Difference between Spir-v, GLSL and HLSL

Question

I'm trying to find out the difference between all the shader langage. I'm working on a game in c++ with Vulkan currently witch mean (if i've read correctly) that every shader i present to Vulkan must be in spir-v extension.

But i've seen sometimes the uses of this library: https://github.com/KhronosGroup/SPIRV-Cross Which can translates spir-v to other langage (GLSL, HLSL or MSL), is it something useful when i try to make a game? Not to work on shader on different platform.

Or maybe i need these differents format to use them or different platform? (which doesn't seem right as vulkan look for spir-v). Nevertheless, i saw that there was a tool MoltenVK to use the shader on Mac. Which mean mac doesn't support correctly vulkan? So what are the pro and cons of these langage? (I mean when creating a game, the user isn't supposed to modify the shader)

I hope my question isn't too fuzzy.

Answer 1

You can't compare SPIR-V to high-level languages like GLSL and HLSL. Those are different things. SPIR-V is an intermediate, platform-independent representation (that's the "I" in SPIR-V), which aims to decouple Vulkan (as the API) from the actual high-level shading language (eg GLSL and HLSL). So (as you noted), Vulkan implementations do not really know about GLSL or HLSL and can only consume SPIR-V shaders.

With this in mind it now pretty much does not matter at all what high-level language you then choose (GLSL, HLSL, something totally different) as long as you have a way of generating SPIR-V from that shading language. For GLSL you eg use the GLSL reference compiler to generate SPIR-V, and for HLSL you can use the DirectX Shader Compiler to generate SPIR-V. Even though HLSL comes from the DirectX world, it has an officially supported SPIR-V compiler backend that's production ready. Whether you use GLSL or HLSL is then mostly a personal choice. HLSL is more common in the commercial space, as the language is more modern than GLSL, with things like templates. But in the end the choice is yours.

As for MacOS: MoltenVK is a MacOS/iOS compatible Vulkan implementation on top of metal. So everything that is true for Vulkan is also true for MoltenVK (on MacOS and iOS). So as for Windows, Android or Linux you provide your shaders in SPIR-V. And as SPIR-V shaders are platform independent, your generated SPIR-V will work on all platforms that support Vulkan (unless you use specific extensions in your shader that are simply not available on a certain platform).

As for SPIR-V cross: It probably won't be something you need when writing a game. As you decided what shading language you use, and then use a compiler for that shading language to generate the SPIR-V that you feed to Vulkan you most probably won't need to convert back from SPIR-V, as all your source shaders are written in a high-level language.