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9
10<h1>Shading Language Support</h1>
11
12<p>
13This page describes the features and status of Mesa's support for the
14<a href="http://opengl.org/documentation/glsl/" target="_parent">
15OpenGL Shading Language</a>.
16</p>
17
18<p>
19Contents
20</p>
21<ul>
22<li><a href="#envvars">Environment variables</a>
23<li><a href="#glsl120">GLSL 1.20 support</a>
24<li><a href="#unsup">Unsupported Features</a>
25<li><a href="#notes">Implementation Notes</a>
26<li><a href="#hints">Programming Hints</a>
27<li><a href="#standalone">Stand-alone GLSL Compiler</a>
28<li><a href="#implementation">Compiler Implementation</a>
29<li><a href="#validation">Compiler Validation</a>
30</ul>
31
32
33<h2 id="envvars">Environment Variables</h2>
34
35<p>
36The <b>MESA_GLSL</b> environment variable can be set to a comma-separated
37list of keywords to control some aspects of the GLSL compiler and shader
38execution.  These are generally used for debugging.
39</p>
40<ul>
41<li><b>dump</b> - print GLSL shader code to stdout at link time
42<li><b>log</b> - log all GLSL shaders to files.
43    The filenames will be "shader_X.vert" or "shader_X.frag" where X
44    the shader ID.
45<li><b>nopt</b> - disable compiler optimizations
46<li><b>opt</b> - force compiler optimizations
47<li><b>uniform</b> - print message to stdout when glUniform is called
48<li><b>nopvert</b> - force vertex shaders to be a simple shader that just transforms
49    the vertex position with ftransform() and passes through the color and
50    texcoord[0] attributes.
51<li><b>nopfrag</b> - force fragment shader to be a simple shader that passes
52    through the color attribute.
53<li><b>useprog</b> - log glUseProgram calls to stderr
54</ul>
55<p>
56Example:  export MESA_GLSL=dump,nopt
57</p>
58
59
60<h2 id="glsl120">GLSL Version</h2>
61
62<p>
63The GLSL compiler currently supports version 1.20 of the shading language.
64</p>
65
66<p>
67Several GLSL extensions are also supported:
68</p>
69<ul>
70<li>GL_ARB_draw_buffers
71<li>GL_ARB_texture_rectangle
72<li>GL_ARB_fragment_coord_conventions
73<li>GL_EXT_texture_array
74</ul>
75
76
77<h2 id="unsup">Unsupported Features</h2>
78
79<p>XXX update this section</p>
80
81<p>
82The following features of the shading language are not yet fully supported
83in Mesa:
84</p>
85
86<ul>
87<li>Linking of multiple shaders does not always work.  Currently, linking
88    is implemented through shader concatenation and re-compiling.  This
89    doesn't always work because of some #pragma and preprocessor issues.
90<li>gl_ClipVertex
91<li>The gl_Color and gl_SecondaryColor varying vars are interpolated
92    without perspective correction
93</ul>
94
95<p>
96All other major features of the shading language should function.
97</p>
98
99
100<h2 id="notes">Implementation Notes</h2>
101
102<ul>
103<li>Shading language programs are compiled into low-level programs
104    very similar to those of GL_ARB_vertex/fragment_program.
105<li>All vector types (vec2, vec3, vec4, bvec2, etc) currently occupy full
106    float[4] registers.
107<li>Float constants and variables are packed so that up to four floats
108    can occupy one program parameter/register.
109<li>All function calls are inlined.
110<li>Shaders which use too many registers will not compile.
111<li>The quality of generated code is pretty good, register usage is fair.
112<li>Shader error detection and reporting of errors (InfoLog) is not
113    very good yet.
114<li>The ftransform() function doesn't necessarily match the results of
115    fixed-function transformation.
116</ul>
117
118<p>
119These issues will be addressed/resolved in the future.
120</p>
121
122
123<h2 id="hints">Programming Hints</h2>
124
125<ul>
126<li>Use the built-in library functions whenever possible.
127    For example, instead of writing this:
128<pre>
129        float x = 1.0 / sqrt(y);
130</pre>
131    Write this:
132<pre>
133        float x = inversesqrt(y);
134</pre>
135</li>
136</ul>
137
138
139<h2 id="standalone">Stand-alone GLSL Compiler</h2>
140
141<p>
142The stand-alone GLSL compiler program can be used to compile GLSL shaders
143into low-level GPU code.
144</p>
145
146<p>
147This tool is useful for:
148</p>
149<ul>
150<li>Inspecting GPU code to gain insight into compilation
151<li>Generating initial GPU code for subsequent hand-tuning
152<li>Debugging the GLSL compiler itself
153</ul>
154
155<p>
156After building Mesa, the compiler can be found at src/glsl/glsl_compiler
157</p>
158
159<p>
160Here's an example of using the compiler to compile a vertex shader and
161emit GL_ARB_vertex_program-style instructions:
162</p>
163<pre>
164    src/glsl/glsl_compiler --dump-ast myshader.vert
165</pre>
166
167Options include
168<ul>
169<li><b>--dump-ast</b> - dump GPU code
170<li><b>--dump-hir</b> - dump high-level IR code
171<li><b>--dump-lir</b> - dump low-level IR code
172<li><b>--link</b> - ???
173</ul>
174
175
176<h2 id="implementation">Compiler Implementation</h2>
177
178<p>
179The source code for Mesa's shading language compiler is in the
180<code>src/glsl/</code> directory.
181</p>
182
183<p>
184XXX provide some info about the compiler....
185</p>
186
187<p>
188The final vertex and fragment programs may be interpreted in software
189(see prog_execute.c) or translated into a specific hardware architecture
190(see drivers/dri/i915/i915_fragprog.c for example).
191</p>
192
193<h3>Code Generation Options</h3>
194
195<p>
196Internally, there are several options that control the compiler's code
197generation and instruction selection.
198These options are seen in the gl_shader_state struct and may be set
199by the device driver to indicate its preferences:
200
201<pre>
202struct gl_shader_state
203{
204   ...
205   /** Driver-selectable options: */
206   GLboolean EmitHighLevelInstructions;
207   GLboolean EmitCondCodes;
208   GLboolean EmitComments;
209};
210</pre>
211
212<dl>
213<dt>EmitHighLevelInstructions</dt>
214<dd>
215This option controls instruction selection for loops and conditionals.
216If the option is set high-level IF/ELSE/ENDIF, LOOP/ENDLOOP, CONT/BRK
217instructions will be emitted.
218Otherwise, those constructs will be implemented with BRA instructions.
219</dd>
220
221<dt>EmitCondCodes</dt>
222<dd>
223If set, condition codes (ala GL_NV_fragment_program) will be used for
224branching and looping.
225Otherwise, ordinary registers will be used (the IF instruction will
226examine the first operand's X component and do the if-part if non-zero).
227This option is only relevant if EmitHighLevelInstructions is set.
228</dd>
229
230<dt>EmitComments</dt>
231<dd>
232If set, instructions will be annoted with comments to help with debugging.
233Extra NOP instructions will also be inserted.
234</dd>
235</dl>
236
237
238<h2 id="validation">Compiler Validation</h2>
239
240<p>
241Developers working on the GLSL compiler should test frequently to avoid
242regressions.
243</p>
244
245<p>
246The <a href="http://piglit.freedesktop.org/" target="_parent">Piglit</a> project
247has many GLSL tests and the
248<a href="http://glean.sf.net" target="_parent">Glean</a> glsl1 test 
249tests GLSL features.
250</p>
251
252<p>
253The Mesa demos repository also has some good GLSL tests.
254</p>
255
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