goto [variable], and address of code labels

[Manu]

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[ pProgram[regs.PC++] ];

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.
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