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   9 <H1>Shading Language Support</H1>
  10
  11 <p>
  12 This page describes the features and status of Mesa's support for the
  13 <a href="http://opengl.org/documentation/glsl/" target="_parent">
  14 OpenGL Shading Language</a>.
  15 </p>
  16
  17 <p>
  18 Last updated on 28 March 2007.
  19 </p>
  20
  21 <p>
  22 Contents
  23 </p>
  24 <ul>
  25 <li><a href="#unsup">Unsupported Features</a>
  26 <li><a href="#notes">Implementation Notes</a>
  27 <li><a href="#hints">Programming Hints</a>
  28 <li><a href="#standalone">Stand-alone GLSL Compiler</a>
  29 <li><a href="#implementation">Compiler Implementation</a>
  30 <li><a href="#validation">Compiler Validation</a>
  31 </ul>
  32
  33
  34 <a name="unsup">
  35 <h2>Unsupported Features</h2>
  36
  37 <p>
  38 The following features of the shading language are not yet supported
  39 in Mesa:
  40 </p>
  41
  42 <ul>
  43 <li>Dereferencing arrays with non-constant indexes
  44 <li>Comparison of user-defined structs
  45 <li>Linking of multiple shaders is not supported
  46 <li>gl_ClipVertex
  47 <li>The derivative functions such as dFdx() are not implemented
  48 <li>The inverse trig functions asin(), acos(), and atan() are not implemented
  49 <li>The gl_Color and gl_SecondaryColor varying vars are interpolated
  50     without perspective correction
  51 <li>Floating point literal suffixes 'f' and 'F' aren't allowed.
  52 </ul>
  53
  54 <p>
  55 All other major features of the shading language should function.
  56 </p>
  57
  58
  59 <a name="notes">
  60 <h2>Implementation Notes</h2>
  61
  62 <ul>
  63 <li>Shading language programs are compiled into low-level programs
  64     very similar to those of GL_ARB_vertex/fragment_program.
  65 <li>All vector types (vec2, vec3, vec4, bvec2, etc) currently occupy full
  66     float[4] registers.
  67 <li>Float constants and variables are packed so that up to four floats
  68     can occupy one program parameter/register.
  69 <li>All function calls are inlined.
  70 <li>Shaders which use too many registers will not compile.
  71 <li>The quality of generated code is pretty good, register usage is fair.
  72 <li>Shader error detection and reporting of errors (InfoLog) is not
  73     very good yet.
  74 <li>The ftransform() function doesn't necessarily match the results of
  75     fixed-function transformation.
  76 </ul>
  77
  78 <p>
  79 These issues will be addressed/resolved in the future.
  80 </p>
  81
  82
  83 <a name="hints">
  84 <h2>Programming Hints</h2>
  85
  86 <ul>
  87 <li>Declare <em>in</em> function parameters as <em>const</em> whenever possible.
  88     This improves the efficiency of function inlining.
  89 </li>
  90 <br>
  91 <li>To reduce register usage, declare variables within smaller scopes.
  92     For example, the following code:
  93 <pre>
  94     void main()
  95     {
  96        vec4 a1, a2, b1, b2;
  97        gl_Position = expression using a1, a2.
  98        gl_Color = expression using b1, b2;
  99     }
 100 </pre>
 101     Can be rewritten as follows to use half as many registers:
 102 <pre>
 103     void main()
 104     {
 105        {
 106           vec4 a1, a2;
 107           gl_Position = expression using a1, a2.
 108        }
 109        {
 110           vec4 b1, b2;
 111           gl_Color = expression using b1, b2;
 112        }
 113     }
 114 </pre>
 115     Alternately, rather than using several float variables, use
 116     a vec4 instead.  Use swizzling and writemasks to access the
 117     components of the vec4 as floats.
 118 </li>
 119 <br>
 120 <li>Use the built-in library functions whenever possible.
 121     For example, instead of writing this:
 122 <pre>
 123         float x = 1.0 / sqrt(y);
 124 </pre>
 125     Write this:
 126 <pre>
 127         float x = inversesqrt(y);
 128 </pre>
 129 <li>
 130    Use ++i when possible as it's more efficient than i++
 131 </li>
 132 </ul>
 133
 134
 135 <a name="standalone">
 136 <h2>Stand-alone GLSL Compiler</h2>
 137
 138 <p>
 139 A unique stand-alone GLSL compiler driver has been added to Mesa.
 140 <p>
 141
 142 <p>
 143 The stand-alone compiler (like a conventional command-line compiler)
 144 is a tool that accepts Shading Language programs and emits low-level
 145 GPU programs.
 146 </p>
 147
 148 <p>
 149 This tool is useful for:
 150 <p>
 151 <ul>
 152 <li>Inspecting GPU code to gain insight into compilation
 153 <li>Generating initial GPU code for subsequent hand-tuning
 154 <li>Debugging the GLSL compiler itself
 155 </ul>
 156
 157 <p>
 158 After building Mesa the glslcompiler should be found in the Mesa/bin/ directory.
 159 If it's not there, it can be built manually:
 160 </p>
 161 <pre>
 162     cd src/mesa/drivers/glslcompiler
 163     make
 164 </pre>
 165
 166
 167 <p>
 168 Here's an example of using the compiler to compile a vertex shader and
 169 emit GL_ARB_vertex_program-style instructions:
 170 </p>
 171 <pre>
 172     bin/glslcompiler --debug --numbers --fs progs/glsl/CH06-brick.frag.txt
 173 </pre>
 174 <p>
 175 results in:
 176 </p>
 177 <pre>
 178 # Fragment Program/Shader
 179   0: RCP TEMP[4].x, UNIFORM[2].xxxx;
 180   1: RCP TEMP[4].y, UNIFORM[2].yyyy;
 181   2: MUL TEMP[3].xy, VARYING[0], TEMP[4];
 182   3: MOV TEMP[1], TEMP[3];
 183   4: MUL TEMP[0].w, TEMP[1].yyyy, CONST[4].xxxx;
 184   5: FRC TEMP[1].z, TEMP[0].wwww;
 185   6: SGT.C TEMP[0].w, TEMP[1].zzzz, CONST[4].xxxx;
 186   7: IF (NE.wwww); # (if false, goto 9);
 187   8:    ADD TEMP[1].x, TEMP[1].xxxx, CONST[4].xxxx;
 188   9: ENDIF;
 189  10: FRC TEMP[1].xy, TEMP[1];
 190  11: SGT TEMP[2].xy, UNIFORM[3], TEMP[1];
 191  12: MUL TEMP[1].z, TEMP[2].xxxx, TEMP[2].yyyy;
 192  13: LRP TEMP[0], TEMP[1].zzzz, UNIFORM[0], UNIFORM[1];
 193  14: MUL TEMP[0].xyz, TEMP[0], VARYING[1].xxxx;
 194  15: MOV OUTPUT[0].xyz, TEMP[0];
 195  16: MOV OUTPUT[0].w, CONST[4].yyyy;
 196  17: END
 197 </pre>
 198
 199 <p>
 200 Note that some shading language constructs (such as uniform and varying
 201 variables) aren't expressible in ARB or NV-style programs.
 202 Therefore, the resulting output is not always legal by definition of
 203 those program languages.
 204 </p>
 205 <p>
 206 Also note that this compiler driver is still under development.
 207 Over time, the correctness of the GPU programs, with respect to the ARB
 208 and NV languagues, should improve.
 209 </p>
 210
 211
 212
 213 <a name="implementation">
 214 <h2>Compiler Implementation</h2>
 215
 216 <p>
 217 The source code for Mesa's shading language compiler is in the
 218 <code>src/mesa/shader/slang/</code> directory.
 219 </p>
 220
 221 <p>
 222 The compiler follows a fairly standard design and basically works as follows:
 223 </p>
 224 <ul>
 225 <li>The input string is tokenized (see grammar.c) and parsed
 226 (see slang_compiler_*.c) to produce an Abstract Syntax Tree (AST).
 227 The nodes in this tree are slang_operation structures
 228 (see slang_compile_operation.h).
 229 The nodes are decorated with symbol table, scoping and datatype information.
 230 <li>The AST is converted into an Intermediate representation (IR) tree
 231 (see the slang_codegen.c file).
 232 The IR nodes represent basic GPU instructions, like add, dot product,
 233 move, etc.
 234 The IR tree is mostly a binary tree, but a few nodes have three or four
 235 children.
 236 In principle, the IR tree could be executed by doing an in-order traversal.
 237 <li>The IR tree is traversed in-order to emit code (see slang_emit.c).
 238 This is also when registers are allocated to store variables and temps.
 239 <li>In the future, a pattern-matching code generator-generator may be
 240 used for code generation.
 241 Programs such as L-BURG (Bottom-Up Rewrite Generator) and Twig look for
 242 patterns in IR trees, compute weights for subtrees and use the weights
 243 to select the best instructions to represent the sub-tree.
 244 <li>The emitted GPU instructions (see prog_instruction.h) are stored in a
 245 gl_program object (see mtypes.h).
 246 <li>When a fragment shader and vertex shader are linked (see slang_link.c)
 247 the varying vars are matched up, uniforms are merged, and vertex
 248 attributes are resolved (rewriting instructions as needed).
 249 </ul>
 250
 251 <p>
 252 The final vertex and fragment programs may be interpreted in software
 253 (see prog_execute.c) or translated into a specific hardware architecture
 254 (see drivers/dri/i915/i915_fragprog.c for example).
 255 </p>
 256
 257 <h3>Code Generation Options</h3>
 258
 259 <p>
 260 Internally, there are several options that control the compiler's code
 261 generation and instruction selection.
 262 These options are seen in the gl_shader_state struct and may be set
 263 by the device driver to indicate its preferences:
 264
 265 <pre>
 266 struct gl_shader_state
 267 {
 268    ...
 269    /** Driver-selectable options: */
 270    GLboolean EmitHighLevelInstructions;
 271    GLboolean EmitCondCodes;
 272    GLboolean EmitComments;
 273 };
 274 </pre>
 275
 276 <ul>
 277 <li>EmitHighLevelInstructions
 278 <br>
 279 This option controls instruction selection for loops and conditionals.
 280 If the option is set high-level IF/ELSE/ENDIF, LOOP/ENDLOOP, CONT/BRK
 281 instructions will be emitted.
 282 Otherwise, those constructs will be implemented with BRA instructions.
 283 </li>
 284
 285 <li>EmitCondCodes
 286 <br>
 287 If set, condition codes (ala GL_NV_fragment_program) will be used for
 288 branching and looping.
 289 Otherwise, ordinary registers will be used (the IF instruction will
 290 examine the first operand's X component and do the if-part if non-zero).
 291 This option is only relevant if EmitHighLevelInstructions is set.
 292 </li>
 293
 294 <li>EmitComments
 295 <br>
 296 If set, instructions will be annoted with comments to help with debugging.
 297 Extra NOP instructions will also be inserted.
 298 </br>
 299
 300 </ul>
 301
 302
 303 <a name="validation">
 304 <h2>Compiler Validation</h2>
 305
 306 <p>
 307 A new <a href="http://glean.sf.net" target="_parent">Glean</a> test has
 308 been create to exercise the GLSL compiler.
 309 </p>
 310 <p>
 311 The <em>glsl1</em> test runs over 150 sub-tests to check that the language
 312 features and built-in functions work properly.
 313 This test should be run frequently while working on the compiler to catch
 314 regressions.
 315 </p>
 316 <p>
 317 The test coverage is reasonably broad and complete but additional tests
 318 should be added.
 319 </p>
 320
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