llvm dev - Feb 2015 - [LLVMdev] Some basic questions regarding MCJIT and Kaleidoscope sample

Hi,

I am building a new JIT compiler for Lua (actually a derivative of
Lua), and am planning to use LLVM for this. I have trying out some
basic functions using LLVM 3.5.1. I have been puzzled by one aspect of
the MCJIT versions of the Kaleidoscope sample, and would hugely
appreciate some insight.

Can a single MCJIT instance be used to manage several modules?
Why is a separate MCJIT instance created for each module?

Unfortunately the tutorial text does not explain the rationale for this.

At the moment in my implementation I am putting each compiled function
in its own module and creating an engine for it. This is so that each
function can be managed independently and linked to garbage collection
in Lua. However I do not know if there is a better way - for example
creating a shared engine across all modules. I would also like to
understand the trade offs with this approach versus other approaches.

Many thanks

Regards
Dibyendu

We (and by 'we' I mean 'Lang' (cc'd)) have recently been
experimenting with
improvements/replacements to the MCJIT in the form of Orc, a new layered,
composable JIT API. There are examples of this in the Kaleidoscope code
samples (llvm/examples/Kaleidoscope/Orc) though not comprehensive tutorials
as yet.

On Tue, Feb 10, 2015 at 1:57 PM, Dibyendu Majumdar <mobile at
majumdar.org.uk>
wrote:
> Hi,
>
> I am building a new JIT compiler for Lua (actually a derivative of
> Lua), and am planning to use LLVM for this. I have trying out some
> basic functions using LLVM 3.5.1. I have been puzzled by one aspect of
> the MCJIT versions of the Kaleidoscope sample, and would hugely
> appreciate some insight.
>
> Can a single MCJIT instance be used to manage several modules?
> Why is a separate MCJIT instance created for each module?
>
> Unfortunately the tutorial text does not explain the rationale for this.
>
> At the moment in my implementation I am putting each compiled function
> in its own module and creating an engine for it. This is so that each
> function can be managed independently and linked to garbage collection
> in Lua. However I do not know if there is a better way - for example
> creating a shared engine across all modules. I would also like to
> understand the trade offs with this approach versus other approaches.
>
> Many thanks
>
> Regards
> Dibyendu
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
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HI Dibyendu,

A single MCJIT instance can notionally manage multiple modules, but there
are caveats (which I'm afraid I don't remember off the top of my head)
that
make it unattractive in practice. I believe most clients opt for something
like the ExecutionEngine-per-Module model used in the Kaleidoscope
tutorials.

As Dave mentioned, I'm also working on some new JIT APIs (Orc) that are
intended to be more feature-rich and easier to use than MCJIT. If you would
like to try them out and have any questions about how they work please
don't hesitate to ask. You'll need to live on the bleeding edge for
those
though - they're not available on 3.5.

Cheers,
Lang.


On Tue, Feb 10, 2015 at 2:14 PM, David Blaikie <dblaikie at gmail.com>
wrote:
> We (and by 'we' I mean 'Lang' (cc'd)) have recently
been experimenting
> with improvements/replacements to the MCJIT in the form of Orc, a new
> layered, composable JIT API. There are examples of this in the Kaleidoscope
> code samples (llvm/examples/Kaleidoscope/Orc) though not comprehensive
> tutorials as yet.
>
> On Tue, Feb 10, 2015 at 1:57 PM, Dibyendu Majumdar <mobile at
majumdar.org.uk
> > wrote:
>
>> Hi,
>>
>> I am building a new JIT compiler for Lua (actually a derivative of
>> Lua), and am planning to use LLVM for this. I have trying out some
>> basic functions using LLVM 3.5.1. I have been puzzled by one aspect of
>> the MCJIT versions of the Kaleidoscope sample, and would hugely
>> appreciate some insight.
>>
>> Can a single MCJIT instance be used to manage several modules?
>> Why is a separate MCJIT instance created for each module?
>>
>> Unfortunately the tutorial text does not explain the rationale for
this.
>>
>> At the moment in my implementation I am putting each compiled function
>> in its own module and creating an engine for it. This is so that each
>> function can be managed independently and linked to garbage collection
>> in Lua. However I do not know if there is a better way - for example
>> creating a shared engine across all modules. I would also like to
>> understand the trade offs with this approach versus other approaches.
>>
>> Many thanks
>>
>> Regards
>> Dibyendu
>> _______________________________________________
>> LLVM Developers mailing list
>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>>
>
>
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Hi David,

On 10 February 2015 at 22:14, David Blaikie  wrote:
> We (and by 'we' I mean 'Lang' (cc'd)) have recently
been experimenting with
> improvements/replacements to the MCJIT in the form of Orc, a new layered,
> composable JIT API. There are examples of this in the Kaleidoscope code
> samples (llvm/examples/Kaleidoscope/Orc) though not comprehensive tutorials
> as yet.
>
Yes I have noticed that there is a replacement for MCJIT using Orc.
But I assume this is still early days for the new api? When will a
production release be available?

Will Orc support the use case described in my post - i.e. - functions
independently JITed and garbage collected? In Lua,  function calls go
via the runtime so I don't need to link functions. Later on I will
look at inlining functions to be able to bypass the Lua runtime when
it makes sense.

The other requirement I have is to be able to access standard Lua
functions (not dynamically linked) from within the JITed function. I
discovered that MCJIT has the bug due to which addGlobalMapping() does
not work. So I am using sys::DynamicLibrary::AddSymbol() to register
the Lua functions.

BTW a newbie question - once I have compiled a function - is there a
need to retain the module and engine - apart from the fact that the
memory allocated to the JITed function is presumably managed by the
engine?

Thanks and Regards

Dibyendu