A coding challenge: was: luaD_precall() question

[Dibyendu Majumdar <mobile@...>]

On 30 June 2017 at 16:56, Dibyendu Majumdar <mobile@majumdar.org.uk> wrote:
> I am looking at an optimization scenario. My goal is to allow certain
> native C functions that take one or two primitive arguments and return
> a primitive, to be called directly by the VM bypassing the normal Lua
> function call sequence. Examples of functions that can be handled this
> way are the maths functions.
>
> My plan is to enhance the CClosure structure with an additional field
> - that will encode the C function's argument types and the return
> type.
>

Assuming that I have a byte to hold extra data for a light C function,
I want to be able to:

a) Work out what the expected arguments and return value are from this
extra byte of data - assuming only up to 2-3 arguments are allowed and
all arguments and return values are primitive types.

b) I need to be able to check that all the actual parameter values on
the Lua stack have the correct types.

c) Call the C function with appropriate signature.

d) Check that the returned value is the right type.

And I want to do all this without having lots of branchy code using
if/else conditions.

Vaguely the strategy I have is to be able to map the information in
the extra byte and the actual types of the parameters to a _vtable_
that has functions with the right signature. To explain, lets say we
only support a function with single double argument, returning a
double. And lets say such a function is denoted by the numeric code 1.
So then I want to map this 1 + the type of value in Lua stack to the
vtable location 1 (first function) if valid - else to 0 if invalid.

I don't know if I have been able to explain ... please let me know if
this is unclear and I will try again.

Regards
Dibyendu