</p>
<p>
-Last updated on 20 Jan 2007.
+Last updated on 15 December 2008.
</p>
+<p>
+Contents
+</p>
+<ul>
+<li><a href="#envvars">Environment variables</a>
+<li><a href="#120">GLSL 1.20 support</a>
+<li><a href="#unsup">Unsupported Features</a>
+<li><a href="#notes">Implementation Notes</a>
+<li><a href="#hints">Programming Hints</a>
+<li><a href="#standalone">Stand-alone GLSL Compiler</a>
+<li><a href="#implementation">Compiler Implementation</a>
+<li><a href="#validation">Compiler Validation</a>
+</ul>
+
+
+
+<a name="envvars">
+<h2>Environment Variables</h2>
+
+<p>
+The <b>MESA_GLSL</b> environment variable can be set to a comma-separated
+list of keywords to control some aspects of the GLSL compiler and shader
+execution. These are generally used for debugging.
+</p>
+<ul>
+<li>dump - print GLSL shader code to stdout at link time
+<li>log - log all GLSL shaders to files.
+ The filenames will be "shader_X.vert" or "shader_X.frag" where X
+ the shader ID.
+<li>nopt - disable compiler optimizations
+<li>opt - force compiler optimizations
+<li>uniform - print message to stdout when glUniform is called
+<li>nopvert - force vertex shaders to be a simple shader that just transforms
+ the vertex position with ftransform() and passes through the color and
+ texcoord[0] attributes.
+<li>nopfrag - force fragment shader to be a simple shader that passes
+ through the color attribute.
+<li>useprog - log glUseProgram calls to stderr
+</ul>
+<p>
+Example: export MESA_GLSL=dump,nopt
+</p>
+
+
+<a name="120">
+<h2>GLSL 1.20 support</h2>
+
+<p>
+GLSL version 1.20 is supported in Mesa 7.3 and later.
+Among the features/differences of GLSL 1.20 are:
+<ul>
+<li><code>mat2x3, mat2x4</code>, etc. types and functions
+<li><code>transpose(), outerProduct(), matrixCompMult()</code> functions
+(but untested)
+<li>precision qualifiers (lowp, mediump, highp)
+<li><code>invariant</code> qualifier
+<li><code>array.length()</code> method
+<li><code>float[5] a;</code> array syntax
+<li><code>centroid</code> qualifier
+<li>unsized array constructors
+<li>initializers for uniforms
+<li>const initializers calling built-in functions
+</ul>
+
+
+
+<a name="unsup">
<h2>Unsupported Features</h2>
<p>
-The following features of the shading language are not yet supported
+The following features of the shading language are not yet fully supported
in Mesa:
</p>
<ul>
-<li>Arrays
-<li>Structs
-<li>Linking of multiple shaders is not supported
-<li>Not all built-in OpenGL state variables are supported yet.
- Common variables such as gl_ModelViewMatrix and gl_NormalMatrix
- are supported.
-<li>Integer operations are not fully implemented (most are implemented
- as floating point).
+<li>Linking of multiple shaders does not always work. Currently, linking
+ is implemented through shader concatenation and re-compiling. This
+ doesn't always work because of some #pragma and preprocessor issues.
+<li>gl_ClipVertex
+<li>The gl_Color and gl_SecondaryColor varying vars are interpolated
+ without perspective correction
</ul>
<p>
</p>
+<a name="notes">
<h2>Implementation Notes</h2>
<ul>
<li>The quality of generated code is pretty good, register usage is fair.
<li>Shader error detection and reporting of errors (InfoLog) is not
very good yet.
-<li>There are massive memory leaks in the compiler.
+<li>The ftransform() function doesn't necessarily match the results of
+ fixed-function transformation.
</ul>
<p>
</p>
+<a name="hints">
<h2>Programming Hints</h2>
<ul>
<pre>
float x = inversesqrt(y);
</pre>
+<li>
+ Use ++i when possible as it's more efficient than i++
+</li>
</ul>
+<a name="standalone">
+<h2>Stand-alone GLSL Compiler</h2>
+
+<p>
+A unique stand-alone GLSL compiler driver has been added to Mesa.
+<p>
+
+<p>
+The stand-alone compiler (like a conventional command-line compiler)
+is a tool that accepts Shading Language programs and emits low-level
+GPU programs.
+</p>
+
+<p>
+This tool is useful for:
+<p>
+<ul>
+<li>Inspecting GPU code to gain insight into compilation
+<li>Generating initial GPU code for subsequent hand-tuning
+<li>Debugging the GLSL compiler itself
+</ul>
+
+<p>
+After building Mesa, the glslcompiler can be built by manually running:
+</p>
+<pre>
+ make realclean
+ make linux
+ cd src/mesa/drivers/glslcompiler
+ make
+</pre>
+
+
+<p>
+Here's an example of using the compiler to compile a vertex shader and
+emit GL_ARB_vertex_program-style instructions:
+</p>
+<pre>
+ bin/glslcompiler --debug --numbers --fs progs/glsl/CH06-brick.frag.txt
+</pre>
+<p>
+results in:
+</p>
+<pre>
+# Fragment Program/Shader
+ 0: RCP TEMP[4].x, UNIFORM[2].xxxx;
+ 1: RCP TEMP[4].y, UNIFORM[2].yyyy;
+ 2: MUL TEMP[3].xy, VARYING[0], TEMP[4];
+ 3: MOV TEMP[1], TEMP[3];
+ 4: MUL TEMP[0].w, TEMP[1].yyyy, CONST[4].xxxx;
+ 5: FRC TEMP[1].z, TEMP[0].wwww;
+ 6: SGT.C TEMP[0].w, TEMP[1].zzzz, CONST[4].xxxx;
+ 7: IF (NE.wwww); # (if false, goto 9);
+ 8: ADD TEMP[1].x, TEMP[1].xxxx, CONST[4].xxxx;
+ 9: ENDIF;
+ 10: FRC TEMP[1].xy, TEMP[1];
+ 11: SGT TEMP[2].xy, UNIFORM[3], TEMP[1];
+ 12: MUL TEMP[1].z, TEMP[2].xxxx, TEMP[2].yyyy;
+ 13: LRP TEMP[0], TEMP[1].zzzz, UNIFORM[0], UNIFORM[1];
+ 14: MUL TEMP[0].xyz, TEMP[0], VARYING[1].xxxx;
+ 15: MOV OUTPUT[0].xyz, TEMP[0];
+ 16: MOV OUTPUT[0].w, CONST[4].yyyy;
+ 17: END
+</pre>
+
+<p>
+Note that some shading language constructs (such as uniform and varying
+variables) aren't expressible in ARB or NV-style programs.
+Therefore, the resulting output is not always legal by definition of
+those program languages.
+</p>
+<p>
+Also note that this compiler driver is still under development.
+Over time, the correctness of the GPU programs, with respect to the ARB
+and NV languagues, should improve.
+</p>
+
+
+
+<a name="implementation">
+<h2>Compiler Implementation</h2>
+
+<p>
+The source code for Mesa's shading language compiler is in the
+<code>src/mesa/shader/slang/</code> directory.
+</p>
+
+<p>
+The compiler follows a fairly standard design and basically works as follows:
+</p>
+<ul>
+<li>The input string is tokenized (see grammar.c) and parsed
+(see slang_compiler_*.c) to produce an Abstract Syntax Tree (AST).
+The nodes in this tree are slang_operation structures
+(see slang_compile_operation.h).
+The nodes are decorated with symbol table, scoping and datatype information.
+<li>The AST is converted into an Intermediate representation (IR) tree
+(see the slang_codegen.c file).
+The IR nodes represent basic GPU instructions, like add, dot product,
+move, etc.
+The IR tree is mostly a binary tree, but a few nodes have three or four
+children.
+In principle, the IR tree could be executed by doing an in-order traversal.
+<li>The IR tree is traversed in-order to emit code (see slang_emit.c).
+This is also when registers are allocated to store variables and temps.
+<li>In the future, a pattern-matching code generator-generator may be
+used for code generation.
+Programs such as L-BURG (Bottom-Up Rewrite Generator) and Twig look for
+patterns in IR trees, compute weights for subtrees and use the weights
+to select the best instructions to represent the sub-tree.
+<li>The emitted GPU instructions (see prog_instruction.h) are stored in a
+gl_program object (see mtypes.h).
+<li>When a fragment shader and vertex shader are linked (see slang_link.c)
+the varying vars are matched up, uniforms are merged, and vertex
+attributes are resolved (rewriting instructions as needed).
+</ul>
+
+<p>
+The final vertex and fragment programs may be interpreted in software
+(see prog_execute.c) or translated into a specific hardware architecture
+(see drivers/dri/i915/i915_fragprog.c for example).
+</p>
+
+<h3>Code Generation Options</h3>
+
+<p>
+Internally, there are several options that control the compiler's code
+generation and instruction selection.
+These options are seen in the gl_shader_state struct and may be set
+by the device driver to indicate its preferences:
+
+<pre>
+struct gl_shader_state
+{
+ ...
+ /** Driver-selectable options: */
+ GLboolean EmitHighLevelInstructions;
+ GLboolean EmitCondCodes;
+ GLboolean EmitComments;
+};
+</pre>
+
+<ul>
+<li>EmitHighLevelInstructions
+<br>
+This option controls instruction selection for loops and conditionals.
+If the option is set high-level IF/ELSE/ENDIF, LOOP/ENDLOOP, CONT/BRK
+instructions will be emitted.
+Otherwise, those constructs will be implemented with BRA instructions.
+</li>
+
+<li>EmitCondCodes
+<br>
+If set, condition codes (ala GL_NV_fragment_program) will be used for
+branching and looping.
+Otherwise, ordinary registers will be used (the IF instruction will
+examine the first operand's X component and do the if-part if non-zero).
+This option is only relevant if EmitHighLevelInstructions is set.
+</li>
+
+<li>EmitComments
+<br>
+If set, instructions will be annoted with comments to help with debugging.
+Extra NOP instructions will also be inserted.
+</br>
+
+</ul>
+
+
+<a name="validation">
+<h2>Compiler Validation</h2>
+
+<p>
+A <a href="http://glean.sf.net" target="_parent">Glean</a> test has
+been create to exercise the GLSL compiler.
+</p>
+<p>
+The <em>glsl1</em> test runs over 170 sub-tests to check that the language
+features and built-in functions work properly.
+This test should be run frequently while working on the compiler to catch
+regressions.
+</p>
+<p>
+The test coverage is reasonably broad and complete but additional tests
+should be added.
+</p>
+
+
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