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Brian9595d192007-01-20 13:40:57 -0700[diff] [blame]1<HTML>
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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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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>
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>
28<li><a href="#standalone">Stand-alone 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>
Brian61d31ae2007-02-17 09:41:19 -0700[diff] [blame]31</ul>
32
33
34<a name="unsup">
Brian9595d192007-01-20 13:40:57 -0700[diff] [blame]35<h2>Unsupported Features</h2>
36
37<p>
38The following features of the shading language are not yet supported
39in Mesa:
40</p>
41
42<ul>
Brian6d4cf6b2007-02-21 16:07:03 -0700[diff] [blame]43<li>Dereferencing arrays with non-constant indexes
Brianb67d9312007-03-26 10:23:50 -0600[diff] [blame]44<li>Comparison of user-defined structs
Brian9595d192007-01-20 13:40:57 -0700[diff] [blame]45<li>Linking of multiple shaders is not supported
Brianb03e1712007-03-09 09:51:55 -0700[diff] [blame]46<li>gl_ClipVertex
Brianba3d3842007-04-19 14:24:29 -0600[diff] [blame]47<li>The derivative functions such as dFdx() are not implemented
Brian7ff72a72007-04-27 15:23:19 -0600[diff] [blame]48<li>The inverse trig functions asin(), acos(), and atan() are not implemented
Brian4b1d1b72007-04-27 15:25:00 -0600[diff] [blame]49<li>The gl_Color and gl_SecondaryColor varying vars are interpolated
50 without perspective correction
Brian5ef3a2c2007-11-27 10:27:55 -0700[diff] [blame]51<li>Floating point literal suffixes 'f' and 'F' aren't allowed.
Brian9595d192007-01-20 13:40:57 -0700[diff] [blame]52</ul>
53
54<p>
55All other major features of the shading language should function.
56</p>
57
58
Brian07e62082007-02-27 16:45:40 -0700[diff] [blame]59<a name="notes">
Brian9595d192007-01-20 13:40:57 -0700[diff] [blame]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.
Brianbbec2fd2007-01-28 12:11:10 -0700[diff] [blame]65<li>All vector types (vec2, vec3, vec4, bvec2, etc) currently occupy full
Brian9595d192007-01-20 13:40:57 -0700[diff] [blame]66 float[4] registers.
Brianbbec2fd2007-01-28 12:11:10 -0700[diff] [blame]67<li>Float constants and variables are packed so that up to four floats
68 can occupy one program parameter/register.
Brian9595d192007-01-20 13:40:57 -0700[diff] [blame]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.
Brian05e6fd82007-03-27 16:05:25 -0600[diff] [blame]74<li>The ftransform() function doesn't necessarily match the results of
75 fixed-function transformation.
Brian9595d192007-01-20 13:40:57 -0700[diff] [blame]76</ul>
77
78<p>
79These issues will be addressed/resolved in the future.
80</p>
81
82
Brian61d31ae2007-02-17 09:41:19 -0700[diff] [blame]83<a name="hints">
Brian9595d192007-01-20 13:40:57 -0700[diff] [blame]84<h2>Programming Hints</h2>
85
86<ul>
Brianbbec2fd2007-01-28 12:11:10 -0700[diff] [blame]87<li>Declare <em>in</em> function parameters as <em>const</em> whenever possible.
Brian9595d192007-01-20 13:40:57 -0700[diff] [blame]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>
Brian7eba12e2007-03-28 17:14:35 -0600[diff] [blame]129<li>
130 Use ++i when possible as it's more efficient than i++
131</li>
Brian9595d192007-01-20 13:40:57 -0700[diff] [blame]132</ul>
133
134
Brian61d31ae2007-02-17 09:41:19 -0700[diff] [blame]135<a name="standalone">
136<h2>Stand-alone Compiler</h2>
137
138<p>
139A unique stand-alone GLSL compiler driver has been added to Mesa.
140<p>
141
142<p>
143The stand-alone compiler (like a conventional command-line compiler)
144is a tool that accepts Shading Language programs and emits low-level
145GPU programs.
146</p>
147
148<p>
149This 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>
Brianff0cc922007-02-22 16:29:48 -0700[diff] [blame]158To build the glslcompiler program (this will be improved someday):
Brian61d31ae2007-02-17 09:41:19 -0700[diff] [blame]159</p>
Brianff0cc922007-02-22 16:29:48 -0700[diff] [blame]160<pre>
161 cd src/mesa
162 make libmesa.a
163 cd drivers/glslcompiler
164 make
165</pre>
166
Brian61d31ae2007-02-17 09:41:19 -0700[diff] [blame]167
168<p>
169Here's an example of using the compiler to compile a vertex shader and
170emit GL_ARB_vertex_program-style instructions:
171</p>
172<pre>
173 glslcompiler --arb --linenumbers --vs vertshader.txt
174</pre>
175<p>
176The output may look similar to this:
177</p>
178<pre>
179!!ARBvp1.0
180 0: MOV result.texcoord[0], vertex.texcoord[0];
181 1: DP4 temp0.x, state.matrix.mvp.row[0], vertex.position;
182 2: DP4 temp0.y, state.matrix.mvp.row[1], vertex.position;
183 3: DP4 temp0.z, state.matrix.mvp.row[2], vertex.position;
184 4: DP4 temp0.w, state.matrix.mvp.row[3], vertex.position;
185 5: MOV result.position, temp0;
186 6: END
187</pre>
188
189<p>
190Note that some shading language constructs (such as uniform and varying
191variables) aren't expressible in ARB or NV-style programs.
192Therefore, the resulting output is not always legal by definition of
193those program languages.
194</p>
195<p>
196Also note that this compiler driver is still under development.
197Over time, the correctness of the GPU programs, with respect to the ARB
198and NV languagues, should improve.
199</p>
200
Brian07e62082007-02-27 16:45:40 -0700[diff] [blame]201
202
203<a name="implementation">
204<h2>Compiler Implementation</h2>
205
206<p>
207The source code for Mesa's shading language compiler is in the
208<code>src/mesa/shader/slang/</code> directory.
209</p>
210
211<p>
212The compiler follows a fairly standard design and basically works as follows:
213</p>
214<ul>
215<li>The input string is tokenized (see grammar.c) and parsed
216(see slang_compiler_*.c) to produce an Abstract Syntax Tree (AST).
217The nodes in this tree are slang_operation structures
218(see slang_compile_operation.h).
219The nodes are decorated with symbol table, scoping and datatype information.
220<li>The AST is converted into an Intermediate representation (IR) tree
221(see the slang_codegen.c file).
222The IR nodes represent basic GPU instructions, like add, dot product,
223move, etc.
224The IR tree is mostly a binary tree, but a few nodes have three or four
225children.
226In principle, the IR tree could be executed by doing an in-order traversal.
227<li>The IR tree is traversed in-order to emit code (see slang_emit.c).
228This is also when registers are allocated to store variables and temps.
229<li>In the future, a pattern-matching code generator-generator may be
230used for code generation.
231Programs such as L-BURG (Bottom-Up Rewrite Generator) and Twig look for
232patterns in IR trees, compute weights for subtrees and use the weights
233to select the best instructions to represent the sub-tree.
234<li>The emitted GPU instructions (see prog_instruction.h) are stored in a
235gl_program object (see mtypes.h).
236<li>When a fragment shader and vertex shader are linked (see slang_link.c)
237the varying vars are matched up, uniforms are merged, and vertex
238attributes are resolved (rewriting instructions as needed).
239</ul>
240
241<p>
242The final vertex and fragment programs may be interpreted in software
243(see prog_execute.c) or translated into a specific hardware architecture
244(see drivers/dri/i915/i915_fragprog.c for example).
245</p>
246
Brian8f9db0f2007-03-23 17:49:19 -0600[diff] [blame]247<h3>Code Generation Options</h3>
248
249<p>
250Internally, there are several options that control the compiler's code
251generation and instruction selection.
252These options are seen in the gl_shader_state struct and may be set
253by the device driver to indicate its preferences:
254
255<pre>
256struct gl_shader_state
257{
258 ...
259 /** Driver-selectable options: */
260 GLboolean EmitHighLevelInstructions;
261 GLboolean EmitCondCodes;
262 GLboolean EmitComments;
263};
264</pre>
265
266<ul>
267<li>EmitHighLevelInstructions
268<br>
269This option controls instruction selection for loops and conditionals.
270If the option is set high-level IF/ELSE/ENDIF, LOOP/ENDLOOP, CONT/BRK
271instructions will be emitted.
272Otherwise, those constructs will be implemented with BRA instructions.
273</li>
274
275<li>EmitCondCodes
276<br>
277If set, condition codes (ala GL_NV_fragment_program) will be used for
278branching and looping.
279Otherwise, ordinary registers will be used (the IF instruction will
280examine the first operand's X component and do the if-part if non-zero).
281This option is only relevant if EmitHighLevelInstructions is set.
282</li>
283
284<li>EmitComments
285<br>
286If set, instructions will be annoted with comments to help with debugging.
287Extra NOP instructions will also be inserted.
288</br>
289
290</ul>
Brian07e62082007-02-27 16:45:40 -0700[diff] [blame]291
292
Brian7eba12e2007-03-28 17:14:35 -0600[diff] [blame]293<a name="validation">
294<h2>Compiler Validation</h2>
295
296<p>
297A new <a href="http://glean.sf.net" target="_parent">Glean</a> test has
298been create to exercise the GLSL compiler.
299</p>
300<p>
Brian8598f552007-04-18 09:30:07 -0600[diff] [blame]301The <em>glsl1</em> test runs over 150 sub-tests to check that the language
Brian7eba12e2007-03-28 17:14:35 -0600[diff] [blame]302features and built-in functions work properly.
303This test should be run frequently while working on the compiler to catch
304regressions.
305</p>
306<p>
307The test coverage is reasonably broad and complete but additional tests
308should be added.
309</p>
310
311
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