It's important to cite prior work in academic papers, nonetheless
Adding C++ lambda support to shader programming would certainly be beneficial, but I don't think it, alone, would provide the modularity, composition, and GPU code specialization features necessary for an effective shader programming model.
That said, I am not an expert in SYCL, so if you have some ideas about how SYCL could meet our design goals, I would appreciate further feedback!
The Rust GPU project is working to satisfy a necessary condition of unified shader programming in Rust: the ability to compile Rust code to GPU-executable kernels. However, in its current state, it does not provide interop between the host (CPU) code and GPU code in a unified way. Programmers still need to create separate host and GPU representations of parameters (and keep these two definitions in sync) and use API calls to communicate between the two halves of the code.
Similarly, Metal's GPU shader code is written in (a subset of) C++, but Metal does not meet our definition of a unified shader programming environment for the same reason.
Our work investigates the next step in this process: once we can translate host language X to GPU-compatible code, what else do we need to accomplish to unify the host and GPU halves of shader code into the same programming environment.
I have replied to some of the comments, and I am happy to answer questions and have further discussion about our work!
The ability to compile C++ code to GPU-executable kernels for use in shader programming is a massive undertaking, and we're glad that the Circle compiler is working to accomplish this task! Instead, our work focuses on the next layer: once we can generate both host- and GPU-compatible shader code from C++, what else is required to achieve useful unified shader programming?
Our work identifies shader specialization as a major missing piece when attempting to unify host and GPU shader code into C++. For reasons we discuss in the paper, current methods available in C++ to potentially support GPU code specialization are insufficient, and Circle does not add any language design provisions to allow dynamic logic in host code to influence compile-time specialization and selection of GPU code, which is central to supporting unified shader specialization. Our work aims to provide this support by co-opting C++ attributes and virtual functions.
Circle also adds other language features on top of C++, including some interesting new metaprogramming features. It may be possible to build a unified shader programming system using these new features, but since these new features are not part of C++, they would not allow us to meet our design goals. The end of Section 7 in our paper includes an expanded discussion of this topic.
I'm happy to answer any questions and have further discussion here!
Anyways I understand that under different constraints (adherence to C++ standard, can't easily implement a whole C++ frontend and interpreter) this becomes much harder.
Basically, the amount of code interfacing with OpenGL should be fairly minimal.
There are higher level graphics libraries on top of OpenGL, Vulkan, Metal, WebGL & DirectX, for example bgfx or sokol, but IIRC they still go C style for the same reason. Then there are language-specific libraries that will be idiomatic, like wgpu for Rust. Finally, you reach entire very opinionated game engines at various levels (Unity, UE, Godot, etc).
(for shaders, MSL is a C++ dialect, but HLSL isn't)
DirectX is a COM API, and although some masochits can make use of it from C, 99% of the world does not, including the official SDK.
Metal is a mix of Objective-C and C++, with Swift bindings thanks Objective-C interop.
None of them are C based like Khronos darlings.
It took NVidia beating them to the ground for OpenCL to ever consider anything other than C and create SPIR.
Then the existing C++ bindings for Vulkan were originally created by NVidia (again).
Now ANARI is yet again a C API, they just cannot move beyond C.
Modern C also lack portability due to UBOs and compiler optimizations, but still good enough so is C++.
I would not consider a C++ API particularly more expressive than a C API.
RAII handles are nice. But that’s about all the expressiveness you get from C++ at the API level.
Languages that provide expressiveness generally do so at the implementation level, in my experience.
Opengl also gets away with because you're almost always working with either floats or integers (for say index buffers) but once you start dealing with containers of containers, a C api ends up really messy with everything being an "object" and having to call function_str(my_obj_handle) with the correct handle - see sentry's api  for an example of what this looks like.
A C++ api for OpenGL would allow for type safe flags, type and bounds checked buffers, RAII handles with all the good (and granted, bad) that comes with them.
When managing resources on separate memory, RAII is also rarely something you’d want. There are different memory management strategies and tight control over memory is very important.
Even if you implement your library in C++ you will need to expose a C API to be compatible with other languages. Not everyone uses C++.
It's the opposite.
When managing resources in system memory, RAII is rarely something I’d want because malloc() is slow.
When managing resources on separate device, RAII is invaluable. These external resources often have API designed like bind/use/unbind, map/update/unmap and similar. You forget that one close/unbind/unmap and you violate your part of the contract i.e. anything may happen including crashes and memory leaks.
DirectX is only barely C++. It's structs and virtual interfaces. They actually provide a full C interface. The API itself is exceptionally C-like IMHO. It does not, for example, return unique_ptr or vectors. There's no std::string or std::function. In fact there's no std:: anything.
Don't get me wrong I like namespaces, constructors, and methods over C-style "namespace", no constructors, and an ocean of loose functions. If people want to create and use C++ wrappers around C APIs that's great. But OP's question was "Why does OpenGL persist with a C style API instead of a more expressive one?".
Writing a C API and wrapping with C++ is very different than writing an "expressive" C++ interface imho.
COM is language agnostic and definitly not C.
A C++ wrapper doesn't need std::anything to be C++.
Barely C++ is still an improvement over bare bones C.
LibGNM(X) and GX2 are also not std::whatever_else, but again build up on not being a bare bones C API stuck in the days of IrisGL.
Finally Metal is a mix of Objective-C and C++, both definitly an improvment.
Then there is the whole issue of they are proper frameworks, not "here is a specification and now go hunting how to load fonts, models, materials" that Khronos does.
If that's all you want, its not hard to write your own RAII wrapper for things anyway.
A C++ API imposes its style on you. Usually badly. Never mind the ABI issues.
Most interfaces could be improved by re-writing them in C. I can’t say the same for the inverse.
That said, OpenGL is a terrible API that is nearly impossible to use correctly. But neither C nor C++ have much impact on its horrifically stateful architecture.
> Never mind ABI issues.
A C ABI is far more brittle than a C++ one.
A C API can called by every language under the sun. C is the lingua franca of computer science.
A C++ API can't even be reliably called by other C++ programs. If I produce a shared library with C API it can be used broadly and easily. If I produce a shared library with a C++ it can only be used by C++ programs compiled with precisely the same settings.
Providing a C++ API really just means "compile this entire library from source". I do indeed compile a lot of great third-party C++ code from source. That's not always possible or even desirable.
If I need to use that code from a different language - for example C# for Unity or matlab for data science - then that C++ API is worthless and needs to be wrapped with a C API.
I've recently been integrating some C++ libraries into C++ based Unreal Engine. It's easier to integrate a C API with Unreal C++ than it is to integrate random C++ project with C++ based Unreal.
The fact that C++ doesn't have a standard build system doesn't help. Integrating a C++ library to compile from source requires adapting random build system A with your build system B. There's a reason C++ projects love single-header (or single header + single cpp) C++ libraries with no additional dependencies. Because they don't require jumping through a million hoops.
And is usually only used as a last resort. I've lost count of the number of libraries reimplemented in different languages just to avoid using a C-style API at all.
> C is the lingua franca of computer science.
Is it? If you were to randomly draw a developer from the body of working programmers, how confident would you really be that they could competently write a good portable C program, or even write one at all? I certainly wouldn't put any money on it.
C++ is the preferred language of web browsers, interpreters, games/game engines, GUIs, heterogeneous programming frameworks, and every relevant C compiler.
And to reiterate, a C ABI is inherently more fragile than a C++ ABI simply due to the lack of name mangling, among other things. The fact that platforms choose to never touch their libc doesn't change that. But if you have to maintain a C library you will feel this.
> Is it? If you were to randomly draw a developer from the body of working programmers, how confident would you really be that they could competently write a good portable C program, or even write one at all? I certainly wouldn't put any money on it.
Perhaps what they meant is more like "C is the lingua franca of linking". CS people or developers may not be great at writing C code, but almost everything can execute it. C++ of course, but also Java and Python. I think even Haskell can call C code, but I'm not certain of that.
It really doesn't mean that. But even if it did it'd still be worth it.
> It makes most c++ libraries completely unusable in embedded, high performance, and high reliability environments
Most libraries are completely unusable in those environments, including most C libraries.
On some platforms you can make even single compiler builds in C++ incompatible if you use stl - e.g. a stl::vector may be quite different in debug and release build which leads naturally to faulty memory acceess.
I made it a library with a single header: https://github.com/redgpu/framework
Supports WASM and WebGL too!
This work is definitely more on the practical or engineering side of things, and we believe it has significant research value as well. Our work echoes motivations from prior works, but the choice to target C++ and its limitations represents a fundamental difference compared to prior works that instead use uncommon/new languages with expanded feature sets. We believe that the synthesis of ideas from prior work, combined with our implementation strategy of co-opting existing language features, allows us to create a cohesive, C++-based system that is a novel contribution to the field.
Nevertheless, these types of "systems" papers are sometimes challenging to review, especially if reviewers are unfamiliar with systems research. The novelty comes from the act of building the system---it lets us see what "just works," see what doesn't (e.g., shader specialization in this case), and figure out solutions to the challenges that arise along the way.
I think Kayvon Fatahalian of Stanford University does a fantastic job of illustrating the value of systems papers here: https://graphics.stanford.edu/~kayvonf/notes/systemspaper/ and the structure of our paper is certainly influenced by this philosophy.
The problem isn't just compiling C++ to glsl. The problem is taking c++ and allowing you to write your shader as part of your program. Right now, you essentially have 2 options: you can write one shader that does everything and use the actual data passed to it to conditionally run what you want, or you build a shader composition system that takes small chunks of code and pieces them together based on what data you want to pass to it. Thats overly simplified, but a kind of high level view. There are a few talks from bungie detailing the work they put into their shader system so that technical artists could have a smooth workflow.
Ultimately, your engine can't handle custom shaders without the engine user also writing code for the cpu side, unless you have a AAA sized engine team that develops a compiler to achieve that.
Even with these layered tools, the host-GPU interop is still something that shader programming systems have to contend with directly, in contrast to "unified" systems like CUDA and C++ AMP in the GPU compute world. As you point out, translating C++ to GPU-compatible code is just one step in the process. Our work looks at the next step: once we can write both host and GPU shader code in C++, what's missing? What we discovered is that by merging the host and GPU halves together in C++, we inadvertently broke the critical shader specialization optimization. So our work presents a method to support shader specialization in a first class way, while also meeting our other design goals.