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3<TITLE>Shading Language Support</TITLE>
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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>
Brianb67d9312007-03-26 10:23:50 -0600[diff] [blame^]18Last updated on 26 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>
Brian61d31ae2007-02-17 09:41:19 -0700[diff] [blame]30</ul>
31
32
33<a name="unsup">
Brian9595d192007-01-20 13:40:57 -0700[diff] [blame]34<h2>Unsupported Features</h2>
35
36<p>
37The following features of the shading language are not yet supported
38in Mesa:
39</p>
40
41<ul>
Brian6d4cf6b2007-02-21 16:07:03 -0700[diff] [blame]42<li>Dereferencing arrays with non-constant indexes
Brianb67d9312007-03-26 10:23:50 -0600[diff] [blame^]43<li>Comparison of user-defined structs
Brian9595d192007-01-20 13:40:57 -0700[diff] [blame]44<li>Linking of multiple shaders is not supported
Brianb03e1712007-03-09 09:51:55 -0700[diff] [blame]45<li>gl_ClipVertex
Brian9595d192007-01-20 13:40:57 -0700[diff] [blame]46</ul>
47
48<p>
49All other major features of the shading language should function.
50</p>
51
52
Brian07e62082007-02-27 16:45:40 -0700[diff] [blame]53<a name="notes">
Brian9595d192007-01-20 13:40:57 -0700[diff] [blame]54<h2>Implementation Notes</h2>
55
56<ul>
57<li>Shading language programs are compiled into low-level programs
58 very similar to those of GL_ARB_vertex/fragment_program.
Brianbbec2fd2007-01-28 12:11:10 -0700[diff] [blame]59<li>All vector types (vec2, vec3, vec4, bvec2, etc) currently occupy full
Brian9595d192007-01-20 13:40:57 -0700[diff] [blame]60 float[4] registers.
Brianbbec2fd2007-01-28 12:11:10 -0700[diff] [blame]61<li>Float constants and variables are packed so that up to four floats
62 can occupy one program parameter/register.
Brian9595d192007-01-20 13:40:57 -0700[diff] [blame]63<li>All function calls are inlined.
64<li>Shaders which use too many registers will not compile.
65<li>The quality of generated code is pretty good, register usage is fair.
66<li>Shader error detection and reporting of errors (InfoLog) is not
67 very good yet.
Brian8f9db0f2007-03-23 17:49:19 -0600[diff] [blame]68<li>There are known memory leaks in the compiler.
Brian9595d192007-01-20 13:40:57 -0700[diff] [blame]69</ul>
70
71<p>
72These issues will be addressed/resolved in the future.
73</p>
74
75
Brian61d31ae2007-02-17 09:41:19 -0700[diff] [blame]76<a name="hints">
Brian9595d192007-01-20 13:40:57 -0700[diff] [blame]77<h2>Programming Hints</h2>
78
79<ul>
Brianbbec2fd2007-01-28 12:11:10 -0700[diff] [blame]80<li>Declare <em>in</em> function parameters as <em>const</em> whenever possible.
Brian9595d192007-01-20 13:40:57 -0700[diff] [blame]81 This improves the efficiency of function inlining.
82</li>
83<br>
84<li>To reduce register usage, declare variables within smaller scopes.
85 For example, the following code:
86<pre>
87 void main()
88 {
89 vec4 a1, a2, b1, b2;
90 gl_Position = expression using a1, a2.
91 gl_Color = expression using b1, b2;
92 }
93</pre>
94 Can be rewritten as follows to use half as many registers:
95<pre>
96 void main()
97 {
98 {
99 vec4 a1, a2;
100 gl_Position = expression using a1, a2.
101 }
102 {
103 vec4 b1, b2;
104 gl_Color = expression using b1, b2;
105 }
106 }
107</pre>
108 Alternately, rather than using several float variables, use
109 a vec4 instead. Use swizzling and writemasks to access the
110 components of the vec4 as floats.
111</li>
112<br>
113<li>Use the built-in library functions whenever possible.
114 For example, instead of writing this:
115<pre>
116 float x = 1.0 / sqrt(y);
117</pre>
118 Write this:
119<pre>
120 float x = inversesqrt(y);
121</pre>
122</ul>
123
124
Brian61d31ae2007-02-17 09:41:19 -0700[diff] [blame]125<a name="standalone">
126<h2>Stand-alone Compiler</h2>
127
128<p>
129A unique stand-alone GLSL compiler driver has been added to Mesa.
130<p>
131
132<p>
133The stand-alone compiler (like a conventional command-line compiler)
134is a tool that accepts Shading Language programs and emits low-level
135GPU programs.
136</p>
137
138<p>
139This tool is useful for:
140<p>
141<ul>
142<li>Inspecting GPU code to gain insight into compilation
143<li>Generating initial GPU code for subsequent hand-tuning
144<li>Debugging the GLSL compiler itself
145</ul>
146
147<p>
Brianff0cc922007-02-22 16:29:48 -0700[diff] [blame]148To build the glslcompiler program (this will be improved someday):
Brian61d31ae2007-02-17 09:41:19 -0700[diff] [blame]149</p>
Brianff0cc922007-02-22 16:29:48 -0700[diff] [blame]150<pre>
151 cd src/mesa
152 make libmesa.a
153 cd drivers/glslcompiler
154 make
155</pre>
156
Brian61d31ae2007-02-17 09:41:19 -0700[diff] [blame]157
158<p>
159Here's an example of using the compiler to compile a vertex shader and
160emit GL_ARB_vertex_program-style instructions:
161</p>
162<pre>
163 glslcompiler --arb --linenumbers --vs vertshader.txt
164</pre>
165<p>
166The output may look similar to this:
167</p>
168<pre>
169!!ARBvp1.0
170 0: MOV result.texcoord[0], vertex.texcoord[0];
171 1: DP4 temp0.x, state.matrix.mvp.row[0], vertex.position;
172 2: DP4 temp0.y, state.matrix.mvp.row[1], vertex.position;
173 3: DP4 temp0.z, state.matrix.mvp.row[2], vertex.position;
174 4: DP4 temp0.w, state.matrix.mvp.row[3], vertex.position;
175 5: MOV result.position, temp0;
176 6: END
177</pre>
178
179<p>
180Note that some shading language constructs (such as uniform and varying
181variables) aren't expressible in ARB or NV-style programs.
182Therefore, the resulting output is not always legal by definition of
183those program languages.
184</p>
185<p>
186Also note that this compiler driver is still under development.
187Over time, the correctness of the GPU programs, with respect to the ARB
188and NV languagues, should improve.
189</p>
190
Brian07e62082007-02-27 16:45:40 -0700[diff] [blame]191
192
193<a name="implementation">
194<h2>Compiler Implementation</h2>
195
196<p>
197The source code for Mesa's shading language compiler is in the
198<code>src/mesa/shader/slang/</code> directory.
199</p>
200
201<p>
202The compiler follows a fairly standard design and basically works as follows:
203</p>
204<ul>
205<li>The input string is tokenized (see grammar.c) and parsed
206(see slang_compiler_*.c) to produce an Abstract Syntax Tree (AST).
207The nodes in this tree are slang_operation structures
208(see slang_compile_operation.h).
209The nodes are decorated with symbol table, scoping and datatype information.
210<li>The AST is converted into an Intermediate representation (IR) tree
211(see the slang_codegen.c file).
212The IR nodes represent basic GPU instructions, like add, dot product,
213move, etc.
214The IR tree is mostly a binary tree, but a few nodes have three or four
215children.
216In principle, the IR tree could be executed by doing an in-order traversal.
217<li>The IR tree is traversed in-order to emit code (see slang_emit.c).
218This is also when registers are allocated to store variables and temps.
219<li>In the future, a pattern-matching code generator-generator may be
220used for code generation.
221Programs such as L-BURG (Bottom-Up Rewrite Generator) and Twig look for
222patterns in IR trees, compute weights for subtrees and use the weights
223to select the best instructions to represent the sub-tree.
224<li>The emitted GPU instructions (see prog_instruction.h) are stored in a
225gl_program object (see mtypes.h).
226<li>When a fragment shader and vertex shader are linked (see slang_link.c)
227the varying vars are matched up, uniforms are merged, and vertex
228attributes are resolved (rewriting instructions as needed).
229</ul>
230
231<p>
232The final vertex and fragment programs may be interpreted in software
233(see prog_execute.c) or translated into a specific hardware architecture
234(see drivers/dri/i915/i915_fragprog.c for example).
235</p>
236
Brian8f9db0f2007-03-23 17:49:19 -0600[diff] [blame]237<h3>Code Generation Options</h3>
238
239<p>
240Internally, there are several options that control the compiler's code
241generation and instruction selection.
242These options are seen in the gl_shader_state struct and may be set
243by the device driver to indicate its preferences:
244
245<pre>
246struct gl_shader_state
247{
248 ...
249 /** Driver-selectable options: */
250 GLboolean EmitHighLevelInstructions;
251 GLboolean EmitCondCodes;
252 GLboolean EmitComments;
253};
254</pre>
255
256<ul>
257<li>EmitHighLevelInstructions
258<br>
259This option controls instruction selection for loops and conditionals.
260If the option is set high-level IF/ELSE/ENDIF, LOOP/ENDLOOP, CONT/BRK
261instructions will be emitted.
262Otherwise, those constructs will be implemented with BRA instructions.
263</li>
264
265<li>EmitCondCodes
266<br>
267If set, condition codes (ala GL_NV_fragment_program) will be used for
268branching and looping.
269Otherwise, ordinary registers will be used (the IF instruction will
270examine the first operand's X component and do the if-part if non-zero).
271This option is only relevant if EmitHighLevelInstructions is set.
272</li>
273
274<li>EmitComments
275<br>
276If set, instructions will be annoted with comments to help with debugging.
277Extra NOP instructions will also be inserted.
278</br>
279
280</ul>
Brian07e62082007-02-27 16:45:40 -0700[diff] [blame]281
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