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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 <h2 id=
"support">GLSL Version
</h2>
70 The GLSL compiler currently supports version
3.30 of the shading language.
74 Several GLSL extensions are also supported:
77 <li>GL_ARB_draw_buffers
78 <li>GL_ARB_fragment_coord_conventions
79 <li>GL_ARB_shader_bit_encoding
83 <h2 id=
"unsup">Unsupported Features
</h2>
85 <p>XXX update this section
</p>
88 The following features of the shading language are not yet fully supported
93 <li>Linking of multiple shaders does not always work. Currently, linking
94 is implemented through shader concatenation and re-compiling. This
95 doesn't always work because of some #pragma and preprocessor issues.
96 <li>The gl_Color and gl_SecondaryColor varying vars are interpolated
97 without perspective correction
101 All other major features of the shading language should function.
105 <h2 id=
"notes">Implementation Notes
</h2>
108 <li>Shading language programs are compiled into low-level programs
109 very similar to those of GL_ARB_vertex/fragment_program.
110 <li>All vector types (vec2, vec3, vec4, bvec2, etc) currently occupy full
112 <li>Float constants and variables are packed so that up to four floats
113 can occupy one program parameter/register.
114 <li>All function calls are inlined.
115 <li>Shaders which use too many registers will not compile.
116 <li>The quality of generated code is pretty good, register usage is fair.
117 <li>Shader error detection and reporting of errors (InfoLog) is not
119 <li>The ftransform() function doesn't necessarily match the results of
120 fixed-function transformation.
124 These issues will be addressed/resolved in the future.
128 <h2 id=
"hints">Programming Hints
</h2>
131 <li>Use the built-in library functions whenever possible.
132 For example, instead of writing this:
134 float x =
1.0 / sqrt(y);
138 float x = inversesqrt(y);
144 <h2 id=
"standalone">Stand-alone GLSL Compiler
</h2>
147 The stand-alone GLSL compiler program can be used to compile GLSL shaders
148 into low-level GPU code.
152 This tool is useful for:
155 <li>Inspecting GPU code to gain insight into compilation
156 <li>Generating initial GPU code for subsequent hand-tuning
157 <li>Debugging the GLSL compiler itself
161 After building Mesa, the compiler can be found at src/glsl/glsl_compiler
165 Here's an example of using the compiler to compile a vertex shader and
166 emit GL_ARB_vertex_program-style instructions:
169 src/glsl/glsl_compiler --dump-ast myshader.vert
174 <li><b>--dump-ast
</b> - dump GPU code
175 <li><b>--dump-hir
</b> - dump high-level IR code
176 <li><b>--dump-lir
</b> - dump low-level IR code
177 <li><b>--link
</b> - ???
181 <h2 id=
"implementation">Compiler Implementation
</h2>
184 The source code for Mesa's shading language compiler is in the
185 <code>src/glsl/
</code> directory.
189 XXX provide some info about the compiler....
193 The final vertex and fragment programs may be interpreted in software
194 (see prog_execute.c) or translated into a specific hardware architecture
195 (see drivers/dri/i915/i915_fragprog.c for example).
198 <h3>Code Generation Options
</h3>
201 Internally, there are several options that control the compiler's code
202 generation and instruction selection.
203 These options are seen in the gl_shader_state struct and may be set
204 by the device driver to indicate its preferences:
207 struct gl_shader_state
210 /** Driver-selectable options: */
211 GLboolean EmitHighLevelInstructions;
212 GLboolean EmitCondCodes;
213 GLboolean EmitComments;
218 <dt>EmitHighLevelInstructions
</dt>
220 This option controls instruction selection for loops and conditionals.
221 If the option is set high-level IF/ELSE/ENDIF, LOOP/ENDLOOP, CONT/BRK
222 instructions will be emitted.
223 Otherwise, those constructs will be implemented with BRA instructions.
226 <dt>EmitCondCodes
</dt>
228 If set, condition codes (ala GL_NV_fragment_program) will be used for
229 branching and looping.
230 Otherwise, ordinary registers will be used (the IF instruction will
231 examine the first operand's X component and do the if-part if non-zero).
232 This option is only relevant if EmitHighLevelInstructions is set.
235 <dt>EmitComments
</dt>
237 If set, instructions will be annotated with comments to help with debugging.
238 Extra NOP instructions will also be inserted.
243 <h2 id=
"validation">Compiler Validation
</h2>
246 Developers working on the GLSL compiler should test frequently to avoid
251 The
<a href=
"http://piglit.freedesktop.org/">Piglit
</a> project
256 The Mesa demos repository also has some good GLSL tests.