313 lines
8.4 KiB
HTML
313 lines
8.4 KiB
HTML
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<HTML>
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<TITLE>Shading Language Support</TITLE>
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<link rel="stylesheet" type="text/css" href="mesa.css"></head>
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<BODY>
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<H1>Shading Language Support</H1>
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<p>
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This page describes the features and status of Mesa's support for the
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<a href="http://opengl.org/documentation/glsl/" target="_parent">
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OpenGL Shading Language</a>.
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</p>
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<p>
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Last updated on 28 March 2007.
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</p>
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<p>
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Contents
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</p>
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<ul>
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<li><a href="#unsup">Unsupported Features</a>
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<li><a href="#notes">Implementation Notes</a>
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<li><a href="#hints">Programming Hints</a>
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<li><a href="#standalone">Stand-alone Compiler</a>
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<li><a href="#implementation">Compiler Implementation</a>
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<li><a href="#validation">Compiler Validation</a>
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</ul>
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<a name="unsup">
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<h2>Unsupported Features</h2>
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<p>
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The following features of the shading language are not yet supported
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in Mesa:
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</p>
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<ul>
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<li>Dereferencing arrays with non-constant indexes
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<li>Comparison of user-defined structs
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<li>Linking of multiple shaders is not supported
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<li>gl_ClipVertex
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<li>The derivative functions such as dFdx() are not implemented
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<li>The inverse trig functions asin(), acos(), and atan() are not implemented
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<li>The gl_Color and gl_SecondaryColor varying vars are interpolated
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without perspective correction
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</ul>
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<p>
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All other major features of the shading language should function.
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</p>
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<a name="notes">
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<h2>Implementation Notes</h2>
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<ul>
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<li>Shading language programs are compiled into low-level programs
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very similar to those of GL_ARB_vertex/fragment_program.
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<li>All vector types (vec2, vec3, vec4, bvec2, etc) currently occupy full
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float[4] registers.
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<li>Float constants and variables are packed so that up to four floats
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can occupy one program parameter/register.
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<li>All function calls are inlined.
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<li>Shaders which use too many registers will not compile.
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<li>The quality of generated code is pretty good, register usage is fair.
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<li>Shader error detection and reporting of errors (InfoLog) is not
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very good yet.
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<li>The ftransform() function doesn't necessarily match the results of
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fixed-function transformation.
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</ul>
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<p>
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These issues will be addressed/resolved in the future.
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</p>
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<a name="hints">
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<h2>Programming Hints</h2>
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<ul>
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<li>Declare <em>in</em> function parameters as <em>const</em> whenever possible.
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This improves the efficiency of function inlining.
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</li>
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<br>
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<li>To reduce register usage, declare variables within smaller scopes.
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For example, the following code:
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<pre>
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void main()
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{
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vec4 a1, a2, b1, b2;
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gl_Position = expression using a1, a2.
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gl_Color = expression using b1, b2;
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}
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</pre>
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Can be rewritten as follows to use half as many registers:
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<pre>
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void main()
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{
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{
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vec4 a1, a2;
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gl_Position = expression using a1, a2.
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}
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{
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vec4 b1, b2;
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gl_Color = expression using b1, b2;
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}
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}
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</pre>
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Alternately, rather than using several float variables, use
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a vec4 instead. Use swizzling and writemasks to access the
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components of the vec4 as floats.
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</li>
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<br>
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<li>Use the built-in library functions whenever possible.
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For example, instead of writing this:
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<pre>
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float x = 1.0 / sqrt(y);
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</pre>
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Write this:
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<pre>
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float x = inversesqrt(y);
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</pre>
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<li>
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Use ++i when possible as it's more efficient than i++
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</li>
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</ul>
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<a name="standalone">
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<h2>Stand-alone Compiler</h2>
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<p>
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A unique stand-alone GLSL compiler driver has been added to Mesa.
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<p>
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<p>
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The stand-alone compiler (like a conventional command-line compiler)
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is a tool that accepts Shading Language programs and emits low-level
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GPU programs.
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</p>
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<p>
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This tool is useful for:
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<p>
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<ul>
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<li>Inspecting GPU code to gain insight into compilation
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<li>Generating initial GPU code for subsequent hand-tuning
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<li>Debugging the GLSL compiler itself
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</ul>
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<p>
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To build the glslcompiler program (this will be improved someday):
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</p>
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<pre>
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cd src/mesa
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make libmesa.a
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cd drivers/glslcompiler
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make
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</pre>
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<p>
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Here's an example of using the compiler to compile a vertex shader and
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emit GL_ARB_vertex_program-style instructions:
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</p>
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<pre>
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glslcompiler --arb --linenumbers --vs vertshader.txt
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</pre>
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<p>
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The output may look similar to this:
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</p>
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<pre>
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!!ARBvp1.0
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0: MOV result.texcoord[0], vertex.texcoord[0];
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1: DP4 temp0.x, state.matrix.mvp.row[0], vertex.position;
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2: DP4 temp0.y, state.matrix.mvp.row[1], vertex.position;
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3: DP4 temp0.z, state.matrix.mvp.row[2], vertex.position;
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4: DP4 temp0.w, state.matrix.mvp.row[3], vertex.position;
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5: MOV result.position, temp0;
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6: END
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</pre>
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<p>
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Note that some shading language constructs (such as uniform and varying
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variables) aren't expressible in ARB or NV-style programs.
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Therefore, the resulting output is not always legal by definition of
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those program languages.
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</p>
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<p>
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Also note that this compiler driver is still under development.
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Over time, the correctness of the GPU programs, with respect to the ARB
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and NV languagues, should improve.
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</p>
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<a name="implementation">
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<h2>Compiler Implementation</h2>
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<p>
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The source code for Mesa's shading language compiler is in the
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<code>src/mesa/shader/slang/</code> directory.
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</p>
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<p>
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The compiler follows a fairly standard design and basically works as follows:
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</p>
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<ul>
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<li>The input string is tokenized (see grammar.c) and parsed
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(see slang_compiler_*.c) to produce an Abstract Syntax Tree (AST).
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The nodes in this tree are slang_operation structures
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(see slang_compile_operation.h).
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The nodes are decorated with symbol table, scoping and datatype information.
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<li>The AST is converted into an Intermediate representation (IR) tree
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(see the slang_codegen.c file).
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The IR nodes represent basic GPU instructions, like add, dot product,
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move, etc.
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The IR tree is mostly a binary tree, but a few nodes have three or four
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children.
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In principle, the IR tree could be executed by doing an in-order traversal.
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<li>The IR tree is traversed in-order to emit code (see slang_emit.c).
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This is also when registers are allocated to store variables and temps.
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<li>In the future, a pattern-matching code generator-generator may be
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used for code generation.
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Programs such as L-BURG (Bottom-Up Rewrite Generator) and Twig look for
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patterns in IR trees, compute weights for subtrees and use the weights
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to select the best instructions to represent the sub-tree.
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<li>The emitted GPU instructions (see prog_instruction.h) are stored in a
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gl_program object (see mtypes.h).
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<li>When a fragment shader and vertex shader are linked (see slang_link.c)
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the varying vars are matched up, uniforms are merged, and vertex
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attributes are resolved (rewriting instructions as needed).
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</ul>
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<p>
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The final vertex and fragment programs may be interpreted in software
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(see prog_execute.c) or translated into a specific hardware architecture
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(see drivers/dri/i915/i915_fragprog.c for example).
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</p>
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<h3>Code Generation Options</h3>
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<p>
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Internally, there are several options that control the compiler's code
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generation and instruction selection.
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These options are seen in the gl_shader_state struct and may be set
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by the device driver to indicate its preferences:
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<pre>
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struct gl_shader_state
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{
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...
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/** Driver-selectable options: */
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GLboolean EmitHighLevelInstructions;
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GLboolean EmitCondCodes;
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GLboolean EmitComments;
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};
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</pre>
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<ul>
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<li>EmitHighLevelInstructions
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<br>
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This option controls instruction selection for loops and conditionals.
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If the option is set high-level IF/ELSE/ENDIF, LOOP/ENDLOOP, CONT/BRK
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instructions will be emitted.
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Otherwise, those constructs will be implemented with BRA instructions.
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</li>
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<li>EmitCondCodes
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<br>
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If set, condition codes (ala GL_NV_fragment_program) will be used for
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branching and looping.
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Otherwise, ordinary registers will be used (the IF instruction will
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examine the first operand's X component and do the if-part if non-zero).
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This option is only relevant if EmitHighLevelInstructions is set.
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</li>
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<li>EmitComments
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<br>
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If set, instructions will be annoted with comments to help with debugging.
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Extra NOP instructions will also be inserted.
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</br>
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</ul>
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<a name="validation">
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<h2>Compiler Validation</h2>
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<p>
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A new <a href="http://glean.sf.net" target="_parent">Glean</a> test has
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been create to exercise the GLSL compiler.
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</p>
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<p>
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The <em>glsl1</em> test runs over 150 sub-tests to check that the language
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features and built-in functions work properly.
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This test should be run frequently while working on the compiler to catch
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regressions.
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</p>
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<p>
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The test coverage is reasonably broad and complete but additional tests
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should be added.
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</p>
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</BODY>
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</HTML>
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