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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="http://opengl.org/documentation/glsl/" target="_parent">
  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="#glsl120">GLSL 1.20 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>nopt</b> - disable compiler optimizations
  53 <li><b>opt</b> - force compiler optimizations
  54 <li><b>uniform</b> - print message to stdout when glUniform is called
  55 <li><b>nopvert</b> - force vertex shaders to be a simple shader that just transforms
  56     the vertex position with ftransform() and passes through the color and
  57     texcoord[0] attributes.
  58 <li><b>nopfrag</b> - force fragment shader to be a simple shader that passes
  59     through the color attribute.
  60 <li><b>useprog</b> - log glUseProgram calls to stderr
  61 </ul>
  62 <p>
  63 Example:  export MESA_GLSL=dump,nopt
  64 </p>
  65
  66
  67 <h2 id="glsl120">GLSL Version</h2>
  68
  69 <p>
  70 The GLSL compiler currently supports version 1.20 of the shading language.
  71 </p>
  72
  73 <p>
  74 Several GLSL extensions are also supported:
  75 </p>
  76 <ul>
  77 <li>GL_ARB_draw_buffers
  78 <li>GL_ARB_texture_rectangle
  79 <li>GL_ARB_fragment_coord_conventions
  80 <li>GL_EXT_texture_array
  81 </ul>
  82
  83
  84 <h2 id="unsup">Unsupported Features</h2>
  85
  86 <p>XXX update this section</p>
  87
  88 <p>
  89 The following features of the shading language are not yet fully supported
  90 in Mesa:
  91 </p>
  92
  93 <ul>
  94 <li>Linking of multiple shaders does not always work.  Currently, linking
  95     is implemented through shader concatenation and re-compiling.  This
  96     doesn't always work because of some #pragma and preprocessor issues.
  97 <li>gl_ClipVertex
  98 <li>The gl_Color and gl_SecondaryColor varying vars are interpolated
  99     without perspective correction
 100 </ul>
 101
 102 <p>
 103 All other major features of the shading language should function.
 104 </p>
 105
 106
 107 <h2 id="notes">Implementation Notes</h2>
 108
 109 <ul>
 110 <li>Shading language programs are compiled into low-level programs
 111     very similar to those of GL_ARB_vertex/fragment_program.
 112 <li>All vector types (vec2, vec3, vec4, bvec2, etc) currently occupy full
 113     float[4] registers.
 114 <li>Float constants and variables are packed so that up to four floats
 115     can occupy one program parameter/register.
 116 <li>All function calls are inlined.
 117 <li>Shaders which use too many registers will not compile.
 118 <li>The quality of generated code is pretty good, register usage is fair.
 119 <li>Shader error detection and reporting of errors (InfoLog) is not
 120     very good yet.
 121 <li>The ftransform() function doesn't necessarily match the results of
 122     fixed-function transformation.
 123 </ul>
 124
 125 <p>
 126 These issues will be addressed/resolved in the future.
 127 </p>
 128
 129
 130 <h2 id="hints">Programming Hints</h2>
 131
 132 <ul>
 133 <li>Use the built-in library functions whenever possible.
 134     For example, instead of writing this:
 135 <pre>
 136         float x = 1.0 / sqrt(y);
 137 </pre>
 138     Write this:
 139 <pre>
 140         float x = inversesqrt(y);
 141 </pre>
 142 </li>
 143 </ul>
 144
 145
 146 <h2 id="standalone">Stand-alone GLSL Compiler</h2>
 147
 148 <p>
 149 The stand-alone GLSL compiler program can be used to compile GLSL shaders
 150 into low-level GPU code.
 151 </p>
 152
 153 <p>
 154 This tool is useful for:
 155 </p>
 156 <ul>
 157 <li>Inspecting GPU code to gain insight into compilation
 158 <li>Generating initial GPU code for subsequent hand-tuning
 159 <li>Debugging the GLSL compiler itself
 160 </ul>
 161
 162 <p>
 163 After building Mesa, the compiler can be found at src/glsl/glsl_compiler
 164 </p>
 165
 166 <p>
 167 Here's an example of using the compiler to compile a vertex shader and
 168 emit GL_ARB_vertex_program-style instructions:
 169 </p>
 170 <pre>
 171     src/glsl/glsl_compiler --dump-ast myshader.vert
 172 </pre>
 173
 174 Options include
 175 <ul>
 176 <li><b>--dump-ast</b> - dump GPU code
 177 <li><b>--dump-hir</b> - dump high-level IR code
 178 <li><b>--dump-lir</b> - dump low-level IR code
 179 <li><b>--link</b> - ???
 180 </ul>
 181
 182
 183 <h2 id="implementation">Compiler Implementation</h2>
 184
 185 <p>
 186 The source code for Mesa's shading language compiler is in the
 187 <code>src/glsl/</code> directory.
 188 </p>
 189
 190 <p>
 191 XXX provide some info about the compiler....
 192 </p>
 193
 194 <p>
 195 The final vertex and fragment programs may be interpreted in software
 196 (see prog_execute.c) or translated into a specific hardware architecture
 197 (see drivers/dri/i915/i915_fragprog.c for example).
 198 </p>
 199
 200 <h3>Code Generation Options</h3>
 201
 202 <p>
 203 Internally, there are several options that control the compiler's code
 204 generation and instruction selection.
 205 These options are seen in the gl_shader_state struct and may be set
 206 by the device driver to indicate its preferences:
 207
 208 <pre>
 209 struct gl_shader_state
 210 {
 211    ...
 212    /** Driver-selectable options: */
 213    GLboolean EmitHighLevelInstructions;
 214    GLboolean EmitCondCodes;
 215    GLboolean EmitComments;
 216 };
 217 </pre>
 218
 219 <dl>
 220 <dt>EmitHighLevelInstructions</dt>
 221 <dd>
 222 This option controls instruction selection for loops and conditionals.
 223 If the option is set high-level IF/ELSE/ENDIF, LOOP/ENDLOOP, CONT/BRK
 224 instructions will be emitted.
 225 Otherwise, those constructs will be implemented with BRA instructions.
 226 </dd>
 227
 228 <dt>EmitCondCodes</dt>
 229 <dd>
 230 If set, condition codes (ala GL_NV_fragment_program) will be used for
 231 branching and looping.
 232 Otherwise, ordinary registers will be used (the IF instruction will
 233 examine the first operand's X component and do the if-part if non-zero).
 234 This option is only relevant if EmitHighLevelInstructions is set.
 235 </dd>
 236
 237 <dt>EmitComments</dt>
 238 <dd>
 239 If set, instructions will be annoted with comments to help with debugging.
 240 Extra NOP instructions will also be inserted.
 241 </dd>
 242 </dl>
 243
 244
 245 <h2 id="validation">Compiler Validation</h2>
 246
 247 <p>
 248 Developers working on the GLSL compiler should test frequently to avoid
 249 regressions.
 250 </p>
 251
 252 <p>
 253 The <a href="http://piglit.freedesktop.org/" target="_parent">Piglit</a> project
 254 has many GLSL tests and the
 255 <a href="http://glean.sf.net" target="_parent">Glean</a> glsl1 test
 256 tests GLSL features.
 257 </p>
 258
 259 <p>
 260 The Mesa demos repository also has some good GLSL tests.
 261 </p>
 262
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