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 SPIR-V, after 8transforming GLSL to conform to the [GL_KHR_vulkan_glsl][GL_KHR_vulkan_glsl] extension spec with 9some additional workarounds and emulation. We emulate OpenGL's different depth range, viewport y 10flipping, default uniforms, and OpenGL 11[line segment rasterization](OpenGLLineSegmentRasterization.md) among others. For more info see 12[TranslatorVulkan.cpp][TranslatorVulkan.cpp]. After initial compilation, the SPIR-V blobs 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 additional code is generated to be removed or modified during SPIR-V 16transformation. 17 18 The translator outputs some feature code conditional to Vulkan specialization constants, which are 19resolved at draw-time. For example, 20[Bresenham line rasterization](OpenGLLineSegmentRasterization.md) emulation. 21 221. **Link Time**: During a call to `glLinkProgram` the Vulkan back-end can know the necessary 23locations and properties to write to connect the shader stage interfaces. However, some draw-time 24information is still necessary to finalize the SPIR-V; for example whether "early fragment tests" 25can be enabled. As an optimization, the SPIR-V is transformed with arbitrary settings and a Vulkan 26pipeline object is created in an attempt to warm the Vulkan pipeline cache. 27 281. **Draw-Time SPIR-V Transformation and Pipeline Creation**: Once the application records a draw 29call, a transformation pass is done on the generated SPIR-V to update the arbitrary descriptor set, 30binding and location indices set in step 1. Additionally, component and various transform feedback 31decorations are added, inactive varyings are removed, early fragment tests are enabled or disabled, 32debug info is removed and pre-rotation is applied. At this time, `VkShaderModule`s are created (and 33cached). The appropriate specialization constants are then resolved and the `VkPipeline` object is 34created (see details [here](PipelineCreation)). Note that we currently don't use 35[SPIRV-Tools][SPIRV-Tools] to perform any SPIR-V optimization. This could be something to improve on 36in the future. 37 38See the below diagram for a high-level view of the shader translation flow: 39 40<!-- Generated from https://bramp.github.io/js-sequence-diagrams/ 41 Note: remove whitespace in - -> arrows. 42participant App 43participant "ANGLE Front-end" 44participant "Vulkan Back-end" 45participant "ANGLE Translator" 46participant "Link-Time SPIR-V Transformer" 47 48App->"ANGLE Front-end": glCompileShader (VS) 49"ANGLE Front-end"->"Vulkan Back-end": ShaderVk::compile 50"Vulkan Back-end"->"ANGLE Translator": sh::Compile 51"ANGLE Translator"- ->"ANGLE Front-end": return SPIR-V 52 53Note right of "ANGLE Front-end": SPIR-V is using bogus\ndecorations to be\ncorrected at link time. 54 55Note right of App: Same for FS, GS, etc... 56 57App->"ANGLE Front-end": glCreateProgram (...) 58App->"ANGLE Front-end": glAttachShader (...) 59App->"ANGLE Front-end": glLinkProgram 60"ANGLE Front-end"->"Vulkan Back-end": ProgramVk::link 61 62Note right of "Vulkan Back-end": ProgramVk inits uniforms,\nlayouts, and descriptors. 63 64"Vulkan Back-end"->"Link-Time SPIR-V Transformer": SpvGetShaderSpirvCode 65"Link-Time SPIR-V Transformer"- ->"Vulkan Back-end": retrieve SPIR-V and determine decorations 66"Vulkan Back-end"- ->"ANGLE Front-end": return success 67 68Note right of App: App execution continues... 69 70App->"ANGLE Front-end": glDrawArrays (any draw) 71"ANGLE Front-end"->"Vulkan Back-end": ContextVk::drawArrays 72 73"Vulkan Back-end"->"Link-Time SPIR-V Transformer": SpvTransformSpirvCode 74"Link-Time SPIR-V Transformer"- ->"Vulkan Back-end": return transformed SPIR-V 75 76Note right of "Vulkan Back-end": We init VkShaderModules\nand VkPipeline then\nrecord the draw. 77 78"Vulkan Back-end"- ->"ANGLE Front-end": return success 79--> 80 81 82 83[GL_KHR_vulkan_glsl]: https://github.com/KhronosGroup/GLSL/blob/main/extensions/khr/GL_KHR_vulkan_glsl.txt 84[SPIRV-Tools]: https://github.com/KhronosGroup/SPIRV-Tools 85[translator]: https://chromium.googlesource.com/angle/angle/+/refs/heads/main/src/compiler/translator/ 86[TranslatorVulkan.cpp]: https://chromium.googlesource.com/angle/angle/+/refs/heads/main/src/compiler/translator/TranslatorVulkan.cpp 87[VkShaderModule]: https://www.khronos.org/registry/vulkan/specs/1.1-extensions/man/html/VkShaderModule.html 88[PipelineCreation]: PipelineCreation.md 89
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