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diff --git a/docs/shading.html b/docs/shading.html new file mode 100644 index 00000000000..40a6d7ac90b --- /dev/null +++ b/docs/shading.html @@ -0,0 +1,286 @@ +<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 17 Feb 2007. +</p> + +<p> +Contents +</p> +<ul> +<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 Compiler</a> +<li><a href="#implementation">Compiler Implementation</a> +</ul> + + +<a name="unsup"> +<h2>Unsupported Features</h2> + +<p> +The following features of the shading language are not yet supported +in Mesa: +</p> + +<ul> +<li>Dereferencing arrays with non-constant indexes +<li>User-defined structs +<li>Linking of multiple shaders is not supported +<li>Integer operations are not fully implemented (most are implemented + as floating point). +<li>gl_ClipVertex +</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>There are known memory leaks in the compiler. +</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> +</ul> + + +<a name="standalone"> +<h2>Stand-alone 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> +To build the glslcompiler program (this will be improved someday): +</p> +<pre> + cd src/mesa + make libmesa.a + cd 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> + glslcompiler --arb --linenumbers --vs vertshader.txt +</pre> +<p> +The output may look similar to this: +</p> +<pre> +!!ARBvp1.0 + 0: MOV result.texcoord[0], vertex.texcoord[0]; + 1: DP4 temp0.x, state.matrix.mvp.row[0], vertex.position; + 2: DP4 temp0.y, state.matrix.mvp.row[1], vertex.position; + 3: DP4 temp0.z, state.matrix.mvp.row[2], vertex.position; + 4: DP4 temp0.w, state.matrix.mvp.row[3], vertex.position; + 5: MOV result.position, temp0; + 6: 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> + + +</BODY> +</HTML> |