Weird issue with SDL3 GPU API and metal

10.2.2025

(NOTE: This post mostly exists so that someone else running into a similar error would find this on Google and be able to solve it faster.)

I ran into this super weird issue on MacOS that caused me a lot of headache while working on my game engine on mac. My current plan is to build the engine on top of the new GPU API provided by SDL3. I compile my shaders from Slang to SPIRV and Metal. While loading the shaders with the GPU API, an error occurs saying that my metal shading language version is too old.

The error looks like this:

ERROR: Creating MTLLibrary failed: Error Domain=MTLLibraryErrorDomain Code=3 "tri.vert.slang:12:3: error: 'vertex' attribute requires Metal language standard macos-metal2.3 or higher

I am writing the engine in a new programming language in closed beta called Jai and I had a C version of the same code to load the shader where this error did not occur with the same compiled shader. I fairly quickly realized, that this is due to some difference in how MacOS related things are configured in Jai vs when building with GCC. These differences seemed to cause the default MSL version to be very low. I went into SDL's source, and found the place where the shader is loaded, and added an additional setting to set the Metal shading language version to the version my shaders required. I then re-compiled SDL3 and re-installed it on my machine and everything started working. The change I did was to the: ./src/gpu/metal/SDL_gpu_metal.m file, and I changed only one function slightly to have the metal language version set.

MTLCompileOptions* options = [[MTLCompileOptions alloc] init];
options.languageVersion = MTLLanguageVersion2_3;

if (format == SDL_GPU_SHADERFORMAT_MSL) {
    NSString *codeString = [[NSString alloc]
        initWithBytes:code
               length:codeSize
             encoding:NSUTF8StringEncoding];
    library = [renderer->device
        newLibraryWithSource:codeString
                     options:options
                       error:&error];