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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>cache_fb
</b> - force cached shaders to be ignored and do a full
54 recompile via the fallback path
</li>
55 <li><b>uniform
</b> - print message to stdout when glUniform is called
56 <li><b>nopvert
</b> - force vertex shaders to be a simple shader that just transforms
57 the vertex position with ftransform() and passes through the color and
58 texcoord[
0] attributes.
59 <li><b>nopfrag
</b> - force fragment shader to be a simple shader that passes
60 through the color attribute.
61 <li><b>useprog
</b> - log glUseProgram calls to stderr
64 Example: export MESA_GLSL=dump,nopt
68 Shaders can be dumped and replaced on runtime for debugging purposes. This
69 feature is not currently supported by SCons build.
71 This is controlled via following environment variables:
73 <li><b>MESA_SHADER_DUMP_PATH
</b> - path where shader sources are dumped
74 <li><b>MESA_SHADER_READ_PATH
</b> - path where replacement shaders are read
76 Note, path set must exist before running for dumping or replacing to work.
77 When both are set, these paths should be different so the dumped shaders do
78 not clobber the replacement shaders. Also, the filenames of the replacement shaders
79 should match the filenames of the corresponding dumped shaders.
82 <h2 id=
"support">GLSL Version
</h2>
85 The GLSL compiler currently supports version
3.30 of the shading language.
89 Several GLSL extensions are also supported:
92 <li>GL_ARB_draw_buffers
93 <li>GL_ARB_fragment_coord_conventions
94 <li>GL_ARB_shader_bit_encoding
98 <h2 id=
"unsup">Unsupported Features
</h2>
100 <p>XXX update this section
</p>
103 The following features of the shading language are not yet fully supported
108 <li>Linking of multiple shaders does not always work. Currently, linking
109 is implemented through shader concatenation and re-compiling. This
110 doesn't always work because of some #pragma and preprocessor issues.
111 <li>The gl_Color and gl_SecondaryColor varying vars are interpolated
112 without perspective correction
116 All other major features of the shading language should function.
120 <h2 id=
"notes">Implementation Notes
</h2>
123 <li>Shading language programs are compiled into low-level programs
124 very similar to those of GL_ARB_vertex/fragment_program.
125 <li>All vector types (vec2, vec3, vec4, bvec2, etc) currently occupy full
127 <li>Float constants and variables are packed so that up to four floats
128 can occupy one program parameter/register.
129 <li>All function calls are inlined.
130 <li>Shaders which use too many registers will not compile.
131 <li>The quality of generated code is pretty good, register usage is fair.
132 <li>Shader error detection and reporting of errors (InfoLog) is not
134 <li>The ftransform() function doesn't necessarily match the results of
135 fixed-function transformation.
139 These issues will be addressed/resolved in the future.
143 <h2 id=
"hints">Programming Hints
</h2>
146 <li>Use the built-in library functions whenever possible.
147 For example, instead of writing this:
149 float x =
1.0 / sqrt(y);
153 float x = inversesqrt(y);
159 <h2 id=
"standalone">Stand-alone GLSL Compiler
</h2>
162 The stand-alone GLSL compiler program can be used to compile GLSL shaders
163 into low-level GPU code.
167 This tool is useful for:
170 <li>Inspecting GPU code to gain insight into compilation
171 <li>Generating initial GPU code for subsequent hand-tuning
172 <li>Debugging the GLSL compiler itself
176 After building Mesa, the compiler can be found at src/compiler/glsl/glsl_compiler
180 Here's an example of using the compiler to compile a vertex shader and
181 emit GL_ARB_vertex_program-style instructions:
184 src/compiler/glsl/glsl_compiler --version XXX --dump-ast myshader.vert
189 <li><b>--dump-ast
</b> - dump GPU code
190 <li><b>--dump-hir
</b> - dump high-level IR code
191 <li><b>--dump-lir
</b> - dump low-level IR code
192 <li><b>--dump-builder
</b> - dump GLSL IR code
193 <li><b>--link
</b> - link shaders
194 <li><b>--just-log
</b> - display only shader / linker info if exist,
195 without any header or separator
196 <li><b>--version
</b> - [Mandatory] define the GLSL version to use
200 <h2 id=
"implementation">Compiler Implementation
</h2>
203 The source code for Mesa's shading language compiler is in the
204 <code>src/compiler/glsl/
</code> directory.
208 XXX provide some info about the compiler....
212 The final vertex and fragment programs may be interpreted in software
213 (see prog_execute.c) or translated into a specific hardware architecture
214 (see drivers/dri/i915/i915_fragprog.c for example).
217 <h2 id=
"validation">Compiler Validation
</h2>
220 Developers working on the GLSL compiler should test frequently to avoid
225 The
<a href=
"https://piglit.freedesktop.org/">Piglit
</a> project
230 The Mesa demos repository also has some good GLSL tests.