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11  <h1>The Mesa 3D Graphics Library</h1>
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16
17<h1>Shading Language Support</h1>
18
19<p>
20This page describes the features and status of Mesa's support for the
21<a href="https://opengl.org/documentation/glsl/">
22OpenGL Shading Language</a>.
23</p>
24
25<p>
26Contents
27</p>
28<ul>
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>
37</ul>
38
39
40<h2 id="envvars">Environment Variables</h2>
41
42<p>
43The <b>MESA_GLSL</b> environment variable can be set to a comma-separated
44list of keywords to control some aspects of the GLSL compiler and shader
45execution.  These are generally used for debugging.
46</p>
47<ul>
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
51    the shader ID.
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
62</ul>
63<p>
64Example:  export MESA_GLSL=dump,nopt
65</p>
66
67<h3 id="replacement">Experimenting with Shader Replacements</h3>
68<p>
69Shaders can be dumped and replaced on runtime for debugging purposes. This
70feature is not currently supported by SCons build.
71
72This is controlled via following environment variables:
73<ul>
74<li><b>MESA_SHADER_DUMP_PATH</b> - path where shader sources are dumped
75<li><b>MESA_SHADER_READ_PATH</b> - path where replacement shaders are read
76</ul>
77Note, path set must exist before running for dumping or replacing to work.
78When both are set, these paths should be different so the dumped shaders do
79not clobber the replacement shaders. Also, the filenames of the replacement shaders
80should match the filenames of the corresponding dumped shaders.
81</p>
82
83<h3 id="capture">Capturing Shaders</h3>
84
85<p>
86Setting <b>MESA_SHADER_CAPTURE_PATH</b> to a directory will cause the compiler
87to write <tt>.shader_test</tt> files for use with
88<a href="https://cgit.freedesktop.org/mesa/shader-db">shader-db</a>, a tool
89which compiler developers can use to gather statistics about shaders
90(instructions, cycles, memory accesses, and so on).
91</p>
92<p>
93Notably, this captures linked GLSL shaders - with all stages together -
94as well as ARB programs.
95</p>
96
97<h2 id="support">GLSL Version</h2>
98
99<p>
100The GLSL compiler currently supports version 3.30 of the shading language.
101</p>
102
103<p>
104Several GLSL extensions are also supported:
105</p>
106<ul>
107<li>GL_ARB_draw_buffers
108<li>GL_ARB_fragment_coord_conventions
109<li>GL_ARB_shader_bit_encoding
110</ul>
111
112
113<h2 id="unsup">Unsupported Features</h2>
114
115<p>XXX update this section</p>
116
117<p>
118The following features of the shading language are not yet fully supported
119in Mesa:
120</p>
121
122<ul>
123<li>Linking of multiple shaders does not always work.  Currently, linking
124    is implemented through shader concatenation and re-compiling.  This
125    doesn't always work because of some #pragma and preprocessor issues.
126<li>The gl_Color and gl_SecondaryColor varying vars are interpolated
127    without perspective correction
128</ul>
129
130<p>
131All other major features of the shading language should function.
132</p>
133
134
135<h2 id="notes">Implementation Notes</h2>
136
137<ul>
138<li>Shading language programs are compiled into low-level programs
139    very similar to those of GL_ARB_vertex/fragment_program.
140<li>All vector types (vec2, vec3, vec4, bvec2, etc) currently occupy full
141    float[4] registers.
142<li>Float constants and variables are packed so that up to four floats
143    can occupy one program parameter/register.
144<li>All function calls are inlined.
145<li>Shaders which use too many registers will not compile.
146<li>The quality of generated code is pretty good, register usage is fair.
147<li>Shader error detection and reporting of errors (InfoLog) is not
148    very good yet.
149<li>The ftransform() function doesn't necessarily match the results of
150    fixed-function transformation.
151</ul>
152
153<p>
154These issues will be addressed/resolved in the future.
155</p>
156
157
158<h2 id="hints">Programming Hints</h2>
159
160<ul>
161<li>Use the built-in library functions whenever possible.
162    For example, instead of writing this:
163<pre>
164        float x = 1.0 / sqrt(y);
165</pre>
166    Write this:
167<pre>
168        float x = inversesqrt(y);
169</pre>
170</li>
171</ul>
172
173
174<h2 id="standalone">Stand-alone GLSL Compiler</h2>
175
176<p>
177The stand-alone GLSL compiler program can be used to compile GLSL shaders
178into low-level GPU code.
179</p>
180
181<p>
182This tool is useful for:
183</p>
184<ul>
185<li>Inspecting GPU code to gain insight into compilation
186<li>Generating initial GPU code for subsequent hand-tuning
187<li>Debugging the GLSL compiler itself
188</ul>
189
190<p>
191After building Mesa, the compiler can be found at src/compiler/glsl/glsl_compiler
192</p>
193
194<p>
195Here's an example of using the compiler to compile a vertex shader and
196emit GL_ARB_vertex_program-style instructions:
197</p>
198<pre>
199    src/compiler/glsl/glsl_compiler --version XXX --dump-ast myshader.vert
200</pre>
201
202Options include
203<ul>
204<li><b>--dump-ast</b> - dump GPU code
205<li><b>--dump-hir</b> - dump high-level IR code
206<li><b>--dump-lir</b> - dump low-level IR code
207<li><b>--dump-builder</b> - dump GLSL IR code
208<li><b>--link</b> - link shaders
209<li><b>--just-log</b> - display only shader / linker info if exist,
210without any header or separator
211<li><b>--version</b> - [Mandatory] define the GLSL version to use
212</ul>
213
214
215<h2 id="implementation">Compiler Implementation</h2>
216
217<p>
218The source code for Mesa's shading language compiler is in the
219<code>src/compiler/glsl/</code> directory.
220</p>
221
222<p>
223XXX provide some info about the compiler....
224</p>
225
226<p>
227The final vertex and fragment programs may be interpreted in software
228(see prog_execute.c) or translated into a specific hardware architecture
229(see drivers/dri/i915/i915_fragprog.c for example).
230</p>
231
232<h2 id="validation">Compiler Validation</h2>
233
234<p>
235Developers working on the GLSL compiler should test frequently to avoid
236regressions.
237</p>
238
239<p>
240The <a href="https://piglit.freedesktop.org/">Piglit</a> project
241has many GLSL tests.
242</p>
243
244<p>
245The Mesa demos repository also has some good GLSL tests.
246</p>
247
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