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

[David Blaikie via llvm-dev]

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)

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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