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11 <h1>The Mesa
3D Graphics Library
</h1>
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17 <h1>Shading Language Support
</h1>
20 This page describes the features and status of Mesa's support for the
21 <a href=
"https://opengl.org/documentation/glsl/">
22 OpenGL Shading Language
</a>.
29 <li><a href=
"#envvars">Environment variables
</a>
30 <li><a href=
"#support">GLSL
1.40 support
</a>
31 <li><a href=
"#unsup">Unsupported Features
</a>
32 <li><a href=
"#notes">Implementation Notes
</a>
33 <li><a href=
"#hints">Programming Hints
</a>
34 <li><a href=
"#standalone">Stand-alone GLSL Compiler
</a>
35 <li><a href=
"#implementation">Compiler Implementation
</a>
36 <li><a href=
"#validation">Compiler Validation
</a>
40 <h2 id=
"envvars">Environment Variables
</h2>
43 The
<b>MESA_GLSL
</b> environment variable can be set to a comma-separated
44 list of keywords to control some aspects of the GLSL compiler and shader
45 execution. These are generally used for debugging.
48 <li><b>dump
</b> - print GLSL shader code to stdout at link time
49 <li><b>log
</b> - log all GLSL shaders to files.
50 The filenames will be
"shader_X.vert" or
"shader_X.frag" where X
52 <li><b>cache_info
</b> - print debug information about shader cache
53 <li><b>uniform
</b> - print message to stdout when glUniform is called
54 <li><b>nopvert
</b> - force vertex shaders to be a simple shader that just transforms
55 the vertex position with ftransform() and passes through the color and
56 texcoord[
0] attributes.
57 <li><b>nopfrag
</b> - force fragment shader to be a simple shader that passes
58 through the color attribute.
59 <li><b>useprog
</b> - log glUseProgram calls to stderr
62 Example: export MESA_GLSL=dump,nopt
66 Shaders can be dumped and replaced on runtime for debugging purposes. Mesa
67 needs to be configured with '--with-sha1' to enable this functionality. This
68 feature is not currently supported by SCons build.
70 This is controlled via following environment variables:
72 <li><b>MESA_SHADER_DUMP_PATH
</b> - path where shader sources are dumped
73 <li><b>MESA_SHADER_READ_PATH
</b> - path where replacement shaders are read
75 Note, path set must exist before running for dumping or replacing to work.
76 When both are set, these paths should be different so the dumped shaders do
77 not clobber the replacement shaders.
80 <h2 id=
"support">GLSL Version
</h2>
83 The GLSL compiler currently supports version
3.30 of the shading language.
87 Several GLSL extensions are also supported:
90 <li>GL_ARB_draw_buffers
91 <li>GL_ARB_fragment_coord_conventions
92 <li>GL_ARB_shader_bit_encoding
96 <h2 id=
"unsup">Unsupported Features
</h2>
98 <p>XXX update this section
</p>
101 The following features of the shading language are not yet fully supported
106 <li>Linking of multiple shaders does not always work. Currently, linking
107 is implemented through shader concatenation and re-compiling. This
108 doesn't always work because of some #pragma and preprocessor issues.
109 <li>The gl_Color and gl_SecondaryColor varying vars are interpolated
110 without perspective correction
114 All other major features of the shading language should function.
118 <h2 id=
"notes">Implementation Notes
</h2>
121 <li>Shading language programs are compiled into low-level programs
122 very similar to those of GL_ARB_vertex/fragment_program.
123 <li>All vector types (vec2, vec3, vec4, bvec2, etc) currently occupy full
125 <li>Float constants and variables are packed so that up to four floats
126 can occupy one program parameter/register.
127 <li>All function calls are inlined.
128 <li>Shaders which use too many registers will not compile.
129 <li>The quality of generated code is pretty good, register usage is fair.
130 <li>Shader error detection and reporting of errors (InfoLog) is not
132 <li>The ftransform() function doesn't necessarily match the results of
133 fixed-function transformation.
137 These issues will be addressed/resolved in the future.
141 <h2 id=
"hints">Programming Hints
</h2>
144 <li>Use the built-in library functions whenever possible.
145 For example, instead of writing this:
147 float x =
1.0 / sqrt(y);
151 float x = inversesqrt(y);
157 <h2 id=
"standalone">Stand-alone GLSL Compiler
</h2>
160 The stand-alone GLSL compiler program can be used to compile GLSL shaders
161 into low-level GPU code.
165 This tool is useful for:
168 <li>Inspecting GPU code to gain insight into compilation
169 <li>Generating initial GPU code for subsequent hand-tuning
170 <li>Debugging the GLSL compiler itself
174 After building Mesa, the compiler can be found at src/compiler/glsl/glsl_compiler
178 Here's an example of using the compiler to compile a vertex shader and
179 emit GL_ARB_vertex_program-style instructions:
182 src/compiler/glsl/glsl_compiler --version XXX --dump-ast myshader.vert
187 <li><b>--dump-ast
</b> - dump GPU code
188 <li><b>--dump-hir
</b> - dump high-level IR code
189 <li><b>--dump-lir
</b> - dump low-level IR code
190 <li><b>--dump-builder
</b> - dump GLSL IR code
191 <li><b>--link
</b> - link shaders
192 <li><b>--just-log
</b> - display only shader / linker info if exist,
193 without any header or separator
194 <li><b>--version
</b> - [Mandatory] define the GLSL version to use
198 <h2 id=
"implementation">Compiler Implementation
</h2>
201 The source code for Mesa's shading language compiler is in the
202 <code>src/compiler/glsl/
</code> directory.
206 XXX provide some info about the compiler....
210 The final vertex and fragment programs may be interpreted in software
211 (see prog_execute.c) or translated into a specific hardware architecture
212 (see drivers/dri/i915/i915_fragprog.c for example).
215 <h2 id=
"validation">Compiler Validation
</h2>
218 Developers working on the GLSL compiler should test frequently to avoid
223 The
<a href=
"https://piglit.freedesktop.org/">Piglit
</a> project
228 The Mesa demos repository also has some good GLSL tests.