Added MesaLib-7.6

author: marha <marha@users.sourceforge.net> 2009-10-08 13:15:52 +0000
committer: marha <marha@users.sourceforge.net> 2009-10-08 13:15:52 +0000
commit: a0c4815433ccd57322f4f7703ca35e9ccfa59250 (patch)
tree: f5213802ec12adb86ec3136001c1c29fe5343700 /mesalib/docs/shading.html
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+<HTML>
+
+<TITLE>Shading Language Support</TITLE>
+
+<link rel="stylesheet" type="text/css" href="mesa.css"></head>
+
+<BODY>
+
+<H1>Shading Language Support</H1>
+
+<p>
+This page describes the features and status of Mesa's support for the
+<a href="http://opengl.org/documentation/glsl/" target="_parent">
+OpenGL Shading Language</a>.
+</p>
+
+<p>
+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:
+</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
+</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 fully supported
+in Mesa:
+</p>
+
+<ul>
+<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>
+All other major features of the shading language should function.
+</p>
+
+
+<a name="notes">
+<h2>Implementation Notes</h2>
+
+<ul>
+<li>Shading language programs are compiled into low-level programs
+    very similar to those of GL_ARB_vertex/fragment_program.
+<li>All vector types (vec2, vec3, vec4, bvec2, etc) currently occupy full
+    float[4] registers.
+<li>Float constants and variables are packed so that up to four floats
+    can occupy one program parameter/register.
+<li>All function calls are inlined.
+<li>Shaders which use too many registers will not compile.
+<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>The ftransform() function doesn't necessarily match the results of
+    fixed-function transformation.
+</ul>
+
+<p>
+These issues will be addressed/resolved in the future.
+</p>
+
+
+<a name="hints">
+<h2>Programming Hints</h2>
+
+<ul>
+<li>Declare <em>in</em> function parameters as <em>const</em> whenever possible.
+    This improves the efficiency of function inlining.
+</li>
+<br>
+<li>To reduce register usage, declare variables within smaller scopes.
+    For example, the following code:
+<pre>
+    void main()
+    {
+       vec4 a1, a2, b1, b2;
+       gl_Position = expression using a1, a2.
+       gl_Color = expression using b1, b2;
+    }
+</pre>
+    Can be rewritten as follows to use half as many registers:
+<pre>
+    void main()
+    {
+       {
+          vec4 a1, a2;
+          gl_Position = expression using a1, a2.
+       }
+       {
+          vec4 b1, b2;
+          gl_Color = expression using b1, b2;
+       }
+    }
+</pre>
+    Alternately, rather than using several float variables, use
+    a vec4 instead.  Use swizzling and writemasks to access the
+    components of the vec4 as floats.
+</li>
+<br>
+<li>Use the built-in library functions whenever possible.
+    For example, instead of writing this:
+<pre>
+        float x = 1.0 / sqrt(y);
+</pre>
+    Write this:
+<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>
+
+
+</BODY>
+</HTML>
author	marha <marha@users.sourceforge.net>	2009-10-08 13:15:52 +0000
committer	marha <marha@users.sourceforge.net>	2009-10-08 13:15:52 +0000
commit	a0c4815433ccd57322f4f7703ca35e9ccfa59250 (patch)
tree	f5213802ec12adb86ec3136001c1c29fe5343700 /mesalib/docs/shading.html
parent	c73dc01b6de45612b24dc2dd34fba24d81ebf46c (diff)
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