ShaderModuleCompilation.md

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Hash : b36e46ab
Author :
Date : 2020-01-08T15:49:18

Vulkan: Line raster emulation through specialization constant

In preparation for compiling shaders early at link time, this change
reworks line raster emulation such that it uses specialization constants
instead of a preprocessor condition.  This means drawing both triangles
and lines with this program will still result in a one-time shader
compilation.

The compilation is still done at draw time in this change.

Bug: angleproject:3394
Change-Id: I0bf91398868d7f7147456533b728906b505192b2
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1992365
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>

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Shader Module Compilation

ANGLE converts application shaders into Vulkan VkShaderModules through a series of steps:

ANGLE Internal Translation: The initial calls to glCompileShader are 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. They are templated with markers that are filled in later at link time.
Link-Time Translation: 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. We still cannot generate VkShaderModules since some ANGLE features like OpenGL line rasterization emulation depend on draw-time information.
Draw-time SPIR-V Generation: Once the application records a draw call we use Khronos’ glslang to convert the Vulkan-compatible GLSL into SPIR-V. The SPIR-V is then compiled into VkShaderModules. For details please see GlslangWrapperVk.cpp. The VkShaderModules are then used by VkPipelines with the appropriate specialization constant values. 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:

Vulkan Shader Translation Flow

Source

src/libANGLE/renderer/vulkan/doc/ShaderModuleCompilation.md

# Shader Module Compilation

ANGLE converts application shaders into Vulkan [VkShaderModules][VkShaderModule] through a series
of steps:

1. **ANGLE Internal Translation**: The initial calls to `glCompileShader` are passed to the [ANGLE
shader translator][translator]. The translator compiles application shaders into Vulkan-compatible
GLSL. Vulkan-compatible GLSL matches the [GL_KHR_vulkan_glsl][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](OpenGLLineSegmentRasterization.md). For more info see
[TranslatorVulkan.cpp][TranslatorVulkan.cpp]. After initial compilation the shaders are not
complete. They are templated with markers that are filled in later at link time.

1. **Link-Time Translation**: 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][GlslangWrapperVk.cpp] helper class. We still
cannot generate `VkShaderModules` since some ANGLE features like
[OpenGL line rasterization](OpenGLLineSegmentRasterization.md) emulation depend on draw-time
information.

1. **Draw-time SPIR-V Generation**: Once the application records a draw call we use Khronos'
[glslang][glslang] to convert the Vulkan-compatible GLSL into SPIR-V.  The SPIR-V is then compiled
into `VkShaderModules`. For details please see [GlslangWrapperVk.cpp][GlslangWrapperVk.cpp]. The
`VkShaderModules` are then used by `VkPipelines` with the appropriate specialization constant
values. Note that we currently don't use [SPIRV-Tools][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:

<!-- Generated from https://bramp.github.io/js-sequence-diagrams/
participant App
participant "ANGLE Front-end"
participant "Vulkan Back-end"
participant "ANGLE Translator"
participant "GlslangWrapperVk"
participant "Glslang"

App->"ANGLE Front-end": glCompileShader (VS)
"ANGLE Front-end"->"Vulkan Back-end": ShaderVk::compile
"Vulkan Back-end"->"ANGLE Translator": sh::Compile
"ANGLE Translator"- ->"ANGLE Front-end": return Vulkan-compatible GLSL

Note right of "ANGLE Front-end": Source is templated\nwith markers to be\nfilled at link time.

Note right of App: Same for FS, GS, etc...

App->"ANGLE Front-end": glCreateProgram (...)
App->"ANGLE Front-end": glAttachShader (...)
App->"ANGLE Front-end": glLinkProgram
"ANGLE Front-end"->"Vulkan Back-end": ProgramVk::link

Note right of "Vulkan Back-end": ProgramVk inits uniforms,\nlayouts, and descriptors.

"Vulkan Back-end"->GlslangWrapperVk: GlslangWrapperVk::GetShaderSource
GlslangWrapperVk- ->"Vulkan Back-end": return filled-in sources

Note right of "Vulkan Back-end": Source is templated with\ndefines to be resolved at\ndraw time.

"Vulkan Back-end"- ->"ANGLE Front-end": return success

Note right of App: App execution continues...

App->"ANGLE Front-end": glDrawArrays (any draw)
"ANGLE Front-end"->"Vulkan Back-end": ContextVk::drawArrays

"Vulkan Back-end"->GlslangWrapperVk: GlslangWrapperVk::GetShaderCode (with defines)
GlslangWrapperVk->Glslang: GlslangToSpv
Glslang- ->"Vulkan Back-end": Return SPIR-V

Note right of "Vulkan Back-end": We init VkShaderModules\nand VkPipeline then\nrecord the draw.

"Vulkan Back-end"- ->"ANGLE Front-end": return success
-->

![Vulkan Shader Translation Flow](https://raw.githubusercontent.com/google/angle/master/src/libANGLE/renderer/vulkan/doc/img/VulkanShaderTranslation.svg?sanitize=true)

[GL_KHR_vulkan_glsl]: https://github.com/KhronosGroup/GLSL/blob/master/extensions/khr/GL_KHR_vulkan_glsl.txt
[glslang]: https://github.com/KhronosGroup/glslang
[GlslangWrapperVk.cpp]: https://chromium.googlesource.com/angle/angle/+/refs/heads/master/src/libANGLE/renderer/vulkan/GlslangWrapperVk.cpp
[SPIRV-Tools]: https://github.com/KhronosGroup/SPIRV-Tools
[translator]: https://chromium.googlesource.com/angle/angle/+/refs/heads/master/src/compiler/translator/
[TranslatorVulkan.cpp]: https://chromium.googlesource.com/angle/angle/+/refs/heads/master/src/compiler/translator/TranslatorVulkan.cpp
[VkShaderModule]: https://www.khronos.org/registry/vulkan/specs/1.1-extensions/man/html/VkShaderModule.html

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