- - -

Shading Language Support

Shading Language

This page describes the features and status of Mesa's support for the - + OpenGL Shading Language.

-Last updated on 17 Feb 2007. -

Contents

Environment variables +
GLSL 1.40 support
Unsupported Features -
Implementation Notes +
Implementation Notes
Programming Hints -
Stand-alone Compiler +
Stand-alone GLSL Compiler +
Compiler Implementation +
Compiler Validation +

+ + +

Environment Variables

+ +

+The MESA_GLSL 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. +

dump - print GLSL shader code to stdout at link time +
log - log all GLSL shaders to files. + The filenames will be "shader_X.vert" or "shader_X.frag" where X + the shader ID. +
cache_info - print debug information about shader cache +
cache_fb - force cached shaders to be ignored and do a full + recompile via the fallback path
uniform - print message to stdout when glUniform is called +
nopvert - 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. +
nopfrag - force fragment shader to be a simple shader that passes + through the color attribute. +
useprog - log glUseProgram calls to stderr +
errors - GLSL compilation and link errors will be reported to stderr. +

+Example: export MESA_GLSL=dump,nopt +

+ +

Experimenting with Shader Replacements

+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: +

MESA_SHADER_DUMP_PATH - path where shader sources are dumped +
MESA_SHADER_READ_PATH - path where replacement shaders are read +

+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. + +

Capturing Shaders

+ +

+Setting MESA_SHADER_CAPTURE_PATH to a directory will cause the compiler +to write .shader_test files for use with +shader-db, a tool +which compiler developers can use to gather statistics about shaders +(instructions, cycles, memory accesses, and so on). +

+Notably, this captures linked GLSL shaders - with all stages together - +as well as ARB programs. +

+ +

GLSL Version

+ +

+The GLSL compiler currently supports version 3.30 of the shading language. +

+ +

+Several GLSL extensions are also supported: +

GL_ARB_draw_buffers +
GL_ARB_fragment_coord_conventions +
GL_ARB_shader_bit_encoding

- -

Unsupported Features

+ +

XXX update this section

-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:

Arrays -
Structs -
Linking of multiple shaders is not supported -
Not all built-in OpenGL state variables are supported yet. - Common variables such as gl_ModelViewMatrix and gl_NormalMatrix - are supported. -
Integer operations are not fully implemented (most are implemented - as floating point). +
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. +
The gl_Color and gl_SecondaryColor varying vars are interpolated + without perspective correction

@@ -53,8 +133,7 @@ All other major features of the shading language should function.

- -

Implementation Notes

Shading language programs are compiled into low-level programs @@ -68,7 +147,8 @@ All other major features of the shading language should function.
The quality of generated code is pretty good, register usage is fair.
Shader error detection and reporting of errors (InfoLog) is not very good yet. -
There are massive memory leaks in the compiler. +
The ftransform() function doesn't necessarily match the results of + fixed-function transformation.

@@ -76,43 +156,9 @@ These issues will be addressed/resolved in the future.

- -

Programming Hints

Declare in function parameters as const whenever possible. - This improves the efficiency of function inlining. -

To reduce register usage, declare variables within smaller scopes. - For example, the following code: -

-    void main()
-    {
-       vec4 a1, a2, b1, b2;
-       gl_Position = expression using a1, a2.
-       gl_Color = expression using b1, b2;
-    }
-

- Can be rewritten as follows to use half as many registers: -

-    void main()
-    {
-       {
-          vec4 a1, a2;
-          gl_Position = expression using a1, a2.
-       }
-       {
-          vec4 b1, b2;
-          gl_Color = expression using b1, b2;
-       }
-    }
-

- Alternately, rather than using several float variables, use - a vec4 instead. Use swizzling and writemasks to access the - components of the vec4 as floats. -

Use the built-in library functions whenever possible. For example, instead of writing this:

@@ -122,25 +168,20 @@ These issues will be addressed/resolved in the future.
          float x = inversesqrt(y);
 
+

- -

Stand-alone Compiler

Stand-alone GLSL Compiler

-A unique stand-alone GLSL compiler driver has been added to Mesa. -

- -

-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.

This tool is useful for: -

Inspecting GPU code to gain insight into compilation
Generating initial GPU code for subsequent hand-tuning @@ -148,7 +189,7 @@ This tool is useful for:

-(compiler build instructions TBD) +After building Mesa, the compiler can be found at src/compiler/glsl/glsl_compiler

@@ -156,33 +197,55 @@ Here's an example of using the compiler to compile a vertex shader and emit GL_ARB_vertex_program-style instructions:

-    glslcompiler --arb --linenumbers --vs vertshader.txt
+    src/compiler/glsl/glsl_compiler --version XXX --dump-ast myshader.vert

+ +Options include +

--dump-ast - dump GPU code +
--dump-hir - dump high-level IR code +
--dump-lir - dump low-level IR code +
--dump-builder - dump GLSL IR code +
--link - link shaders +
--just-log - display only shader / linker info if exist, +without any header or separator +
--version - [Mandatory] define the GLSL version to use +

+ + +

Compiler Implementation

-The output may look similar to this: +The source code for Mesa's shading language compiler is in the +src/compiler/glsl/ directory. +

+ +

+XXX provide some info about the compiler....

-!!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
-

-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).

+ +

Compiler Validation

+ +

+Developers working on the GLSL compiler should test frequently to avoid +regressions. +

+ +

+The Piglit project +has many GLSL tests. +

-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.

- - +