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.
