Hi,
Despite the time I have spent on Lua I still do not understand how the
code generator works unfortunately. I have got by so far as most of
the new bytecodes I added were just variations of existing bytecodes.
Now that I am trying to implement the type assertion operator for the
first time I am having to invoke certain functions mentioned below -
and I feel unsure of what I am doing. I would hugely appreciate any
help.
My aim is to generate a bytecode instruction that asserts the type of
an expression. I am treating the operator has a unary operator. Unlike
the existing unary operators the difference is that in this case the
bytecode has only one operand A which identifies a register - and the
bytecode asserts the type of the value held in that register. As this
is a unary operator then it can be used within expressions.
So I am modelling my implementation based on codenot() in lcode.c.
Below is a trimmed version.
I guess that all I need here is 1) to assign a register to 'e' if not
assigned, 2) generate the bytecode for type assertion and 3) ensure
that everything else carries on as before. Below seems to work but I
do not quite understand the steps.
My questions are:
a) What does luaK_dischargevars() do?
b) What does discharge2anyreg() do? It seems to also call luaK_dischargevars().
c) I find that if I call freeexp() below it causes bad code generation
- presumably because the register gets freed?
static void codecast(FuncState *fs, UnOpr op, expdesc *e) {
luaK_dischargevars(fs, e);
switch (e->k) {
case VRELOCABLE:
case VNONRELOC: {
discharge2anyreg(fs, e);
/* freeexp(fs, e); */
OpCode opcode;
ravitype_t tt;
if (op == OPR_TO_NUMBER && e->ravi_type != RAVI_TNUMFLT) {
opcode = OP_RAVI_TOFLT;
tt = RAVI_TNUMFLT;
}
else {
/* nothing to do*/
return;
}
luaK_codeABC(fs, opcode, e->u.info, 0, 0);
e->ravi_type = tt;
e->k = VNONRELOC;
return;
}
default: break;
}
luaX_syntaxerror(fs->ls, "invalid type assertion"); /* cannot happen */
}
Thanks and Regards
Dibyendu
