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-<TITLE>Shading Language Support</TITLE>
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+ The Mesa 3D Graphics Library
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-
-<H1>Shading Language Support</H1>
+<h1>Shading Language</h1>
<p>
This page describes the features and status of Mesa's support for the
-<a href="http://opengl.org/documentation/glsl/" target="_parent">
+<a href="https://opengl.org/documentation/glsl/">
OpenGL Shading Language</a>.
</p>
-<p>
-Last updated on 17 Feb 2007.
-</p>
-
<p>
Contents
</p>
<ul>
+<li><a href="#envvars">Environment variables</a>
+<li><a href="#support">GLSL 1.40 support</a>
<li><a href="#unsup">Unsupported Features</a>
-<li><a href="#impl">Implementation Notes</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="#standalone">Stand-alone GLSL Compiler</a>
+<li><a href="#implementation">Compiler Implementation</a>
+<li><a href="#validation">Compiler Validation</a>
+</ul>
+
+
+<h2 id="envvars">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><b>dump</b> - print GLSL shader code to stdout at link time
+<li><b>log</b> - log all GLSL shaders to files.
+ The filenames will be "shader_X.vert" or "shader_X.frag" where X
+ the shader ID.
+<li><b>cache_info</b> - print debug information about shader cache
+<li><b>cache_fb</b> - force cached shaders to be ignored and do a full
+ recompile via the fallback path</li>
+<li><b>uniform</b> - print message to stdout when glUniform is called
+<li><b>nopvert</b> - 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><b>nopfrag</b> - force fragment shader to be a simple shader that passes
+ through the color attribute.
+<li><b>useprog</b> - log glUseProgram calls to stderr
+<li><b>errors</b> - GLSL compilation and link errors will be reported to stderr.
+</ul>
+<p>
+Example: export MESA_GLSL=dump,nopt
+</p>
+
+<h3 id="replacement">Experimenting with Shader Replacements</h3>
+<p>
+Shaders can be dumped and replaced on runtime for debugging purposes. This
+feature is not currently supported by SCons build.
+
+This is controlled via following environment variables:
+</p>
+<ul>
+<li><b>MESA_SHADER_DUMP_PATH</b> - path where shader sources are dumped
+<li><b>MESA_SHADER_READ_PATH</b> - path where replacement shaders are read
+</ul>
+Note, path set must exist before running for dumping or replacing to work.
+When both are set, these paths should be different so the dumped shaders do
+not clobber the replacement shaders. Also, the filenames of the replacement shaders
+should match the filenames of the corresponding dumped shaders.
+
+<h3 id="capture">Capturing Shaders</h3>
+
+<p>
+Setting <b>MESA_SHADER_CAPTURE_PATH</b> to a directory will cause the compiler
+to write <code>.shader_test</code> files for use with
+<a href="https://gitlab.freedesktop.org/mesa/shader-db">shader-db</a>, a tool
+which compiler developers can use to gather statistics about shaders
+(instructions, cycles, memory accesses, and so on).
+</p>
+<p>
+Notably, this captures linked GLSL shaders - with all stages together -
+as well as ARB programs.
+</p>
+
+<h2 id="support">GLSL Version</h2>
+
+<p>
+The GLSL compiler currently supports version 3.30 of the shading language.
+</p>
+
+<p>
+Several GLSL extensions are also supported:
+</p>
+<ul>
+<li>GL_ARB_draw_buffers
+<li>GL_ARB_fragment_coord_conventions
+<li>GL_ARB_shader_bit_encoding
</ul>
-<a name="unsup">
-<h2>Unsupported Features</h2>
+<h2 id="unsup">Unsupported Features</h2>
+
+<p>XXX update this section</p>
<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>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>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>The gl_Color and gl_SecondaryColor varying vars are interpolated
+ without perspective correction
</ul>
<p>
</p>
-<a name="impl">
-<h2>Implementation Notes</h2>
+<h2 id="notes">Implementation Notes</h2>
<ul>
<li>Shading language programs are compiled into low-level programs
<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>
+<h2 id="hints">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>
<pre>
float x = inversesqrt(y);
</pre>
+</li>
</ul>
-<a name="standalone">
-<h2>Stand-alone Compiler</h2>
+<h2 id="standalone">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.
+The stand-alone GLSL compiler program can be used to compile GLSL shaders
+into low-level GPU code.
</p>
<p>
This tool is useful for:
-<p>
+</p>
<ul>
<li>Inspecting GPU code to gain insight into compilation
<li>Generating initial GPU code for subsequent hand-tuning
</ul>
<p>
-(compiler build instructions TBD)
+After building Mesa, the compiler can be found at src/compiler/glsl/glsl_compiler
</p>
<p>
emit GL_ARB_vertex_program-style instructions:
</p>
<pre>
- glslcompiler --arb --linenumbers --vs vertshader.txt
+ src/compiler/glsl/glsl_compiler --version XXX --dump-ast myshader.vert
</pre>
+
+Options include
+<ul>
+<li><b>--dump-ast</b> - dump GPU code
+<li><b>--dump-hir</b> - dump high-level IR code
+<li><b>--dump-lir</b> - dump low-level IR code
+<li><b>--dump-builder</b> - dump GLSL IR code
+<li><b>--link</b> - link shaders
+<li><b>--just-log</b> - display only shader / linker info if exist,
+without any header or separator
+<li><b>--version</b> - [Mandatory] define the GLSL version to use
+</ul>
+
+
+<h2 id="implementation">Compiler Implementation</h2>
+
<p>
-The output may look similar to this:
+The source code for Mesa's shading language compiler is in the
+<code>src/compiler/glsl/</code> directory.
+</p>
+
+<p>
+XXX provide some info about the compiler....
</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.
+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>
+
+<h2 id="validation">Compiler Validation</h2>
+
+<p>
+Developers working on the GLSL compiler should test frequently to avoid
+regressions.
+</p>
+
+<p>
+The <a href="https://piglit.freedesktop.org/">Piglit</a> project
+has many GLSL tests.
+</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.
+The Mesa demos repository also has some good GLSL tests.
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