Shader Build Pipeline: How To Use A Different Version Of AZSLc

Whether you need to validate a new AZSLc package, or simply try a different version of AZSLc for your local engine build, there's a way to do so without having to hack the CMake 3rdParty Package Fetch system. Set the registry key:
"/O3DE/Atom/AzslCompilerOverridePath"
to the absolute path to the AZSLc binary you may want to work with.

Registry keys can be defined within any *.setreg file under one of the Registry/ subfolders of O3DE. For the sake of organization there's already a setreg file that consolidates property keys related with the Shader Build Pipeline:
o3de/Gems/Atom/Asset/Shader/Registry/atom_shaders.setreg, with the following default content:

{
    "O3DE": {
        "Atom": {
            "Shaders": {
                "BuildVariants": true,
                "Build": {
                    "ConfigPath": "@gemroot:AtomShader@/Config"
                }
            }
        }
    }
}

As an example let's assume you checkout, modify and compile the latest version of AZSLc from AZSLc on Github in the following local directory /mnt/data/GIT/o3de-azslc. The Release version of the executable would be located at /mnt/data/GIT/o3de-azslc/bin/linux/release/azslc. To use this version of AZSLc, this would be the required changes in the setreg file mentioned above:

{
    "O3DE": {
        "Atom": {
            "Shaders": {
                "BuildVariants": true,
                "Build": {
                    "ConfigPath": "@gemroot:AtomShader@/Config"
                }
            },
            "AzslCompilerOverridePath": "/mnt/data/GIT/o3de-azslc/bin/linux/release/azslc"
        }
    }
}