Would it be possible for K N to have a pure bitcode target F

Question

Would it be possible for K N to have a pure bitcode target From my very limited understanding of LLVM it sounds like the main platform specific parts of bitcode are in the form of linking external symbols which may not be available on all platforms It seems like it should be possible to compile and publish a library as raw bitcode which would be helpful as new targets get added in the future

Landry Norris · Answer

It seems like something like this would be crucial to the eventual goal of custom cross compilation If we were able to customize the compilation there likely won t be any published libraries that are compatible

Landry Norris · Answer

When I did some experimenting with the cli compiler a while back I saw that there s a flag that produces raw bitcode which we presumably could pass to a compatible LLVM then link against a custom built K N runtime but I never tested this theory

andylamax · Answer

I am no expert but aren t you talking about klib

Landry Norris · Answer

I believe the klib format packages bitcode but it currently is tied to a specific target such as iosArm64 linuxX64 etc As far as I m aware there s not a way to build a klib that s just bitcode with no specific target and publish it The idea being that any K N target could consume this klib

ephemient · Answer

LLVM IR is not meant to be cross platform all sorts of platform specific ABI such as integer sizes and calling conventions leak into it

andylamax · Answer

I get your point now But while it is something I desire having an abstract bitcode one that will work for all targets might be just too much abstraction as some platforms are really very different i e JS and ios How would you have bitcode that works same for both

Landry Norris · Answer

JS would not be covered by LLVM bitcode I m more so wondering if there s a good way to make sure libraries are compatible with future K N targets Right now when a new target is added like the new watch target in 1 8 0 libraries have to be updated to support them

ephemient · Answer

<https youtrack jetbrains com issue KT 52666> is perhaps what you are looking for

Landry Norris · Answer

Since bitcode requires knowledge of calling conventions pure bitcode may not be the solution

ephemient · Answer

I believe the long term plan is to ship the Kotlin compiler s IR

ephemient · Answer

there is no existing IR that is suitable

Landry Norris · Answer

I remember talk about a longer term goal of allowing custom cross compilation I may be thinking of KT 43974 If this is added I don t see how you d be able to use existing libraries without some form of IR publishing

Troels Lund · Answer

The documentation gives you some answers here <https kotlinlang org docs native libraries html library format> From the above klibs contain both platform specific bitcode and platform independent Kotlin IR The IR is generated from Kotlin source while the bitcode is from any interop sources

napperley · Answer

Hypothetically if Kotlin Native had some Bare Metal targets eg RP2040 <https www raspberrypi com documentation microcontrollers rp2040 html> how would the bit code library stuff work on these targets There is a reason the bit code is target specific

ephemient · Answer

not sure what the the issue with bare metal is the common stdlib doesn t have I O or other OS interaction you need a memory allocator but you can t run Kotlin without one regardless of stdlib or not

Landry Norris · Answer

I d assume any type of target independent IR would not allow you to depend on a library that defines just a set of targets likely no okio or compose for the reasons mentioned above It would mainly have value for libraries such as unit conversion text processing math and other more abstract concepts It would be nice to have these libraries without having to build them with new targets yourself

napperley · Answer

Isn t the K N memory allocator separate from the Kotlin Std lib

Landry Norris · Answer

The memory allocator is part of the K N runtime

napperley · Answer

I thought that would be the case

napperley · Answer

On the K N side what is part of the Kotlin Std lib as a summary Presumably there isn t anything in the Std lib that would prevent it being used with a Bare Metal target as a show stopper Would be good if a member of the Kotlin team can confirm this

Landry Norris · Answer

<https github com JetBrains kotlin tree master libraries stdlib|https github com JetBrains kotlin tree master libraries stdlib> I believe most of the stdlib is multiplatform now K N hasn t had its own stdlib in a little while

napperley · Answer

Is K N s version of the Std lib the Common module version

Troels Lund · Answer

Not entirely There are parts of the standard library that is only available in K N like lt https kotlin native|kotlin native gt and Most of these exist because the programming model has to be slightly different for native targets but otherwise only expose functionality that is otherwise available in a slightly different form for other targets

Landry Norris · Answer

Is the kotlin native package part of the stdlib I d assumed it was a separate endorsed library

ephemient · Answer

there are several platform specific parts of stdlib it s all in the docs

ephemient · Answer

for example you can see that <https kotlinlang org api latest jvm stdlib kotlin system > is available on JVM and native but not JS

ephemient · Answer

the colors tell you what s available where

napperley · Answer

Are the K N parts of the Std lib kotlin native kotlin native concurrent and kotlin native ref OS agnostic i e don t depend on an OS

Troels Lund · Answer

They should behave the same on all K N platforms if that s what you re asking

napperley · Answer

Not exactly the answer I was after Was wanting to know if any parts of the Kotlin Std lib depend on using an OS

ephemient · Answer

that is not relevant to the original topic anyway if it s in stdlib it s an external call from the klib regardless of how it s implemented

ephemient · Answer

but just look at the APIs

Landry Norris · Answer

If you re looking to run bare metal it s going to be far from easy The OS provides libunwind libpthread and several other hard to write libraries

ephemient · Answer

they presume an environment where dynamic memory allocation atomics and threads are possible

Landry Norris · Answer

Kotlin 1 6 really only needed libunwind if you used the experimental stms gc but that s sadly not the case anymore Until they make a more minimal runtime you need an OS

ephemient · Answer

do you count whatever supplies that as an OS eh

Landry Norris · Answer

It s part of an OS

ephemient · Answer

it s certainly possible to implement those on raw UEFI

ephemient · Answer

whether you count that as an OS or not is a bit arguable

napperley · Answer

Do note that UEFI is only applicable to AMD Intel based hardware

Landry Norris · Answer

Ironically it s more than anyone on the OSDev wiki ever does It s hard to find a reference OS there with libunwind support They always say to just use a real OS at that point

ephemient · Answer

UEFI is part of ARM BBR as well

napperley · Answer

The libunwind lib is only applicable to CPP

ephemient · Answer

the Kotlin Native runtime uses C++ and needs some form of tracing over Kotlin frames as well

napperley · Answer

What do you mean about Kotlin frames

Landry Norris · Answer

The symbols in libunwind are needed for K N I tried building a K N binary for bare metal back in 1 6 I got close but decided to wait until the new gc was stable

Landry Norris · Answer

Frames are part of the ABI It s basically everything that gets pushed to the stack on a method call

Landry Norris · Answer

Return address params etc

ephemient · Answer

I think it should be possible to replace libunwind with any other backtracer appropriate to the environment I m not sure if it can be stubbed out completely

ephemient · Answer

but that s also not part of the OS unless you count SEH as part of the OS

Landry Norris · Answer

Some osdev examples stub out libunwind but they often use languages where you can disable exceptions The kotlin team has been very opposed to this idea

napperley · Answer

In the Kotlin code base there is an option to disable back traces on a per target basis which should get around the libunwind issue

Landry Norris · Answer

OS is often a hard term to define

Landry Norris · Answer

libunwind is also needed for exception support at all not just getting the trace

Landry Norris · Answer

It may be possible to replace libunwind if you rebuild the K N runtime or just not use unwinding at all but the official runtime needs the symbols for it

Landry Norris · Answer

Some day when I have time and can focus on one side project I d like to make a more minimal runtime without every feature and try to have the compiler output bitcode then use llc to compile to binary but that d be a lot of work

napperley · Answer

The libunwind lib appears to be standard across ALL Bare Metal ARM targets in the official ARM GCC toolchain there are two files unwind h amp unwind arm common h

ephemient · Answer

libunwind and libgcc unwind are two different things

ephemient · Answer

also there s the one in llvm

napperley · Answer

Some of the K N targets use a GCC based tool chain eg the unofficial Zephyr based target uses the official ARM GCC tool chain

napperley · Answer

By the way the libunwind lib is available for some Bare Metal ARM targets eg ARMv6 m <https reviews llvm org D22292 id=63759>