Shading Language

This page describes the features and status of Mesa’s support for the OpenGL Shading Language.

Environment Variables

The MESA_GLSL environment variable can be set to a comma-separated list of keywords to control some aspects of the GLSL compiler and shader execution. These are generally used for debugging.

  • dump - print GLSL shader code to stdout at link time

  • log - log all GLSL shaders to files. The filenames will be “shader_X.vert” or “shader_X.frag” where X the shader ID.

  • cache_info - print debug information about shader cache

  • cache_fb - force cached shaders to be ignored and do a full recompile via the fallback path

  • uniform - print message to stdout when glUniform is called

  • nopvert - force vertex shaders to be a simple shader that just transforms the vertex position with ftransform() and passes through the color and texcoord[0] attributes.

  • nopfrag - force fragment shader to be a simple shader that passes through the color attribute.

  • useprog - log glUseProgram calls to stderr

  • errors - GLSL compilation and link errors will be reported to stderr.

Example: export MESA_GLSL=dump,nopt

Experimenting with Shader Replacements

Shaders can be dumped and replaced on runtime for debugging purposes. This feature is not currently supported by SCons build. This is controlled via following environment variables:

  • MESA_SHADER_DUMP_PATH - path where shader sources are dumped

  • MESA_SHADER_READ_PATH - path where replacement shaders are read

Note, path set must exist before running for dumping or replacing to work. When both are set, these paths should be different so the dumped shaders do not clobber the replacement shaders. Also, the filenames of the replacement shaders should match the filenames of the corresponding dumped shaders.

Capturing Shaders

Setting MESA_SHADER_CAPTURE_PATH to a directory will cause the compiler to write .shader_test files for use with shader-db, a tool which compiler developers can use to gather statistics about shaders (instructions, cycles, memory accesses, and so on).

Notably, this captures linked GLSL shaders - with all stages together - as well as ARB programs.

GLSL Version

The GLSL compiler currently supports version 3.30 of the shading language.

Several GLSL extensions are also supported:

  • GL_ARB_draw_buffers

  • GL_ARB_fragment_coord_conventions

  • GL_ARB_shader_bit_encoding

Unsupported Features

XXX update this section

The following features of the shading language are not yet fully supported in Mesa:

  • Linking of multiple shaders does not always work. Currently, linking is implemented through shader concatenation and re-compiling. This doesn’t always work because of some #pragma and preprocessor issues.

  • The gl_Color and gl_SecondaryColor varying vars are interpolated without perspective correction

All other major features of the shading language should function.

Implementation Notes

  • Shading language programs are compiled into low-level programs very similar to those of GL_ARB_vertex/fragment_program.

  • All vector types (vec2, vec3, vec4, bvec2, etc) currently occupy full float[4] registers.

  • Float constants and variables are packed so that up to four floats can occupy one program parameter/register.

  • All function calls are inlined.

  • Shaders which use too many registers will not compile.

  • The quality of generated code is pretty good, register usage is fair.

  • Shader error detection and reporting of errors (InfoLog) is not very good yet.

  • The ftransform() function doesn’t necessarily match the results of fixed-function transformation.

These issues will be addressed/resolved in the future.

Programming Hints

  • Use the built-in library functions whenever possible. For example, instead of writing this:

    float x = 1.0 / sqrt(y);

    Write this:

    float x = inversesqrt(y);

Stand-alone GLSL Compiler

The stand-alone GLSL compiler program can be used to compile GLSL shaders into low-level GPU code.

This tool is useful for:

  • Inspecting GPU code to gain insight into compilation

  • Generating initial GPU code for subsequent hand-tuning

  • Debugging the GLSL compiler itself

After building Mesa, the compiler can be found at src/compiler/glsl/glsl_compiler

Here’s an example of using the compiler to compile a vertex shader and emit GL_ARB_vertex_program-style instructions:

src/compiler/glsl/glsl_compiler --version XXX --dump-ast myshader.vert

Options include

  • –dump-ast - dump GPU code

  • –dump-hir - dump high-level IR code

  • –dump-lir - dump low-level IR code

  • –dump-builder - dump GLSL IR code

  • –link - link shaders

  • –just-log - display only shader / linker info if exist, without any header or separator

  • –version - [Mandatory] define the GLSL version to use

Compiler Implementation

The source code for Mesa’s shading language compiler is in the src/compiler/glsl/ directory.

XXX provide some info about the compiler….

The final vertex and fragment programs may be interpreted in software (see prog_execute.c) or translated into a specific hardware architecture (see drivers/dri/i915/i915_fragprog.c for example).

Compiler Validation

Developers working on the GLSL compiler should test frequently to avoid regressions.

The Piglit project has many GLSL tests.

The Mesa demos repository also has some good GLSL tests.