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

[Rajesh S R via llvm-dev]

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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