expression. The end 
result should be identical.