docs/shading.html

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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>
  20 This page describes the features and status of Mesa's support for the
  21 <a href="https://opengl.org/documentation/glsl/">
  22 OpenGL Shading Language</a>.
  23 </p>
  24
  25 <p>
  26 Contents
  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>
  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.
  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>
  64 Example:  export MESA_GLSL=dump,nopt
  65 </p>
  66
  67 <p>
  68 Shaders can be dumped and replaced on runtime for debugging purposes. Mesa
  69 needs to be configured with '--with-sha1' to enable this functionality. This
  70 feature is not currently supported by SCons build.
  71
  72 This 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>
  77 Note, path set must exist before running for dumping or replacing to work.
  78 When both are set, these paths should be different so the dumped shaders do
  79 not clobber the replacement shaders.
  80 </p>
  81
  82 <h2 id="support">GLSL Version</h2>
  83
  84 <p>
  85 The GLSL compiler currently supports version 3.30 of the shading language.
  86 </p>
  87
  88 <p>
  89 Several GLSL extensions are also supported:
  90 </p>
  91 <ul>
  92 <li>GL_ARB_draw_buffers
  93 <li>GL_ARB_fragment_coord_conventions
  94 <li>GL_ARB_shader_bit_encoding
  95 </ul>
  96
  97
  98 <h2 id="unsup">Unsupported Features</h2>
  99
 100 <p>XXX update this section</p>
 101
 102 <p>
 103 The following features of the shading language are not yet fully supported
 104 in Mesa:
 105 </p>
 106
 107 <ul>
 108 <li>Linking of multiple shaders does not always work.  Currently, linking
 109     is implemented through shader concatenation and re-compiling.  This
 110     doesn't always work because of some #pragma and preprocessor issues.
 111 <li>The gl_Color and gl_SecondaryColor varying vars are interpolated
 112     without perspective correction
 113 </ul>
 114
 115 <p>
 116 All other major features of the shading language should function.
 117 </p>
 118
 119
 120 <h2 id="notes">Implementation Notes</h2>
 121
 122 <ul>
 123 <li>Shading language programs are compiled into low-level programs
 124     very similar to those of GL_ARB_vertex/fragment_program.
 125 <li>All vector types (vec2, vec3, vec4, bvec2, etc) currently occupy full
 126     float[4] registers.
 127 <li>Float constants and variables are packed so that up to four floats
 128     can occupy one program parameter/register.
 129 <li>All function calls are inlined.
 130 <li>Shaders which use too many registers will not compile.
 131 <li>The quality of generated code is pretty good, register usage is fair.
 132 <li>Shader error detection and reporting of errors (InfoLog) is not
 133     very good yet.
 134 <li>The ftransform() function doesn't necessarily match the results of
 135     fixed-function transformation.
 136 </ul>
 137
 138 <p>
 139 These issues will be addressed/resolved in the future.
 140 </p>
 141
 142
 143 <h2 id="hints">Programming Hints</h2>
 144
 145 <ul>
 146 <li>Use the built-in library functions whenever possible.
 147     For example, instead of writing this:
 148 <pre>
 149         float x = 1.0 / sqrt(y);
 150 </pre>
 151     Write this:
 152 <pre>
 153         float x = inversesqrt(y);
 154 </pre>
 155 </li>
 156 </ul>
 157
 158
 159 <h2 id="standalone">Stand-alone GLSL Compiler</h2>
 160
 161 <p>
 162 The stand-alone GLSL compiler program can be used to compile GLSL shaders
 163 into low-level GPU code.
 164 </p>
 165
 166 <p>
 167 This tool is useful for:
 168 </p>
 169 <ul>
 170 <li>Inspecting GPU code to gain insight into compilation
 171 <li>Generating initial GPU code for subsequent hand-tuning
 172 <li>Debugging the GLSL compiler itself
 173 </ul>
 174
 175 <p>
 176 After building Mesa, the compiler can be found at src/compiler/glsl/glsl_compiler
 177 </p>
 178
 179 <p>
 180 Here's an example of using the compiler to compile a vertex shader and
 181 emit GL_ARB_vertex_program-style instructions:
 182 </p>
 183 <pre>
 184     src/compiler/glsl/glsl_compiler --version XXX --dump-ast myshader.vert
 185 </pre>
 186
 187 Options include
 188 <ul>
 189 <li><b>--dump-ast</b> - dump GPU code
 190 <li><b>--dump-hir</b> - dump high-level IR code
 191 <li><b>--dump-lir</b> - dump low-level IR code
 192 <li><b>--dump-builder</b> - dump GLSL IR code
 193 <li><b>--link</b> - link shaders
 194 <li><b>--just-log</b> - display only shader / linker info if exist,
 195 without any header or separator
 196 <li><b>--version</b> - [Mandatory] define the GLSL version to use
 197 </ul>
 198
 199
 200 <h2 id="implementation">Compiler Implementation</h2>
 201
 202 <p>
 203 The source code for Mesa's shading language compiler is in the
 204 <code>src/compiler/glsl/</code> directory.
 205 </p>
 206
 207 <p>
 208 XXX provide some info about the compiler....
 209 </p>
 210
 211 <p>
 212 The final vertex and fragment programs may be interpreted in software
 213 (see prog_execute.c) or translated into a specific hardware architecture
 214 (see drivers/dri/i915/i915_fragprog.c for example).
 215 </p>
 216
 217 <h2 id="validation">Compiler Validation</h2>
 218
 219 <p>
 220 Developers working on the GLSL compiler should test frequently to avoid
 221 regressions.
 222 </p>
 223
 224 <p>
 225 The <a href="https://piglit.freedesktop.org/">Piglit</a> project
 226 has many GLSL tests.
 227 </p>
 228
 229 <p>
 230 The Mesa demos repository also has some good GLSL tests.
 231 </p>
 232
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