[llvm-dev] Using bytecode version of std::sort for JIT generated data type

Tue Jul 9 18:47:31 PDT 2019

clang is the tool you'd use to get LLVM IR from C source code
Looks like you can use CloneModule:
https://llvm.org/doxygen/CloneModule_8cpp_source.html to clone a module. So
you could have your "standard library" in a module, you load the module up
- then every time you want to JIT something that might use it - you clone
it, then JIT the new things into that module and compile it.

On Tue, Jul 9, 2019 at 6:31 PM Rajesh S R <srrajesh1989 at gmail.com> wrote:

> Do you have some pointers on how to do it? Take a file like sort.cc above
> and generate a JIT module. Does llc
> <https://llvm.org/docs/CommandGuide/llc.html> help with this?
> What library function can we use to load an existing IR module file into
> my JIT runtime module to compile them together?
>
> On Tue, Jul 9, 2019 at 4:17 PM David Blaikie <dblaikie at gmail.com> wrote:
>
>> Ah, no, sort wouldn't be compiled down to machine code - it'd be in LLVM
>> IR. You'd import that into the module where the user code was (or copy this
>> "standard library" LLVM IR module and then add the to-be-JITed code to that
>> copy, then compile it all together).
>>
>> On Mon, Jul 8, 2019 at 8:45 PM Rajesh S R <srrajesh1989 at gmail.com> wrote:
>>
>>>
>>>
>>> On Mon, Jul 8, 2019 at 4:39 PM David Blaikie <dblaikie at gmail.com> wrote:
>>>
>>>> There isn't any magic command for this - you'd have to write some C++
>>>> code/a custom LLVM optimization pass.
>>>>
>>>> Though, that said - perhaps it should just be a runtime parameter where
>>>> you rely on LLVM to inline/optimize things away? You could do some
>>>> relatively smaller code generation - generate a function that calls into
>>>> the generic function (that takes the functior by pointer) and have the
>>>> generic function be always_inline, so it gets inlined into the outer,
>>>> type-specific function, without you having to write code to create all that
>>>> IR on every use (instead relying on the inliner to create it for you,
>>>> essentially)
>>>>
>>>
>>> I don't understand what you mean here. If sort is already generated into
>>> machine code based on Compare(void*, void*) { return false; } how can we
>>> inject our function here?
>>>
>>>>
>>>> On Mon, Jul 8, 2019 at 4:33 PM Rajesh S R <srrajesh1989 at gmail.com>
>>>> wrote:
>>>>
>>>>> Thanks David!
>>>>>
>>>>> I am not clear on how to achieve this though. Could you give more info
>>>>> on this?
>>>>>
>>>>> I expect something like this:
>>>>>
>>>>> sort.cc file:
>>>>>
>>>>> bool Compare(void* a, void* b) {
>>>>>   return false;
>>>>> }
>>>>>
>>>>> void SortFunc(void* arr, int len) {
>>>>>    std::sort(arr, len, &Compare)
>>>>> }
>>>>>
>>>>> $MAGIC_COMMAND sort.cc -o a.llvm
>>>>>
>>>>> a.llvm is a bytecode which can be loaded at runtime in my JIT module
>>>>> and write some code to replace "Compare" function with our own "Compare"
>>>>> function.
>>>>>
>>>>>
>>>>>
>>>>> On Mon, Jul 8, 2019 at 1:17 PM David Blaikie <dblaikie at gmail.com>
>>>>> wrote:
>>>>>
>>>>>> That's an option, for sure - having llvm::Functions you could clone
>>>>>> into your module, replace the call to the Compare function to a call to the
>>>>>> specific comparison you want & the let the optimizers do their work.
>>>>>>
>>>>>> On Wed, Jul 3, 2019 at 4:23 PM Rajesh S R <srrajesh1989 at gmail.com>
>>>>>> wrote:
>>>>>>
>>>>>>> Thanks David! I understand that std::sort doesn't exist without
>>>>>>> types especially at bytecode layer.
>>>>>>>
>>>>>>> What I was thinking was something like the following:
>>>>>>>
>>>>>>> Compile std::sort with a thunk function Compare(void*, void*)
>>>>>>> {rerturn false} into bytecode with an option say noinline and always make
>>>>>>> the function call or even a simple unoptimized bytecode which guarantees
>>>>>>> that Compare exists as a function without any inlining.
>>>>>>>
>>>>>>> At run time implement a new Compare function for your type and
>>>>>>> replace the Compare function with the new Compare implemented with JIT. Now
>>>>>>> the JIT runtime has whole program in bytecode which it can aggressively
>>>>>>> optimize.
>>>>>>> A good way to realize this "thunking" into LLVM JIT can enable lots
>>>>>>> of optimized algorithms to be ported into JIT without having to re-invent
>>>>>>> them for each JIT runtime.
>>>>>>>
>>>>>>> Thoughts?
>>>>>>>
>>>>>>> On Wed, Jul 3, 2019 at 3:56 PM David Blaikie <dblaikie at gmail.com>
>>>>>>> wrote:
>>>>>>>
>>>>>>>> If you consider how std::sort works - it doesn't exist in the
>>>>>>>> machine (or even LLVM IR) level without a specific type - so if you were to
>>>>>>>> support something like std::sort in your language (JITed or not), it means
>>>>>>>> some form of specialization of your types/functions based on other types.
>>>>>>>>
>>>>>>>> So, yes, something like "own Sort function inside JIT" - if your
>>>>>>>> language doesn't support a way to write this in the language itself (if it
>>>>>>>> doesn't have anything like C++ templates or an equivalent generic thing).
>>>>>>>>
>>>>>>>> Otherwise you can do something like qsort (which uses a function
>>>>>>>> pointer to parameterize the comparison) & perhaps force or encourage the
>>>>>>>> optimizer to inline and collapse away this indirection.
>>>>>>>>
>>>>>>>> On Wed, Jul 3, 2019 at 3:43 PM Rajesh S R via llvm-dev <
>>>>>>>> llvm-dev at lists.llvm.org> wrote:
>>>>>>>>
>>>>>>>>> Hi LLVM devs,
>>>>>>>>> The performance of C++ std::sort comes from being able to inline
>>>>>>>>> the comparator. For a JIT generated data type, using the comparator as a
>>>>>>>>> function call from std::sort may not be ideal. So, i was wondering how can
>>>>>>>>> we make a JIT-sort which is as good as statically compiled std::sort with
>>>>>>>>> comparator inlined.
>>>>>>>>>
>>>>>>>>> What is the recommended way to pass a an existing function like
>>>>>>>>> std::sort into JIT so that it can optimize the whole program? For example,
>>>>>>>>> how do we generate the bytecode for existing function (say some external
>>>>>>>>> tools) and give to JIT runtime. Is there some code sample?
>>>>>>>>>
>>>>>>>>> The alternative is to rollout my own Sort function inside JIT, but
>>>>>>>>> i don't want to do that and want to take this as an opportunity to learn
>>>>>>>>> the general approach of passing existing function definitions into JIT to
>>>>>>>>> do whole program optimization like inlining.
>>>>>>>>>
>>>>>>>>> I found this stackoverflow question
>>>>>>>>> <https://stackoverflow.com/questions/10587250/fast-to-compile-efficient-sort-algorithm-for-jit-compilation>which
>>>>>>>>> is related to what I am asking for, but I don't see any final conclusion on
>>>>>>>>> this beyond incurring a function call for each comparison.
>>>>>>>>>
>>>>>>>>> Thanks!
>>>>>>>>>
>>>>>>>>> Rajesh S R
>>>>>>>>> _______________________________________________
>>>>>>>>> LLVM Developers mailing list
>>>>>>>>> llvm-dev at lists.llvm.org
>>>>>>>>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>>>>>>
>>>>>>>>
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