docs/shading.html

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  11   The Mesa 3D Graphics Library
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  16
  17 <h1>Shading Language</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 <li><b>errors</b> - GLSL compilation and link errors will be reported to stderr.
  63 </ul>
  64 <p>
  65 Example:  export MESA_GLSL=dump,nopt
  66 </p>
  67
  68 <h3 id="replacement">Experimenting with Shader Replacements</h3>
  69 <p>
  70 Shaders can be dumped and replaced on runtime for debugging purposes. This
  71 feature is not currently supported by SCons build.
  72
  73 This 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>
  79 Note, path set must exist before running for dumping or replacing to work.
  80 When both are set, these paths should be different so the dumped shaders do
  81 not clobber the replacement shaders. Also, the filenames of the replacement shaders
  82 should match the filenames of the corresponding dumped shaders.
  83
  84 <h3 id="capture">Capturing Shaders</h3>
  85
  86 <p>
  87 Setting <b>MESA_SHADER_CAPTURE_PATH</b> to a directory will cause the compiler
  88 to write <code>.shader_test</code> files for use with
  89 <a href="https://gitlab.freedesktop.org/mesa/shader-db">shader-db</a>, a tool
  90 which compiler developers can use to gather statistics about shaders
  91 (instructions, cycles, memory accesses, and so on).
  92 </p>
  93 <p>
  94 Notably, this captures linked GLSL shaders - with all stages together -
  95 as well as ARB programs.
  96 </p>
  97
  98 <h2 id="support">GLSL Version</h2>
  99
 100 <p>
 101 The GLSL compiler currently supports version 3.30 of the shading language.
 102 </p>
 103
 104 <p>
 105 Several 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>
 119 The following features of the shading language are not yet fully supported
 120 in 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>
 132 All 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>
 155 These 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>
 178 The stand-alone GLSL compiler program can be used to compile GLSL shaders
 179 into low-level GPU code.
 180 </p>
 181
 182 <p>
 183 This 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>
 192 After building Mesa, the compiler can be found at src/compiler/glsl/glsl_compiler
 193 </p>
 194
 195 <p>
 196 Here's an example of using the compiler to compile a vertex shader and
 197 emit 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
 203 Options 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,
 211 without 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>
 219 The source code for Mesa's shading language compiler is in the
 220 <code>src/compiler/glsl/</code> directory.
 221 </p>
 222
 223 <p>
 224 XXX provide some info about the compiler....
 225 </p>
 226
 227 <p>
 228 The 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>
 236 Developers working on the GLSL compiler should test frequently to avoid
 237 regressions.
 238 </p>
 239
 240 <p>
 241 The <a href="https://piglit.freedesktop.org/">Piglit</a> project
 242 has many GLSL tests.
 243 </p>
 244
 245 <p>
 246 The Mesa demos repository also has some good GLSL tests.
 247 </p>
 248
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