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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 <h3 id="replacement">Experimenting with Shader Replacements</h3>
  68 <p>
  69 Shaders can be dumped and replaced on runtime for debugging purposes. 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. Also, the filenames of the replacement shaders
  80 should match the filenames of the corresponding dumped shaders.
  81 </p>
  82
  83 <h3 id="capture">Capturing Shaders</h3>
  84
  85 <p>
  86 Setting <b>MESA_SHADER_CAPTURE_PATH</b> to a directory will cause the compiler
  87 to write <tt>.shader_test</tt> files for use with
  88 <a href="https://gitlab.freedesktop.org/mesa/shader-db">shader-db</a>, a tool
  89 which compiler developers can use to gather statistics about shaders
  90 (instructions, cycles, memory accesses, and so on).
  91 </p>
  92 <p>
  93 Notably, this captures linked GLSL shaders - with all stages together -
  94 as well as ARB programs.
  95 </p>
  96
  97 <h2 id="support">GLSL Version</h2>
  98
  99 <p>
 100 The GLSL compiler currently supports version 3.30 of the shading language.
 101 </p>
 102
 103 <p>
 104 Several 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>
 118 The following features of the shading language are not yet fully supported
 119 in 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>
 131 All 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>
 154 These 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>
 177 The stand-alone GLSL compiler program can be used to compile GLSL shaders
 178 into low-level GPU code.
 179 </p>
 180
 181 <p>
 182 This 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>
 191 After building Mesa, the compiler can be found at src/compiler/glsl/glsl_compiler
 192 </p>
 193
 194 <p>
 195 Here's an example of using the compiler to compile a vertex shader and
 196 emit 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
 202 Options 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,
 210 without 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>
 218 The source code for Mesa's shading language compiler is in the
 219 <code>src/compiler/glsl/</code> directory.
 220 </p>
 221
 222 <p>
 223 XXX provide some info about the compiler....
 224 </p>
 225
 226 <p>
 227 The 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>
 235 Developers working on the GLSL compiler should test frequently to avoid
 236 regressions.
 237 </p>
 238
 239 <p>
 240 The <a href="https://piglit.freedesktop.org/">Piglit</a> project
 241 has many GLSL tests.
 242 </p>
 243
 244 <p>
 245 The Mesa demos repository also has some good GLSL tests.
 246 </p>
 247
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