2007-01-20 20:40:57 +00:00
|
|
|
<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>
|
2007-02-17 16:41:19 +00:00
|
|
|
Last updated on 17 Feb 2007.
|
2007-01-20 20:40:57 +00:00
|
|
|
</p>
|
|
|
|
|
|
2007-02-17 16:41:19 +00:00
|
|
|
<p>
|
|
|
|
|
Contents
|
|
|
|
|
</p>
|
|
|
|
|
<ul>
|
|
|
|
|
<li><a href="#unsup">Unsupported Features</a>
|
2007-02-27 23:45:40 +00:00
|
|
|
<li><a href="#notes">Implementation Notes</a>
|
2007-02-17 16:41:19 +00:00
|
|
|
<li><a href="#hints">Programming Hints</a>
|
|
|
|
|
<li><a href="#standalone">Stand-alone Compiler</a>
|
2007-02-27 23:45:40 +00:00
|
|
|
<li><a href="#implementation">Compiler Implementation</a>
|
2007-02-17 16:41:19 +00:00
|
|
|
</ul>
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
<a name="unsup">
|
2007-01-20 20:40:57 +00:00
|
|
|
<h2>Unsupported Features</h2>
|
|
|
|
|
|
|
|
|
|
<p>
|
|
|
|
|
The following features of the shading language are not yet supported
|
|
|
|
|
in Mesa:
|
|
|
|
|
</p>
|
|
|
|
|
|
|
|
|
|
<ul>
|
2007-02-21 23:07:03 +00:00
|
|
|
<li>Dereferencing arrays with non-constant indexes
|
|
|
|
|
<li>User-defined structs
|
2007-01-20 20:40:57 +00:00
|
|
|
<li>Linking of multiple shaders is not supported
|
|
|
|
|
<li>Integer operations are not fully implemented (most are implemented
|
|
|
|
|
as floating point).
|
2007-03-09 16:51:55 +00:00
|
|
|
<li>gl_ClipVertex
|
2007-01-20 20:40:57 +00:00
|
|
|
</ul>
|
|
|
|
|
|
|
|
|
|
<p>
|
|
|
|
|
All other major features of the shading language should function.
|
|
|
|
|
</p>
|
|
|
|
|
|
|
|
|
|
|
2007-02-27 23:45:40 +00:00
|
|
|
<a name="notes">
|
2007-01-20 20:40:57 +00:00
|
|
|
<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.
|
2007-01-28 19:11:10 +00:00
|
|
|
<li>All vector types (vec2, vec3, vec4, bvec2, etc) currently occupy full
|
2007-01-20 20:40:57 +00:00
|
|
|
float[4] registers.
|
2007-01-28 19:11:10 +00:00
|
|
|
<li>Float constants and variables are packed so that up to four floats
|
|
|
|
|
can occupy one program parameter/register.
|
2007-01-20 20:40:57 +00:00
|
|
|
<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 massive memory leaks in the compiler.
|
|
|
|
|
</ul>
|
|
|
|
|
|
|
|
|
|
<p>
|
|
|
|
|
These issues will be addressed/resolved in the future.
|
|
|
|
|
</p>
|
|
|
|
|
|
|
|
|
|
|
2007-02-17 16:41:19 +00:00
|
|
|
<a name="hints">
|
2007-01-20 20:40:57 +00:00
|
|
|
<h2>Programming Hints</h2>
|
|
|
|
|
|
|
|
|
|
<ul>
|
2007-01-28 19:11:10 +00:00
|
|
|
<li>Declare <em>in</em> function parameters as <em>const</em> whenever possible.
|
2007-01-20 20:40:57 +00:00
|
|
|
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>
|
|
|
|
|
|
|
|
|
|
|
2007-02-17 16:41:19 +00:00
|
|
|
<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>
|
2007-02-22 23:29:48 +00:00
|
|
|
To build the glslcompiler program (this will be improved someday):
|
2007-02-17 16:41:19 +00:00
|
|
|
</p>
|
2007-02-22 23:29:48 +00:00
|
|
|
<pre>
|
|
|
|
|
cd src/mesa
|
|
|
|
|
make libmesa.a
|
|
|
|
|
cd drivers/glslcompiler
|
|
|
|
|
make
|
|
|
|
|
</pre>
|
|
|
|
|
|
2007-02-17 16:41:19 +00:00
|
|
|
|
|
|
|
|
<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>
|
|
|
|
|
|
2007-02-27 23:45:40 +00:00
|
|
|
|
|
|
|
|
|
|
|
|
|
<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>
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
2007-01-20 20:40:57 +00:00
|
|
|
</BODY>
|
|
|
|
|
</HTML>
|
