1# Shader Module Compilation 2 3ANGLE converts application shaders into Vulkan [VkShaderModules][VkShaderModule] through a series 4of steps: 5 61. **ANGLE Internal Translation**: The initial calls to `glCompileShader` are passed to the [ANGLE 7shader translator][translator]. The translator compiles application shaders into Vulkan-compatible 8GLSL. Vulkan-compatible GLSL matches the [GL_KHR_vulkan_glsl][GL_KHR_vulkan_glsl] extension spec 9with some additional workarounds and emulation. We emulate OpenGL's different depth range, viewport 10y flipping, default uniforms, and OpenGL 11[line segment rasterization](OpenGLLineSegmentRasterization.md). For more info see 12[TranslatorVulkan.cpp][TranslatorVulkan.cpp]. After initial compilation the shaders are not 13complete. The translator initially assigns resources and in/out variables arbitrary descriptor set, 14binding and location indices. The correct values are determined at link time. For the sake of 15transform feedback, some markers are left in the shader for link-time substitution. 16 17 The translator outputs some feature code conditional to Vulkan specialization constants, which are 18resolved at draw-time. For example, 19[Bresenham line rasterization](OpenGLLineSegmentRasterization.md) emulation. 20 211. **Link-Time Compilation and Transformation**: During a call to `glLinkProgram` the Vulkan 22back-end can know the necessary locations and properties to write to connect the shader stage 23interfaces. We get the completed shader source using ANGLE's 24[GlslangWrapperVk][GlslangWrapperVk.cpp] helper class. At this time, we use Khronos' 25[glslang][glslang] to convert the Vulkan-compatible GLSL into SPIR-V. A transformation pass is done 26on the generated SPIR-V to update the arbitrary descriptor set, binding and location indices set in 27step 1. Additionally, component and various transform feedback decorations are added and inactive 28varyings are removed from the shader interface. We currently don't generate `VkShaderModules` at 29this time, but that could be a future optimization. 30 311. **Draw-time Pipeline Creation**: Once the application records a draw call, the SPIR-V is compiled 32into `VkShaderModule`s. The appropriate specialization constants are then resolved and the 33`VkPipeline` object is created. Note that we currently don't use [SPIRV-Tools][SPIRV-Tools] to 34perform any SPIR-V optimization. This could be something to improve on in the future. 35 36See the below diagram for a high-level view of the shader translation flow: 37 38<!-- Generated from https://bramp.github.io/js-sequence-diagrams/ 39 Note: remove whitespace in - -> arrows. 40participant App 41participant "ANGLE Front-end" 42participant "Vulkan Back-end" 43participant "ANGLE Translator" 44participant "GlslangWrapperVk" 45participant "Glslang" 46 47App->"ANGLE Front-end": glCompileShader (VS) 48"ANGLE Front-end"->"Vulkan Back-end": ShaderVk::compile 49"Vulkan Back-end"->"ANGLE Translator": sh::Compile 50"ANGLE Translator"- ->"ANGLE Front-end": return Vulkan-compatible GLSL 51 52Note right of "ANGLE Front-end": Source is using bogus\nVulkan qualifiers to be\ncorrected at link time. 53 54Note right of App: Same for FS, GS, etc... 55 56App->"ANGLE Front-end": glCreateProgram (...) 57App->"ANGLE Front-end": glAttachShader (...) 58App->"ANGLE Front-end": glLinkProgram 59"ANGLE Front-end"->"Vulkan Back-end": ProgramVk::link 60 61Note right of "Vulkan Back-end": ProgramVk inits uniforms,\nlayouts, and descriptors. 62 63"Vulkan Back-end"->GlslangWrapperVk: GlslangWrapperVk::GetShaderSpirvCode 64GlslangWrapperVk->Glslang: GlslangToSpv 65Glslang- ->GlslangWrapperVk: Return SPIR-V 66 67Note right of GlslangWrapperVk: Transform SPIR-V 68 69GlslangWrapperVk- ->"Vulkan Back-end": return transformed SPIR-V 70"Vulkan Back-end"- ->"ANGLE Front-end": return success 71 72Note right of App: App execution continues... 73 74App->"ANGLE Front-end": glDrawArrays (any draw) 75"ANGLE Front-end"->"Vulkan Back-end": ContextVk::drawArrays 76 77Note right of "Vulkan Back-end": We init VkShaderModules\nand VkPipeline then\nrecord the draw. 78 79"Vulkan Back-end"- ->"ANGLE Front-end": return success 80--> 81 82 83 84[GL_KHR_vulkan_glsl]: https://github.com/KhronosGroup/GLSL/blob/master/extensions/khr/GL_KHR_vulkan_glsl.txt 85[glslang]: https://github.com/KhronosGroup/glslang 86[GlslangWrapperVk.cpp]: https://chromium.googlesource.com/angle/angle/+/refs/heads/master/src/libANGLE/renderer/vulkan/GlslangWrapperVk.cpp 87[SPIRV-Tools]: https://github.com/KhronosGroup/SPIRV-Tools 88[translator]: https://chromium.googlesource.com/angle/angle/+/refs/heads/master/src/compiler/translator/ 89[TranslatorVulkan.cpp]: https://chromium.googlesource.com/angle/angle/+/refs/heads/master/src/compiler/translator/TranslatorVulkan.cpp 90[VkShaderModule]: https://www.khronos.org/registry/vulkan/specs/1.1-extensions/man/html/VkShaderModule.html 91
){kind=link}