SPIR-V Transpiler

Note: This library is currently considered experimental until shader compilation is verified by engine unit tests, see the Testing section below for more details.

A dart library for transpiling a subset of SPIR-V to the shader languages used by Flutter internally.

• SkSL
• GLSL ES 100
• GLSL ES 300

All exported symbols are documented in lib/spirv.dart.

The supported subset of SPIR-V is specified in lib/src/constants.dart.

If you're using GLSL to generate SPIR-V with glslangValidator or shaderc, the code will need to adhere to the following rules.

• There must be a single vec4 output at location 0.
• The output can only be written to from the main function.
• gl_FragCoord can only be read from the main function, and its z and w components have no meaning.
• Control flow is prohibited aside from function calls and return. if, while, for, switch, etc.
• No inputs from other shader stages.
• Only float, float-vector types, and square float-matrix types.
• Only square matrices are supported.
• Only built-in functions present in GLSL ES 100 are used.
• Debug symbols must be stripped, you can use the spirv-opt --strip-debug flag.

These rules may become less strict in future versions. Confirmant SPIR-V should succesfully transpile from the current version onwards. In other words, a spir-v shader you use now that meets these rules should keep working, but the output of the transpiler may change for that shader.

Support for textures, control flow, and structured types is planned, but not currently included.

Testing

Exception Tests

These tests rely on the .spvasm (SPIR-V Assembly) and .glsl files contained under test/exception_shaders in this directory. They are compiled to binary SPIR-V using spirv-asm, from the SwiftShader dependency. They are tested by testing/dart/spirv_exception_test.dart as part of the normal suite of dart tests. The purpose of these tests is to exercise every explicit failure path for shader transpilation. Each glsl or spvasm file should include a comment describing the failure that it is testing. The given files should be valid apart from the single failure case they are testing.

To test the exception tests directly: ./testing/run_tests.py --type dart --dart-filter spirv_exception_test.dart

Pixel Tests

Pixel tests should run as part of unit-testing for each implementation of dart:ui. Currently, FragmentShader is only supported in C++. These tests aim to validate the correctness of transpilation to each target language. Each shader should render the color green for a correct transpilation, and any other color for failure. They will be a GLSL files that are compiled to SPIR-V via shaderc. Therefor, the fragColor should resolve to vec4(0.0, 1.0, 0.0, 1.0) for all tests.

In each test, the uniform a is initialized with the value of 1.0. This is important so that expressions are not simplified during GLSL to SPIR-V compilation, which may result in the removal of the op being tested.

To test the pixel tests directly: ./testing/run_tests.py --type dart --dart-filter fragment_shader_test.dart

A Note on Test Isolation

Even the simplest GLSL program tests several instructions, so no test us completely isolated to a single op. Also, some of the GLSL 450 op tests will use addition in subtraction, along with the actual op being tested. However, the GLSL program for each test file is kept as simple as possible, to satisfy these conditions: pass if the op works, and fail if the op does not work. In some tests, it is sufficient to only call the GLSL op once, while other may need more calls to more completelty test the op. Many ops support scalars, vectors, or a combination as parameters. Most tests default to using scalars as params, but vec2, vec3, and vec4 parameters are also tested.

• vec2 is tested as a paramter in glsl_op_normalize.glsl
• vec3 is tested as a parameter in glsl_op_cross.glsl
• vec4 is tested as a parameter in glsl_op_length.glsl

Adding New Tests

To add a new test, add a glsl (fragment shader tests) or spvasm (spirv exception tests) src file to a lib/spirv/test/ subfolder, and add the file as a source to the corresponding BUILD.gn.

• New files in exception_shaders are automatically tested in testing/dart/spirv_exception_test.
• New files in supported_op_shaders and supported_glsl_op_shaders are automatically tested in testing/dart/fragment_shader_test.
• New files in general_shaders are not automatically tested and must add a new manual test case in testing/dart/fragment_shader_test.