[LLVMdev] [RFC] MCJIT usage models
Yaron Keren
yaron.keren at gmail.com
Mon Dec 9 21:46:33 PST 2013
Hi Andy,
My use case is quite similar to what Keno described. I am using clang + JIT
to dynamically compile C++ functions generated in response to user
interaction. Generated functions may be unloaded or modified.
I would like to break down the old JIT code into three major parts.
1) The old JIT has its own code emitter, which duplicates code from lib/MC
and does not generate debug info and other limitations.
2) The old JIT supports lazy compilation.
3) The old JIT has its own function-level "dynamic linker"/memory manager,
supporting function replacement with low-overhead JMP stubs.
Now 1) is clearly a problem of code duplication. I'm not sure why a
different emitter was created for the JIT but it would seem it's possible
to reuse the lib/MC code just like MCJIT does. 2) takes much code all over
the JIT code and at least for my use case it could just be removed.
If 1) and 2) are solved and removed we are left with the (relatively small)
"dynamic linker"/hack code only. I'd say the way this linker/loader works
is much better fit for use cases such as my and Keno's than a classic
linker like the ELF loader.
The JIT linker works at function level rather than module level, supports
automatic stub generation and relinking management. We could wrap functions
in modules for the ELF loader, handle the stubs ourselves etc but this is
tricky code as Keno said, requires hard-learned knowledge and feels like
teaching an elephant to dance. The ELF loader is really designed for very
different requirements.
I'd like to see the old JIT function-level "dynamic linker" code preserved
somehow as a ready alternative to a "classic" linker, especially useful
when used in combination clang to dynamically run C/C++ functions.
Yaron
2013/12/10 Keno Fischer <kfischer at college.harvard.edu>
> With Julia, we're obviously very much in the first use case. As you know,
> we pretty much have a working version of Julia on top of MCJIT, but there's
> still a few kinks to work out, which I'll talk about in a separate email.
>
> One think which I remember you asking at the BOF is what MCJIT currently
> can't do well that the old JIT did, so I'd like to offer up an example.
> With the old JIT, I used Clang to do dynamic code generation to interface
> to C++ easily. Now you might argue that is either a problem with clang or
> rather a misuse of clang, but I'd like to think that we should keep the
> tools as flexible as possible, so applications like this can emerge. With
> the old JIT, I'd incrementally compile functions as Clang added them to the
> Module, but with MCJIT that kind of stuff is rather tricky. What I ended up
> doing was having Clang emit into a shadow module that never gets codegen'd
> and when a function is requested, pulling that function and it's closure
> out of the shadow module into the current MCJIT module. Perhaps
> functionality like that should be more readily available in base LLVM to be
> able to use MCJIT with clients not necessarily designed for use with MCJIT.
>
>
>
>
> On Mon, Dec 9, 2013 at 1:08 PM, Kaylor, Andrew <andrew.kaylor at intel.com>wrote:
>
>> Below is an outline of various usage models for MCJIT that I put
>> together based on conversations at last month’s LLVM Developer Meeting. If
>> you’re using or thinking about using MCJIT and your use case doesn’t seem
>> to fit in one of the categories below then either I didn’t talk to you or I
>> didn’t understand what you’re doing.
>>
>>
>>
>> In any case, I’d like to see this get worked into a shape suitable for
>> inclusion in the LLVM documentation. I imagine it serving as a guide both
>> to those who are new to using MCJIT and to those who are developing and
>> maintaining MCJIT. If you’re using MCJIT the latter (yes, the latter) case
>> is particularly important to you right now as having your use case properly
>> represented in this document is the best way to ensure that it is
>> adequately considered when changes are made to MCJIT and when the decision
>> is made as to when we are ready to deprecate the old JIT engine (probably
>> in the 3.5 release, BTW).
>>
>>
>>
>> So here’s what I’m asking for: if you are currently using MCJIT or
>> considering using MCJIT, can you please find the use case that best fits
>> your program and comment on how well the outline describes it. If you
>> understand what I’m saying below but you see something that is missing,
>> please let me know. If you aren’t sure what I’m saying or you don’t know
>> how MCJIT might address your particular issues, please let me know that
>> too. If you think my outline is too sketchy and you need me to elaborate
>> before you can provide meaningful feedback, please let me know about that.
>> If you think it’s the best piece of documentation you’ve read all year and
>> you can’t wait to read it again, that’s good information too.
>>
>>
>>
>> Thanks in advance for any and all feedback.
>>
>>
>>
>> -Andy
>>
>>
>>
>>
>> ------------------------------------------------------------------------------------------
>>
>>
>>
>> Models for MCJIT use
>>
>>
>>
>> 1. Interactive dynamic code generation
>>
>> - user types code which is compiled as needed for execution
>>
>> - example: Kaleidoscope
>>
>> - compilation speed probably isn't critical
>>
>> - use one MCJIT instance with many modules
>>
>> - create new modules on compilation
>>
>> - MCJIT handles linking between modules
>>
>> - external references still need prototypes
>>
>> - we can at least provide a module pass to automate it
>>
>> - memory overhead may be an issue but MCJIT can fix that
>>
>> - see model 2 for pre-defined library
>>
>> - if processing a large script pre-optimize before passing modules to
>> MCJIT
>>
>>
>>
>> 2. Code generation for external target execution
>>
>> - client generates code to be injected into an external process
>>
>> - example: LLDB expression evaluation
>>
>> - target may be another local or remote
>>
>> - target architecture may not match host architecture
>>
>> - may use one or more instances of MCJIT (client preference)
>>
>> - MCJIT handles address remapping on request
>>
>> - custom memory manager handles code/data transfer
>>
>> - speed/memory requirements may vary
>>
>>
>>
>> 3. Large pre-defined module compilation and execution
>>
>> - code/IR is loaded from disk and prepared for execution
>>
>> - example: Intel(R) OpenCL SDK
>>
>> - compilation speed matters but isn't critical
>>
>> - initial startup time is somewhat important
>>
>> - execution speed is critical
>>
>> - memory consumption isn't an issue
>>
>> - tool integration may be important
>>
>> - use one MCJIT instance with multiple (but usually) few modules
>>
>> - use object caching for commonly used code
>>
>> - for very large, sparsely used libraries pre-link modules
>>
>> - object and archive support may be useful
>>
>>
>>
>> 4. Hot function replacement
>>
>> - client uses MCJIT to optimize frequently executed code
>>
>> - example: WebKit
>>
>> - compilation time is not critical
>>
>> - execution speed is critical
>>
>> - steady state memory consumption is very important
>>
>> - client handles pre-JIT interpretation/execution
>>
>> - MCJIT instances may be created as needed
>>
>> - custom memory manager transfers code memory ownership after
>> compilation
>>
>> - MCJIT instance is deleted when no longer needed
>>
>> - client handles function replacement and lifetime management
>>
>>
>>
>> 5. On demand "one-time" execution
>>
>> - client provides a library of code which is used by small,
>> disposable functions
>>
>> - example: database query?
>>
>> - initial load time isn't important
>>
>> - execution time is critical
>>
>> - if library code is fixed, load as shared library
>>
>> - if library code must be generated use a separate instance of MCJIT
>> to hold the library
>>
>> - this instance can support multiple modules
>>
>> - use a custom memory manager to link with functions in this
>> module
>>
>> - object caching and archive support may be useful in this case
>>
>> - if inlining/lto is more important than compile time keep library in
>> an IR module and pre-link just before invoking MCJIT
>>
>> - create one instance of MCJIT as needed and destroy after execution
>>
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