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>uniform</b> - print message to stdout when glUniform is called
  54 <li><b>nopvert</b> - force vertex shaders to be a simple shader that just transforms
  55     the vertex position with ftransform() and passes through the color and
  56     texcoord[0] attributes.
  57 <li><b>nopfrag</b> - force fragment shader to be a simple shader that passes
  58     through the color attribute.
  59 <li><b>useprog</b> - log glUseProgram calls to stderr
  60 </ul>
  61 <p>
  62 Example:  export MESA_GLSL=dump,nopt
  63 </p>
  64
  65 <p>
  66 Shaders can be dumped and replaced on runtime for debugging purposes. Mesa
  67 needs to be configured with '--with-sha1' to enable this functionality. This
  68 feature is not currently supported by SCons build.
  69
  70 This is controlled via following environment variables:
  71 <ul>
  72 <li><b>MESA_SHADER_DUMP_PATH</b> - path where shader sources are dumped
  73 <li><b>MESA_SHADER_READ_PATH</b> - path where replacement shaders are read
  74 </ul>
  75 Note, path set must exist before running for dumping or replacing to work.
  76 When both are set, these paths should be different so the dumped shaders do
  77 not clobber the replacement shaders.
  78 </p>
  79
  80 <h2 id="support">GLSL Version</h2>
  81
  82 <p>
  83 The GLSL compiler currently supports version 3.30 of the shading language.
  84 </p>
  85
  86 <p>
  87 Several GLSL extensions are also supported:
  88 </p>
  89 <ul>
  90 <li>GL_ARB_draw_buffers
  91 <li>GL_ARB_fragment_coord_conventions
  92 <li>GL_ARB_shader_bit_encoding
  93 </ul>
  94
  95
  96 <h2 id="unsup">Unsupported Features</h2>
  97
  98 <p>XXX update this section</p>
  99
 100 <p>
 101 The following features of the shading language are not yet fully supported
 102 in Mesa:
 103 </p>
 104
 105 <ul>
 106 <li>Linking of multiple shaders does not always work.  Currently, linking
 107     is implemented through shader concatenation and re-compiling.  This
 108     doesn't always work because of some #pragma and preprocessor issues.
 109 <li>The gl_Color and gl_SecondaryColor varying vars are interpolated
 110     without perspective correction
 111 </ul>
 112
 113 <p>
 114 All other major features of the shading language should function.
 115 </p>
 116
 117
 118 <h2 id="notes">Implementation Notes</h2>
 119
 120 <ul>
 121 <li>Shading language programs are compiled into low-level programs
 122     very similar to those of GL_ARB_vertex/fragment_program.
 123 <li>All vector types (vec2, vec3, vec4, bvec2, etc) currently occupy full
 124     float[4] registers.
 125 <li>Float constants and variables are packed so that up to four floats
 126     can occupy one program parameter/register.
 127 <li>All function calls are inlined.
 128 <li>Shaders which use too many registers will not compile.
 129 <li>The quality of generated code is pretty good, register usage is fair.
 130 <li>Shader error detection and reporting of errors (InfoLog) is not
 131     very good yet.
 132 <li>The ftransform() function doesn't necessarily match the results of
 133     fixed-function transformation.
 134 </ul>
 135
 136 <p>
 137 These issues will be addressed/resolved in the future.
 138 </p>
 139
 140
 141 <h2 id="hints">Programming Hints</h2>
 142
 143 <ul>
 144 <li>Use the built-in library functions whenever possible.
 145     For example, instead of writing this:
 146 <pre>
 147         float x = 1.0 / sqrt(y);
 148 </pre>
 149     Write this:
 150 <pre>
 151         float x = inversesqrt(y);
 152 </pre>
 153 </li>
 154 </ul>
 155
 156
 157 <h2 id="standalone">Stand-alone GLSL Compiler</h2>
 158
 159 <p>
 160 The stand-alone GLSL compiler program can be used to compile GLSL shaders
 161 into low-level GPU code.
 162 </p>
 163
 164 <p>
 165 This tool is useful for:
 166 </p>
 167 <ul>
 168 <li>Inspecting GPU code to gain insight into compilation
 169 <li>Generating initial GPU code for subsequent hand-tuning
 170 <li>Debugging the GLSL compiler itself
 171 </ul>
 172
 173 <p>
 174 After building Mesa, the compiler can be found at src/compiler/glsl/glsl_compiler
 175 </p>
 176
 177 <p>
 178 Here's an example of using the compiler to compile a vertex shader and
 179 emit GL_ARB_vertex_program-style instructions:
 180 </p>
 181 <pre>
 182     src/compiler/glsl/glsl_compiler --version XXX --dump-ast myshader.vert
 183 </pre>
 184
 185 Options include
 186 <ul>
 187 <li><b>--dump-ast</b> - dump GPU code
 188 <li><b>--dump-hir</b> - dump high-level IR code
 189 <li><b>--dump-lir</b> - dump low-level IR code
 190 <li><b>--dump-builder</b> - dump GLSL IR code
 191 <li><b>--link</b> - link shaders
 192 <li><b>--just-log</b> - display only shader / linker info if exist,
 193 without any header or separator
 194 <li><b>--version</b> - [Mandatory] define the GLSL version to use
 195 </ul>
 196
 197
 198 <h2 id="implementation">Compiler Implementation</h2>
 199
 200 <p>
 201 The source code for Mesa's shading language compiler is in the
 202 <code>src/compiler/glsl/</code> directory.
 203 </p>
 204
 205 <p>
 206 XXX provide some info about the compiler....
 207 </p>
 208
 209 <p>
 210 The final vertex and fragment programs may be interpreted in software
 211 (see prog_execute.c) or translated into a specific hardware architecture
 212 (see drivers/dri/i915/i915_fragprog.c for example).
 213 </p>
 214
 215 <h2 id="validation">Compiler Validation</h2>
 216
 217 <p>
 218 Developers working on the GLSL compiler should test frequently to avoid
 219 regressions.
 220 </p>
 221
 222 <p>
 223 The <a href="https://piglit.freedesktop.org/">Piglit</a> project
 224 has many GLSL tests.
 225 </p>
 226
 227 <p>
 228 The Mesa demos repository also has some good GLSL tests.
 229 </p>
 230
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