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3<TITLE>Shading Language Support</TITLE>
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5<link rel="stylesheet" type="text/css" href="mesa.css"></head>
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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>
Brian7eba12e2007-03-28 17:14:35 -0600[diff] [blame]18Last updated on 28 March 2007.
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>
25<li><a href="#unsup">Unsupported Features</a>
Brian07e62082007-02-27 16:45:40 -0700[diff] [blame]26<li><a href="#notes">Implementation Notes</a>
Brian61d31ae2007-02-17 09:41:19 -0700[diff] [blame]27<li><a href="#hints">Programming Hints</a>
Brian Paul3bfedb72008-07-17 15:40:10 -0600[diff] [blame]28<li><a href="#standalone">Stand-alone GLSL Compiler</a>
Brian07e62082007-02-27 16:45:40 -0700[diff] [blame]29<li><a href="#implementation">Compiler Implementation</a>
Brian7eba12e2007-03-28 17:14:35 -0600[diff] [blame]30<li><a href="#validation">Compiler Validation</a>
Brian Paul1a5c99f2008-07-29 16:46:08 -0600[diff] [blame]31<li><a href="#120">GLSL 1.20 support</a>
Brian61d31ae2007-02-17 09:41:19 -0700[diff] [blame]32</ul>
33
34
35<a name="unsup">
Brian9595d192007-01-20 13:40:57 -0700[diff] [blame]36<h2>Unsupported Features</h2>
37
38<p>
39The following features of the shading language are not yet supported
40in Mesa:
41</p>
42
43<ul>
Brian6d4cf6b2007-02-21 16:07:03 -0700[diff] [blame]44<li>Dereferencing arrays with non-constant indexes
Brianb67d9312007-03-26 10:23:50 -0600[diff] [blame]45<li>Comparison of user-defined structs
Brian9595d192007-01-20 13:40:57 -0700[diff] [blame]46<li>Linking of multiple shaders is not supported
Brianb03e1712007-03-09 09:51:55 -0700[diff] [blame]47<li>gl_ClipVertex
Brianba3d3842007-04-19 14:24:29 -0600[diff] [blame]48<li>The derivative functions such as dFdx() are not implemented
Brian7ff72a72007-04-27 15:23:19 -0600[diff] [blame]49<li>The inverse trig functions asin(), acos(), and atan() are not implemented
Brian4b1d1b72007-04-27 15:25:00 -0600[diff] [blame]50<li>The gl_Color and gl_SecondaryColor varying vars are interpolated
51 without perspective correction
Brian5ef3a2c2007-11-27 10:27:55 -0700[diff] [blame]52<li>Floating point literal suffixes 'f' and 'F' aren't allowed.
Brian9595d192007-01-20 13:40:57 -0700[diff] [blame]53</ul>
54
55<p>
56All other major features of the shading language should function.
57</p>
58
59
Brian07e62082007-02-27 16:45:40 -0700[diff] [blame]60<a name="notes">
Brian9595d192007-01-20 13:40:57 -0700[diff] [blame]61<h2>Implementation Notes</h2>
62
63<ul>
64<li>Shading language programs are compiled into low-level programs
65 very similar to those of GL_ARB_vertex/fragment_program.
Brianbbec2fd2007-01-28 12:11:10 -0700[diff] [blame]66<li>All vector types (vec2, vec3, vec4, bvec2, etc) currently occupy full
Brian9595d192007-01-20 13:40:57 -0700[diff] [blame]67 float[4] registers.
Brianbbec2fd2007-01-28 12:11:10 -0700[diff] [blame]68<li>Float constants and variables are packed so that up to four floats
69 can occupy one program parameter/register.
Brian9595d192007-01-20 13:40:57 -0700[diff] [blame]70<li>All function calls are inlined.
71<li>Shaders which use too many registers will not compile.
72<li>The quality of generated code is pretty good, register usage is fair.
73<li>Shader error detection and reporting of errors (InfoLog) is not
74 very good yet.
Brian05e6fd82007-03-27 16:05:25 -0600[diff] [blame]75<li>The ftransform() function doesn't necessarily match the results of
76 fixed-function transformation.
Brian9595d192007-01-20 13:40:57 -0700[diff] [blame]77</ul>
78
79<p>
80These issues will be addressed/resolved in the future.
81</p>
82
83
Brian61d31ae2007-02-17 09:41:19 -0700[diff] [blame]84<a name="hints">
Brian9595d192007-01-20 13:40:57 -0700[diff] [blame]85<h2>Programming Hints</h2>
86
87<ul>
Brianbbec2fd2007-01-28 12:11:10 -0700[diff] [blame]88<li>Declare <em>in</em> function parameters as <em>const</em> whenever possible.
Brian9595d192007-01-20 13:40:57 -0700[diff] [blame]89 This improves the efficiency of function inlining.
90</li>
91<br>
92<li>To reduce register usage, declare variables within smaller scopes.
93 For example, the following code:
94<pre>
95 void main()
96 {
97 vec4 a1, a2, b1, b2;
98 gl_Position = expression using a1, a2.
99 gl_Color = expression using b1, b2;
100 }
101</pre>
102 Can be rewritten as follows to use half as many registers:
103<pre>
104 void main()
105 {
106 {
107 vec4 a1, a2;
108 gl_Position = expression using a1, a2.
109 }
110 {
111 vec4 b1, b2;
112 gl_Color = expression using b1, b2;
113 }
114 }
115</pre>
116 Alternately, rather than using several float variables, use
117 a vec4 instead. Use swizzling and writemasks to access the
118 components of the vec4 as floats.
119</li>
120<br>
121<li>Use the built-in library functions whenever possible.
122 For example, instead of writing this:
123<pre>
124 float x = 1.0 / sqrt(y);
125</pre>
126 Write this:
127<pre>
128 float x = inversesqrt(y);
129</pre>
Brian7eba12e2007-03-28 17:14:35 -0600[diff] [blame]130<li>
131 Use ++i when possible as it's more efficient than i++
132</li>
Brian9595d192007-01-20 13:40:57 -0700[diff] [blame]133</ul>
134
135
Brian61d31ae2007-02-17 09:41:19 -0700[diff] [blame]136<a name="standalone">
Brian Paul3bfedb72008-07-17 15:40:10 -0600[diff] [blame]137<h2>Stand-alone GLSL Compiler</h2>
Brian61d31ae2007-02-17 09:41:19 -0700[diff] [blame]138
139<p>
140A unique stand-alone GLSL compiler driver has been added to Mesa.
141<p>
142
143<p>
144The stand-alone compiler (like a conventional command-line compiler)
145is a tool that accepts Shading Language programs and emits low-level
146GPU programs.
147</p>
148
149<p>
150This tool is useful for:
151<p>
152<ul>
153<li>Inspecting GPU code to gain insight into compilation
154<li>Generating initial GPU code for subsequent hand-tuning
155<li>Debugging the GLSL compiler itself
156</ul>
157
158<p>
Brian Paul77497eb2008-07-21 09:01:21 -0600[diff] [blame]159After building Mesa, the glslcompiler can be built by manually running:
Brian61d31ae2007-02-17 09:41:19 -0700[diff] [blame]160</p>
Brianff0cc922007-02-22 16:29:48 -0700[diff] [blame]161<pre>
Brian Paul3bfedb72008-07-17 15:40:10 -0600[diff] [blame]162 cd src/mesa/drivers/glslcompiler
Brianff0cc922007-02-22 16:29:48 -0700[diff] [blame]163 make
164</pre>
165
Brian61d31ae2007-02-17 09:41:19 -0700[diff] [blame]166
167<p>
168Here's an example of using the compiler to compile a vertex shader and
169emit GL_ARB_vertex_program-style instructions:
170</p>
171<pre>
Brian Paul3bfedb72008-07-17 15:40:10 -0600[diff] [blame]172 bin/glslcompiler --debug --numbers --fs progs/glsl/CH06-brick.frag.txt
Brian61d31ae2007-02-17 09:41:19 -0700[diff] [blame]173</pre>
174<p>
Brian Paul3bfedb72008-07-17 15:40:10 -0600[diff] [blame]175results in:
Brian61d31ae2007-02-17 09:41:19 -0700[diff] [blame]176</p>
177<pre>
Brian Paul3bfedb72008-07-17 15:40:10 -0600[diff] [blame]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
Brian61d31ae2007-02-17 09:41:19 -0700[diff] [blame]197</pre>
198
199<p>
200Note that some shading language constructs (such as uniform and varying
201variables) aren't expressible in ARB or NV-style programs.
202Therefore, the resulting output is not always legal by definition of
203those program languages.
204</p>
205<p>
206Also note that this compiler driver is still under development.
207Over time, the correctness of the GPU programs, with respect to the ARB
208and NV languagues, should improve.
209</p>
210
Brian07e62082007-02-27 16:45:40 -0700[diff] [blame]211
212
213<a name="implementation">
214<h2>Compiler Implementation</h2>
215
216<p>
217The source code for Mesa's shading language compiler is in the
218<code>src/mesa/shader/slang/</code> directory.
219</p>
220
221<p>
222The 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).
227The nodes in this tree are slang_operation structures
228(see slang_compile_operation.h).
229The 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).
232The IR nodes represent basic GPU instructions, like add, dot product,
233move, etc.
234The IR tree is mostly a binary tree, but a few nodes have three or four
235children.
236In 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).
238This is also when registers are allocated to store variables and temps.
239<li>In the future, a pattern-matching code generator-generator may be
240used for code generation.
241Programs such as L-BURG (Bottom-Up Rewrite Generator) and Twig look for
242patterns in IR trees, compute weights for subtrees and use the weights
243to select the best instructions to represent the sub-tree.
244<li>The emitted GPU instructions (see prog_instruction.h) are stored in a
245gl_program object (see mtypes.h).
246<li>When a fragment shader and vertex shader are linked (see slang_link.c)
247the varying vars are matched up, uniforms are merged, and vertex
248attributes are resolved (rewriting instructions as needed).
249</ul>
250
251<p>
252The 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
Brian8f9db0f2007-03-23 17:49:19 -0600[diff] [blame]257<h3>Code Generation Options</h3>
258
259<p>
260Internally, there are several options that control the compiler's code
261generation and instruction selection.
262These options are seen in the gl_shader_state struct and may be set
263by the device driver to indicate its preferences:
264
265<pre>
266struct 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>
279This option controls instruction selection for loops and conditionals.
280If the option is set high-level IF/ELSE/ENDIF, LOOP/ENDLOOP, CONT/BRK
281instructions will be emitted.
282Otherwise, those constructs will be implemented with BRA instructions.
283</li>
284
285<li>EmitCondCodes
286<br>
287If set, condition codes (ala GL_NV_fragment_program) will be used for
288branching and looping.
289Otherwise, ordinary registers will be used (the IF instruction will
290examine the first operand's X component and do the if-part if non-zero).
291This option is only relevant if EmitHighLevelInstructions is set.
292</li>
293
294<li>EmitComments
295<br>
296If set, instructions will be annoted with comments to help with debugging.
297Extra NOP instructions will also be inserted.
298</br>
299
300</ul>
Brian07e62082007-02-27 16:45:40 -0700[diff] [blame]301
302
Brian7eba12e2007-03-28 17:14:35 -0600[diff] [blame]303<a name="validation">
304<h2>Compiler Validation</h2>
305
306<p>
307A new <a href="http://glean.sf.net" target="_parent">Glean</a> test has
308been create to exercise the GLSL compiler.
309</p>
310<p>
Brian8598f552007-04-18 09:30:07 -0600[diff] [blame]311The <em>glsl1</em> test runs over 150 sub-tests to check that the language
Brian7eba12e2007-03-28 17:14:35 -0600[diff] [blame]312features and built-in functions work properly.
313This test should be run frequently while working on the compiler to catch
314regressions.
315</p>
316<p>
317The test coverage is reasonably broad and complete but additional tests
318should be added.
319</p>
320
321
Brian Paul1a5c99f2008-07-29 16:46:08 -0600[diff] [blame]322
323<a name="120">
324<h2>GLSL 1.20 support</h2>
325
326<p>
327Support for GLSL version 1.20 is underway. Status as follows.
328</p>
329
330<h3>Supported</h3>
331<ul>
332<li><code>mat2x3, mat2x4</code>, etc. types and functions
333<li><code>transpose(), outerProduct(), matrixCompMult()</code> functions
334(but untested)
335<li>precision qualifiers (lowp, mediump, highp)
336</ul>
337
338<h3>Partially Complete</h3>
339<ul>
340<li><code>invariant</code> qualifier
341</ul>
342
343<h3>Not Completed</h3>
344<ul>
345<li><code>array.length()</code> method
346<li><code>float[5] a;</code> array syntax
347<li><code>centroid</code> qualifier
348<li>unsized array constructors
349<li>initializers for uniforms
350<li>const initializers calling built-in functions
351</ul>
352
353
354
355
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