[llvm-dev] Orc/MCJIT: Relocations vs pointers to functions

[Lang Hames via llvm-dev]

Hi Pawel,

IIRC MCJIT uses SectionMemoryManager by default.


That's right, and it will use that to provide whatever functionality
(memory management or symbol resolution) the user doesn't explicitly
override.

Can I just provide my implementation of RuntimeDyld::SymbolResolver or do I
> have to inherit from whole SectionMemoryManager?


You can just subclass RuntimeDyld::SymbolResolver and call
setSymbolResolver, and let MCJIT take care of the memory management.

If/when you switch over to ORC you'll need to provide both a symbol
resolver and memory manager with each call to addModuleSet, but for the
memory manager that's usually just a case of calling
llvm::make_unique<SectionMemoryManager>().

- Lang.

On Thu, Jun 2, 2016 at 7:18 AM, Paweł Bylica <chfast at gmail.com> wrote:

> Quick question, using MCJIT/LLVM 3.8:
> EngineBuilder has 3 methods:
>
>   EngineBuilder
> &setMCJITMemoryManager(std::unique_ptr<RTDyldMemoryManager> mcjmm);
>
>   EngineBuilder&
>   setMemoryManager(std::unique_ptr<MCJITMemoryManager> MM);
>
>   EngineBuilder&
>   setSymbolResolver(std::unique_ptr<RuntimeDyld::SymbolResolver> SR);
>
> IIRC MCJIT uses SectionMemoryManager by default. Can I just provide my
> implementation of RuntimeDyld::SymbolResolver or do I have to inherit
> from whole SectionMemoryManager?
>
> On Thu, May 12, 2016 at 9:06 PM Paweł Bylica <chfast at gmail.com> wrote:
>
>> Thanks!
>>
>> Currently using MCJIT. But migration to ORC is on my TODO list.
>>
>> - Paweł
>>
>> On Thu, May 12, 2016 at 8:30 PM Lang Hames <lhames at gmail.com> wrote:
>>
>>> Hi Pawel,
>>>
>>> Option (1) and (3) are very similar, but using custom resolution (option
>>> 3) guarantees that JIT'd code can't accidentally end up depending on
>>> functions in your JIT that you didn't mean to expose. Having a smaller
>>> symbol lookup space may improve performance too.
>>> Option (2) would work, but there's no advantage vs option (3).
>>>
>>> So I recommend option 3. :)
>>>
>>> If you're using MCJIT you can override the findSymbol method on the
>>> MemoryManager. If you're using ORC you can pass a custom resolver to
>>> addModuleSet.
>>>
>>> Cheers,
>>> Lang.
>>>
>>>
>>> On Wed, May 11, 2016 at 6:47 AM, Paweł Bylica <chfast at gmail.com> wrote:
>>>
>>>> Hi LLVM, Lang.
>>>>
>>>> I'm looking for a advice here. And I truly understand very little what
>>>> the relocations are and how they work.
>>>>
>>>> The problem I want to solve is the case where a jitted code has to call
>>>> back the host application to query additional data. I can think of 3
>>>> possible solutions:
>>>>
>>>>    1. Use built-in relocation resolver (in default memory manager?)
>>>>    and allow the JIT to find the callback function by name. The host
>>>>    application needs to contain symbols that the JIT will search for. You can
>>>>    have only single implementation of them. The JIT will need to search in the
>>>>    set of all symbols in the executable.
>>>>    2. Pass addresses of callback functions as pointers to functions to
>>>>    a jitted function. The generated code should use pointer to functions
>>>>    instead of predefined function names in calls.
>>>>    3. Create you own Memory Manager that will provide addresses to
>>>>    callback functions. Because the set of callback functions is known upfront
>>>>    and quite small that seems to be better than 1.
>>>>
>>>> Can you help me to evaluate the solutions?
>>>>
>>>> - Paweł
>>>>
>>>
>>>
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