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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=
"http://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>nopt
</b> - disable compiler optimizations
53 <li><b>opt
</b> - force compiler optimizations
54 <li><b>uniform
</b> - print message to stdout when glUniform is called
55 <li><b>nopvert
</b> - force vertex shaders to be a simple shader that just transforms
56 the vertex position with ftransform() and passes through the color and
57 texcoord[
0] attributes.
58 <li><b>nopfrag
</b> - force fragment shader to be a simple shader that passes
59 through the color attribute.
60 <li><b>useprog
</b> - log glUseProgram calls to stderr
63 Example: export MESA_GLSL=dump,nopt
67 Shaders can be dumped and replaced on runtime for debugging purposes. Mesa
68 needs to be configured with '--with-sha1' to enable this functionality. 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.
81 <h2 id=
"support">GLSL Version
</h2>
84 The GLSL compiler currently supports version
3.30 of the shading language.
88 Several GLSL extensions are also supported:
91 <li>GL_ARB_draw_buffers
92 <li>GL_ARB_fragment_coord_conventions
93 <li>GL_ARB_shader_bit_encoding
97 <h2 id=
"unsup">Unsupported Features
</h2>
99 <p>XXX update this section
</p>
102 The following features of the shading language are not yet fully supported
107 <li>Linking of multiple shaders does not always work. Currently, linking
108 is implemented through shader concatenation and re-compiling. This
109 doesn't always work because of some #pragma and preprocessor issues.
110 <li>The gl_Color and gl_SecondaryColor varying vars are interpolated
111 without perspective correction
115 All other major features of the shading language should function.
119 <h2 id=
"notes">Implementation Notes
</h2>
122 <li>Shading language programs are compiled into low-level programs
123 very similar to those of GL_ARB_vertex/fragment_program.
124 <li>All vector types (vec2, vec3, vec4, bvec2, etc) currently occupy full
126 <li>Float constants and variables are packed so that up to four floats
127 can occupy one program parameter/register.
128 <li>All function calls are inlined.
129 <li>Shaders which use too many registers will not compile.
130 <li>The quality of generated code is pretty good, register usage is fair.
131 <li>Shader error detection and reporting of errors (InfoLog) is not
133 <li>The ftransform() function doesn't necessarily match the results of
134 fixed-function transformation.
138 These issues will be addressed/resolved in the future.
142 <h2 id=
"hints">Programming Hints
</h2>
145 <li>Use the built-in library functions whenever possible.
146 For example, instead of writing this:
148 float x =
1.0 / sqrt(y);
152 float x = inversesqrt(y);
158 <h2 id=
"standalone">Stand-alone GLSL Compiler
</h2>
161 The stand-alone GLSL compiler program can be used to compile GLSL shaders
162 into low-level GPU code.
166 This tool is useful for:
169 <li>Inspecting GPU code to gain insight into compilation
170 <li>Generating initial GPU code for subsequent hand-tuning
171 <li>Debugging the GLSL compiler itself
175 After building Mesa, the compiler can be found at src/glsl/glsl_compiler
179 Here's an example of using the compiler to compile a vertex shader and
180 emit GL_ARB_vertex_program-style instructions:
183 src/glsl/glsl_compiler --dump-ast myshader.vert
188 <li><b>--dump-ast
</b> - dump GPU code
189 <li><b>--dump-hir
</b> - dump high-level IR code
190 <li><b>--dump-lir
</b> - dump low-level IR code
191 <li><b>--link
</b> - ???
195 <h2 id=
"implementation">Compiler Implementation
</h2>
198 The source code for Mesa's shading language compiler is in the
199 <code>src/glsl/
</code> directory.
203 XXX provide some info about the compiler....
207 The final vertex and fragment programs may be interpreted in software
208 (see prog_execute.c) or translated into a specific hardware architecture
209 (see drivers/dri/i915/i915_fragprog.c for example).
212 <h3>Code Generation Options
</h3>
215 Internally, there are several options that control the compiler's code
216 generation and instruction selection.
217 These options are seen in the gl_shader_state struct and may be set
218 by the device driver to indicate its preferences:
221 struct gl_shader_state
224 /** Driver-selectable options: */
225 GLboolean EmitHighLevelInstructions;
226 GLboolean EmitCondCodes;
227 GLboolean EmitComments;
232 <dt>EmitHighLevelInstructions
</dt>
234 This option controls instruction selection for loops and conditionals.
235 If the option is set high-level IF/ELSE/ENDIF, LOOP/ENDLOOP, CONT/BRK
236 instructions will be emitted.
237 Otherwise, those constructs will be implemented with BRA instructions.
240 <dt>EmitCondCodes
</dt>
242 If set, condition codes (ala GL_NV_fragment_program) will be used for
243 branching and looping.
244 Otherwise, ordinary registers will be used (the IF instruction will
245 examine the first operand's X component and do the if-part if non-zero).
246 This option is only relevant if EmitHighLevelInstructions is set.
249 <dt>EmitComments
</dt>
251 If set, instructions will be annotated with comments to help with debugging.
252 Extra NOP instructions will also be inserted.
257 <h2 id=
"validation">Compiler Validation
</h2>
260 Developers working on the GLSL compiler should test frequently to avoid
265 The
<a href=
"http://piglit.freedesktop.org/">Piglit
</a> project
270 The Mesa demos repository also has some good GLSL tests.