ANGLE converts application shaders into Vulkan VkShaderModules through a series of steps:
glCompileShaderare passed to the ANGLE shader translator. The translator compiles application shaders into Vulkan-compatible GLSL. Vulkan-compatible GLSL matches the GL_KHR_vulkan_glsl extension spec with some additional workarounds and emulation. We emulate OpenGL's different depth range, viewport y flipping, default uniforms, and OpenGL line segment rasterization. For more info see TranslatorVulkan.cpp. After initial compilation the shaders are not complete. The translator initially assigns resources and in/out variables arbitrary descriptor set, binding and location indices. The correct values are determined at link time. For the sake of transform feedback, some markers are left in the shader for link-time substitution.
The translator outputs some feature code conditional to Vulkan specialization constants, which are resolved at draw-time. For example, Bresenham line rasterization emulation.
Link-Time Compilation and Transformation: During a call to
glLinkProgram the Vulkan back-end can know the necessary locations and properties to write to connect the shader stage interfaces. We get the completed shader source using ANGLE‘s GlslangWrapperVk helper class. At this time, we use Khronos’ glslang to convert the Vulkan-compatible GLSL into SPIR-V. A transformation pass is done on the generated SPIR-V to update the arbitrary descriptor set, binding and location indices set in step 1. Additionally, component and various transform feedback decorations are added and inactive varyings are removed from the shader interface. We currently don't generate
VkShaderModules at this time, but that could be a future optimization.
Draw-time Pipeline Creation: Once the application records a draw call, the SPIR-V is compiled into
VkShaderModules. The appropriate specialization constants are then resolved and the
VkPipeline object is created. Note that we currently don't use SPIRV-Tools to perform any SPIR-V optimization. This could be something to improve on in the future.
See the below diagram for a high-level view of the shader translation flow: