Re: Some JIT compilation challenges

[Dibyendu Majumdar <mobile@...>]

On 10 February 2016 at 12:32, Hisham <h@hisham.hm> wrote:
> On 8 February 2016 at 20:02, Dibyendu Majumdar <mobile@majumdar.org.uk> wrote:
>> LLVM has the concept of a Module which equates to a 'translation unit'
>> - within which there can be many functions.
>>
>> In Ravi at present each function is put into its own module - this has
>> the advantage that a function is independent of every other function
>> and when it becomes garbage, the collector releases resources.
>>
>> The model in Ravi doesn't scale very well - at least in terms of
>> memory usage. I assume that each module in LLVM carries some overhead.
>> It would be nice if multiple functions could be put into a module.
>
> This is guesswork, but I think that in idiomatic Lua function lifetime
> falls into two main categories:
>
> * long-lived functions that are part of a Lua module - their lifetime
> is associated to the lifetime of the enclosing module table, which is
> usually the lifetime of the program
> * short-lived closures that are created "inline" by other functions -
> code that does stuff such as `return function() ... end`, where the
> resulting function is used and then thrown away
>
> For the former, I think you shouldn't worry about GC a lot. I'd make a
> Lua module one big LLVM translation unit, add all declared functions
> there and attach the lifetime of the LLVM unit to the Lua module
> table.
>
> For the latter, I think in most cases these short-lived functions
> won't be worth JITting anyway.
>

Thanks Hisham, this sounds like a sensible approach.

Regards
Dibyendu