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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<li><b>errors</b> - GLSL compilation and link errors will be reported to stderr.
63</ul>
64<p>
65Example:  export MESA_GLSL=dump,nopt
66</p>
67
68<h3 id="replacement">Experimenting with Shader Replacements</h3>
69<p>
70Shaders can be dumped and replaced on runtime for debugging purposes. This
71feature is not currently supported by SCons build.
72
73This is controlled via following environment variables:
74</p>
75<ul>
76<li><b>MESA_SHADER_DUMP_PATH</b> - path where shader sources are dumped
77<li><b>MESA_SHADER_READ_PATH</b> - path where replacement shaders are read
78</ul>
79Note, path set must exist before running for dumping or replacing to work. 
80When both are set, these paths should be different so the dumped shaders do 
81not clobber the replacement shaders. Also, the filenames of the replacement shaders
82should match the filenames of the corresponding dumped shaders.
83
84<h3 id="capture">Capturing Shaders</h3>
85
86<p>
87Setting <b>MESA_SHADER_CAPTURE_PATH</b> to a directory will cause the compiler
88to write <tt>.shader_test</tt> files for use with
89<a href="https://gitlab.freedesktop.org/mesa/shader-db">shader-db</a>, a tool
90which compiler developers can use to gather statistics about shaders
91(instructions, cycles, memory accesses, and so on).
92</p>
93<p>
94Notably, this captures linked GLSL shaders - with all stages together -
95as well as ARB programs.
96</p>
97
98<h2 id="support">GLSL Version</h2>
99
100<p>
101The GLSL compiler currently supports version 3.30 of the shading language.
102</p>
103
104<p>
105Several GLSL extensions are also supported:
106</p>
107<ul>
108<li>GL_ARB_draw_buffers
109<li>GL_ARB_fragment_coord_conventions
110<li>GL_ARB_shader_bit_encoding
111</ul>
112
113
114<h2 id="unsup">Unsupported Features</h2>
115
116<p>XXX update this section</p>
117
118<p>
119The following features of the shading language are not yet fully supported
120in Mesa:
121</p>
122
123<ul>
124<li>Linking of multiple shaders does not always work.  Currently, linking
125    is implemented through shader concatenation and re-compiling.  This
126    doesn't always work because of some #pragma and preprocessor issues.
127<li>The gl_Color and gl_SecondaryColor varying vars are interpolated
128    without perspective correction
129</ul>
130
131<p>
132All other major features of the shading language should function.
133</p>
134
135
136<h2 id="notes">Implementation Notes</h2>
137
138<ul>
139<li>Shading language programs are compiled into low-level programs
140    very similar to those of GL_ARB_vertex/fragment_program.
141<li>All vector types (vec2, vec3, vec4, bvec2, etc) currently occupy full
142    float[4] registers.
143<li>Float constants and variables are packed so that up to four floats
144    can occupy one program parameter/register.
145<li>All function calls are inlined.
146<li>Shaders which use too many registers will not compile.
147<li>The quality of generated code is pretty good, register usage is fair.
148<li>Shader error detection and reporting of errors (InfoLog) is not
149    very good yet.
150<li>The ftransform() function doesn't necessarily match the results of
151    fixed-function transformation.
152</ul>
153
154<p>
155These issues will be addressed/resolved in the future.
156</p>
157
158
159<h2 id="hints">Programming Hints</h2>
160
161<ul>
162<li>Use the built-in library functions whenever possible.
163    For example, instead of writing this:
164<pre>
165        float x = 1.0 / sqrt(y);
166</pre>
167    Write this:
168<pre>
169        float x = inversesqrt(y);
170</pre>
171</li>
172</ul>
173
174
175<h2 id="standalone">Stand-alone GLSL Compiler</h2>
176
177<p>
178The stand-alone GLSL compiler program can be used to compile GLSL shaders
179into low-level GPU code.
180</p>
181
182<p>
183This tool is useful for:
184</p>
185<ul>
186<li>Inspecting GPU code to gain insight into compilation
187<li>Generating initial GPU code for subsequent hand-tuning
188<li>Debugging the GLSL compiler itself
189</ul>
190
191<p>
192After building Mesa, the compiler can be found at src/compiler/glsl/glsl_compiler
193</p>
194
195<p>
196Here's an example of using the compiler to compile a vertex shader and
197emit GL_ARB_vertex_program-style instructions:
198</p>
199<pre>
200    src/compiler/glsl/glsl_compiler --version XXX --dump-ast myshader.vert
201</pre>
202
203Options include
204<ul>
205<li><b>--dump-ast</b> - dump GPU code
206<li><b>--dump-hir</b> - dump high-level IR code
207<li><b>--dump-lir</b> - dump low-level IR code
208<li><b>--dump-builder</b> - dump GLSL IR code
209<li><b>--link</b> - link shaders
210<li><b>--just-log</b> - display only shader / linker info if exist,
211without any header or separator
212<li><b>--version</b> - [Mandatory] define the GLSL version to use
213</ul>
214
215
216<h2 id="implementation">Compiler Implementation</h2>
217
218<p>
219The source code for Mesa's shading language compiler is in the
220<code>src/compiler/glsl/</code> directory.
221</p>
222
223<p>
224XXX provide some info about the compiler....
225</p>
226
227<p>
228The final vertex and fragment programs may be interpreted in software
229(see prog_execute.c) or translated into a specific hardware architecture
230(see drivers/dri/i915/i915_fragprog.c for example).
231</p>
232
233<h2 id="validation">Compiler Validation</h2>
234
235<p>
236Developers working on the GLSL compiler should test frequently to avoid
237regressions.
238</p>
239
240<p>
241The <a href="https://piglit.freedesktop.org/">Piglit</a> project
242has many GLSL tests.
243</p>
244
245<p>
246The Mesa demos repository also has some good GLSL tests.
247</p>
248
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