Gitiles
Code Review Sign In
gerrit-public.fairphone.software / fp2-dev / platform / external / mesa3d / 85fb3e402744bbfd67ca9c79c98f54dd8ad169f5 / . / docs / shading.html
blob: 96060f1a752f475c95619f992f3a1183b3b9acfc [file] [log] [blame]
Brian9595d192007-01-20 13:40:57 -0700[diff] [blame]1<HTML>
2
3<TITLE>Shading Language Support</TITLE>
4
5<link rel="stylesheet" type="text/css" href="mesa.css"></head>
6
7<BODY>
8
9<H1>Shading Language Support</H1>
10
11<p>
12This page describes the features and status of Mesa's support for the
13<a href="http://opengl.org/documentation/glsl/" target="_parent">
14OpenGL Shading Language</a>.
15</p>
16
17<p>
Brian Paulc4341fe2008-12-15 18:30:40 -0700[diff] [blame]18Last updated on 15 December 2008.
Brian9595d192007-01-20 13:40:57 -0700[diff] [blame]19</p>
20
Brian61d31ae2007-02-17 09:41:19 -0700[diff] [blame]21<p>
22Contents
23</p>
24<ul>
Brian Paul439909a2009-04-07 11:09:53 -0600[diff] [blame]25<li><a href="#envvars">Environment variables</a>
Brian Paulc4341fe2008-12-15 18:30:40 -0700[diff] [blame]26<li><a href="#120">GLSL 1.20 support</a>
Brian61d31ae2007-02-17 09:41:19 -0700[diff] [blame]27<li><a href="#unsup">Unsupported Features</a>
Brian07e62082007-02-27 16:45:40 -0700[diff] [blame]28<li><a href="#notes">Implementation Notes</a>
Brian61d31ae2007-02-17 09:41:19 -0700[diff] [blame]29<li><a href="#hints">Programming Hints</a>
Brian Paul3bfedb72008-07-17 15:40:10 -0600[diff] [blame]30<li><a href="#standalone">Stand-alone GLSL Compiler</a>
Brian07e62082007-02-27 16:45:40 -0700[diff] [blame]31<li><a href="#implementation">Compiler Implementation</a>
Brian7eba12e2007-03-28 17:14:35 -0600[diff] [blame]32<li><a href="#validation">Compiler Validation</a>
Brian61d31ae2007-02-17 09:41:19 -0700[diff] [blame]33</ul>
34
35
Brian Paulc4341fe2008-12-15 18:30:40 -0700[diff] [blame]36
Brian Paul439909a2009-04-07 11:09:53 -0600[diff] [blame]37<a name="envvars">
38<h2>Environment Variables</h2>
39
40<p>
41The <b>MESA_GLSL</b> environment variable can be set to a comma-separated
Brian Paul85fb3e42009-10-14 11:28:28 -0600[diff] [blame^]42list of keywords to control some aspects of the GLSL compiler and shader
43execution. These are generally used for debugging.
Brian Paul439909a2009-04-07 11:09:53 -0600[diff] [blame]44</p>
45<ul>
46<li>dump - print GLSL shader code to stdout at link time
47<li>log - log all GLSL shaders to files.
48 The filenames will be "shader_X.vert" or "shader_X.frag" where X
49 the shader ID.
50<li>nopt - disable compiler optimizations
51<li>opt - force compiler optimizations
52<li>uniform - print message to stdout when glUniform is called
Brian Paul85fb3e42009-10-14 11:28:28 -0600[diff] [blame^]53<li>nopvert - force vertex shaders to be a simple shader that just transforms
54 the vertex position with ftransform() and passes through the color and
55 texcoord[0] attributes.
56<li>nopfrag - force fragment shader to be a simple shader that passes
57 through the color attribute.
Brian Paul439909a2009-04-07 11:09:53 -0600[diff] [blame]58</ul>
59<p>
60Example: export MESA_GLSL=dump,nopt
61</p>
62
63
Brian Paulc4341fe2008-12-15 18:30:40 -0700[diff] [blame]64<a name="120">
65<h2>GLSL 1.20 support</h2>
66
67<p>
Brian Paul439909a2009-04-07 11:09:53 -0600[diff] [blame]68GLSL version 1.20 is supported in Mesa 7.3 and later.
Brian Paulc4341fe2008-12-15 18:30:40 -0700[diff] [blame]69Among the features/differences of GLSL 1.20 are:
70<ul>
71<li><code>mat2x3, mat2x4</code>, etc. types and functions
72<li><code>transpose(), outerProduct(), matrixCompMult()</code> functions
73(but untested)
74<li>precision qualifiers (lowp, mediump, highp)
75<li><code>invariant</code> qualifier
76<li><code>array.length()</code> method
77<li><code>float[5] a;</code> array syntax
78<li><code>centroid</code> qualifier
79<li>unsized array constructors
80<li>initializers for uniforms
81<li>const initializers calling built-in functions
82</ul>
83
84
85
Brian61d31ae2007-02-17 09:41:19 -0700[diff] [blame]86<a name="unsup">
Brian9595d192007-01-20 13:40:57 -0700[diff] [blame]87<h2>Unsupported Features</h2>
88
89<p>
Brian Paul439909a2009-04-07 11:09:53 -0600[diff] [blame]90The following features of the shading language are not yet fully supported
Brian9595d192007-01-20 13:40:57 -0700[diff] [blame]91in Mesa:
92</p>
93
94<ul>
Brian Paul439909a2009-04-07 11:09:53 -0600[diff] [blame]95<li>Linking of multiple shaders does not always work. Currently, linking
96 is implemented through shader concatenation and re-compiling. This
97 doesn't always work because of some #pragma and preprocessor issues.
Brianb03e1712007-03-09 09:51:55 -0700[diff] [blame]98<li>gl_ClipVertex
Brian4b1d1b72007-04-27 15:25:00 -0600[diff] [blame]99<li>The gl_Color and gl_SecondaryColor varying vars are interpolated
100 without perspective correction
Brian9595d192007-01-20 13:40:57 -0700[diff] [blame]101</ul>
102
103<p>
104All other major features of the shading language should function.
105</p>
106
107
Brian07e62082007-02-27 16:45:40 -0700[diff] [blame]108<a name="notes">
Brian9595d192007-01-20 13:40:57 -0700[diff] [blame]109<h2>Implementation Notes</h2>
110
111<ul>
112<li>Shading language programs are compiled into low-level programs
113 very similar to those of GL_ARB_vertex/fragment_program.
Brianbbec2fd2007-01-28 12:11:10 -0700[diff] [blame]114<li>All vector types (vec2, vec3, vec4, bvec2, etc) currently occupy full
Brian9595d192007-01-20 13:40:57 -0700[diff] [blame]115 float[4] registers.
Brianbbec2fd2007-01-28 12:11:10 -0700[diff] [blame]116<li>Float constants and variables are packed so that up to four floats
117 can occupy one program parameter/register.
Brian9595d192007-01-20 13:40:57 -0700[diff] [blame]118<li>All function calls are inlined.
119<li>Shaders which use too many registers will not compile.
120<li>The quality of generated code is pretty good, register usage is fair.
121<li>Shader error detection and reporting of errors (InfoLog) is not
122 very good yet.
Brian05e6fd82007-03-27 16:05:25 -0600[diff] [blame]123<li>The ftransform() function doesn't necessarily match the results of
124 fixed-function transformation.
Brian9595d192007-01-20 13:40:57 -0700[diff] [blame]125</ul>
126
127<p>
128These issues will be addressed/resolved in the future.
129</p>
130
131
Brian61d31ae2007-02-17 09:41:19 -0700[diff] [blame]132<a name="hints">
Brian9595d192007-01-20 13:40:57 -0700[diff] [blame]133<h2>Programming Hints</h2>
134
135<ul>
Brianbbec2fd2007-01-28 12:11:10 -0700[diff] [blame]136<li>Declare <em>in</em> function parameters as <em>const</em> whenever possible.
Brian9595d192007-01-20 13:40:57 -0700[diff] [blame]137 This improves the efficiency of function inlining.
138</li>
139<br>
140<li>To reduce register usage, declare variables within smaller scopes.
141 For example, the following code:
142<pre>
143 void main()
144 {
145 vec4 a1, a2, b1, b2;
146 gl_Position = expression using a1, a2.
147 gl_Color = expression using b1, b2;
148 }
149</pre>
150 Can be rewritten as follows to use half as many registers:
151<pre>
152 void main()
153 {
154 {
155 vec4 a1, a2;
156 gl_Position = expression using a1, a2.
157 }
158 {
159 vec4 b1, b2;
160 gl_Color = expression using b1, b2;
161 }
162 }
163</pre>
164 Alternately, rather than using several float variables, use
165 a vec4 instead. Use swizzling and writemasks to access the
166 components of the vec4 as floats.
167</li>
168<br>
169<li>Use the built-in library functions whenever possible.
170 For example, instead of writing this:
171<pre>
172 float x = 1.0 / sqrt(y);
173</pre>
174 Write this:
175<pre>
176 float x = inversesqrt(y);
177</pre>
Brian7eba12e2007-03-28 17:14:35 -0600[diff] [blame]178<li>
179 Use ++i when possible as it's more efficient than i++
180</li>
Brian9595d192007-01-20 13:40:57 -0700[diff] [blame]181</ul>
182
183
Brian61d31ae2007-02-17 09:41:19 -0700[diff] [blame]184<a name="standalone">
Brian Paul3bfedb72008-07-17 15:40:10 -0600[diff] [blame]185<h2>Stand-alone GLSL Compiler</h2>
Brian61d31ae2007-02-17 09:41:19 -0700[diff] [blame]186
187<p>
188A unique stand-alone GLSL compiler driver has been added to Mesa.
189<p>
190
191<p>
192The stand-alone compiler (like a conventional command-line compiler)
193is a tool that accepts Shading Language programs and emits low-level
194GPU programs.
195</p>
196
197<p>
198This tool is useful for:
199<p>
200<ul>
201<li>Inspecting GPU code to gain insight into compilation
202<li>Generating initial GPU code for subsequent hand-tuning
203<li>Debugging the GLSL compiler itself
204</ul>
205
206<p>
Brian Paul77497eb2008-07-21 09:01:21 -0600[diff] [blame]207After building Mesa, the glslcompiler can be built by manually running:
Brian61d31ae2007-02-17 09:41:19 -0700[diff] [blame]208</p>
Brianff0cc922007-02-22 16:29:48 -0700[diff] [blame]209<pre>
Brian Paul439909a2009-04-07 11:09:53 -0600[diff] [blame]210 make realclean
211 make linux
Brian Paul3bfedb72008-07-17 15:40:10 -0600[diff] [blame]212 cd src/mesa/drivers/glslcompiler
Brianff0cc922007-02-22 16:29:48 -0700[diff] [blame]213 make
214</pre>
215
Brian61d31ae2007-02-17 09:41:19 -0700[diff] [blame]216
217<p>
218Here's an example of using the compiler to compile a vertex shader and
219emit GL_ARB_vertex_program-style instructions:
220</p>
221<pre>
Brian Paul3bfedb72008-07-17 15:40:10 -0600[diff] [blame]222 bin/glslcompiler --debug --numbers --fs progs/glsl/CH06-brick.frag.txt
Brian61d31ae2007-02-17 09:41:19 -0700[diff] [blame]223</pre>
224<p>
Brian Paul3bfedb72008-07-17 15:40:10 -0600[diff] [blame]225results in:
Brian61d31ae2007-02-17 09:41:19 -0700[diff] [blame]226</p>
227<pre>
Brian Paul3bfedb72008-07-17 15:40:10 -0600[diff] [blame]228# Fragment Program/Shader
229 0: RCP TEMP[4].x, UNIFORM[2].xxxx;
230 1: RCP TEMP[4].y, UNIFORM[2].yyyy;
231 2: MUL TEMP[3].xy, VARYING[0], TEMP[4];
232 3: MOV TEMP[1], TEMP[3];
233 4: MUL TEMP[0].w, TEMP[1].yyyy, CONST[4].xxxx;
234 5: FRC TEMP[1].z, TEMP[0].wwww;
235 6: SGT.C TEMP[0].w, TEMP[1].zzzz, CONST[4].xxxx;
236 7: IF (NE.wwww); # (if false, goto 9);
237 8: ADD TEMP[1].x, TEMP[1].xxxx, CONST[4].xxxx;
238 9: ENDIF;
239 10: FRC TEMP[1].xy, TEMP[1];
240 11: SGT TEMP[2].xy, UNIFORM[3], TEMP[1];
241 12: MUL TEMP[1].z, TEMP[2].xxxx, TEMP[2].yyyy;
242 13: LRP TEMP[0], TEMP[1].zzzz, UNIFORM[0], UNIFORM[1];
243 14: MUL TEMP[0].xyz, TEMP[0], VARYING[1].xxxx;
244 15: MOV OUTPUT[0].xyz, TEMP[0];
245 16: MOV OUTPUT[0].w, CONST[4].yyyy;
246 17: END
Brian61d31ae2007-02-17 09:41:19 -0700[diff] [blame]247</pre>
248
249<p>
250Note that some shading language constructs (such as uniform and varying
251variables) aren't expressible in ARB or NV-style programs.
252Therefore, the resulting output is not always legal by definition of
253those program languages.
254</p>
255<p>
256Also note that this compiler driver is still under development.
257Over time, the correctness of the GPU programs, with respect to the ARB
258and NV languagues, should improve.
259</p>
260
Brian07e62082007-02-27 16:45:40 -0700[diff] [blame]261
262
263<a name="implementation">
264<h2>Compiler Implementation</h2>
265
266<p>
267The source code for Mesa's shading language compiler is in the
268<code>src/mesa/shader/slang/</code> directory.
269</p>
270
271<p>
272The compiler follows a fairly standard design and basically works as follows:
273</p>
274<ul>
275<li>The input string is tokenized (see grammar.c) and parsed
276(see slang_compiler_*.c) to produce an Abstract Syntax Tree (AST).
277The nodes in this tree are slang_operation structures
278(see slang_compile_operation.h).
279The nodes are decorated with symbol table, scoping and datatype information.
280<li>The AST is converted into an Intermediate representation (IR) tree
281(see the slang_codegen.c file).
282The IR nodes represent basic GPU instructions, like add, dot product,
283move, etc.
284The IR tree is mostly a binary tree, but a few nodes have three or four
285children.
286In principle, the IR tree could be executed by doing an in-order traversal.
287<li>The IR tree is traversed in-order to emit code (see slang_emit.c).
288This is also when registers are allocated to store variables and temps.
289<li>In the future, a pattern-matching code generator-generator may be
290used for code generation.
291Programs such as L-BURG (Bottom-Up Rewrite Generator) and Twig look for
292patterns in IR trees, compute weights for subtrees and use the weights
293to select the best instructions to represent the sub-tree.
294<li>The emitted GPU instructions (see prog_instruction.h) are stored in a
295gl_program object (see mtypes.h).
296<li>When a fragment shader and vertex shader are linked (see slang_link.c)
297the varying vars are matched up, uniforms are merged, and vertex
298attributes are resolved (rewriting instructions as needed).
299</ul>
300
301<p>
302The final vertex and fragment programs may be interpreted in software
303(see prog_execute.c) or translated into a specific hardware architecture
304(see drivers/dri/i915/i915_fragprog.c for example).
305</p>
306
Brian8f9db0f2007-03-23 17:49:19 -0600[diff] [blame]307<h3>Code Generation Options</h3>
308
309<p>
310Internally, there are several options that control the compiler's code
311generation and instruction selection.
312These options are seen in the gl_shader_state struct and may be set
313by the device driver to indicate its preferences:
314
315<pre>
316struct gl_shader_state
317{
318 ...
319 /** Driver-selectable options: */
320 GLboolean EmitHighLevelInstructions;
321 GLboolean EmitCondCodes;
322 GLboolean EmitComments;
323};
324</pre>
325
326<ul>
327<li>EmitHighLevelInstructions
328<br>
329This option controls instruction selection for loops and conditionals.
330If the option is set high-level IF/ELSE/ENDIF, LOOP/ENDLOOP, CONT/BRK
331instructions will be emitted.
332Otherwise, those constructs will be implemented with BRA instructions.
333</li>
334
335<li>EmitCondCodes
336<br>
337If set, condition codes (ala GL_NV_fragment_program) will be used for
338branching and looping.
339Otherwise, ordinary registers will be used (the IF instruction will
340examine the first operand's X component and do the if-part if non-zero).
341This option is only relevant if EmitHighLevelInstructions is set.
342</li>
343
344<li>EmitComments
345<br>
346If set, instructions will be annoted with comments to help with debugging.
347Extra NOP instructions will also be inserted.
348</br>
349
350</ul>
Brian07e62082007-02-27 16:45:40 -0700[diff] [blame]351
352
Brian7eba12e2007-03-28 17:14:35 -0600[diff] [blame]353<a name="validation">
354<h2>Compiler Validation</h2>
355
356<p>
Brian Paulc4341fe2008-12-15 18:30:40 -0700[diff] [blame]357A <a href="http://glean.sf.net" target="_parent">Glean</a> test has
Brian7eba12e2007-03-28 17:14:35 -0600[diff] [blame]358been create to exercise the GLSL compiler.
359</p>
360<p>
Brian Paulc4341fe2008-12-15 18:30:40 -0700[diff] [blame]361The <em>glsl1</em> test runs over 170 sub-tests to check that the language
Brian7eba12e2007-03-28 17:14:35 -0600[diff] [blame]362features and built-in functions work properly.
363This test should be run frequently while working on the compiler to catch
364regressions.
365</p>
366<p>
367The test coverage is reasonably broad and complete but additional tests
368should be added.
369</p>
370
371
Brian9595d192007-01-20 13:40:57 -0700[diff] [blame]372</BODY>
373</HTML>