- - -

Shading Language Support

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

-Last updated on 15 December 2008. -

Contents

GLSL 1.20 support +
Environment variables +
GLSL 1.40 support
Unsupported Features
Implementation Notes
Programming Hints @@ -32,40 +37,76 @@ Contents

Environment Variables

- -

GLSL 1.20 support

+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. +
nopt - disable compiler optimizations +
opt - force compiler optimizations +
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 +

+Example: export MESA_GLSL=dump,nopt +

-GLSL version 1.20 is supported in Mesa 7.3. -Among the features/differences of GLSL 1.20 are: +Shaders can be dumped and replaced on runtime for debugging purposes. Mesa +needs to be configured with '--with-sha1' to enable this functionality. This +feature is not currently supported by SCons build. + +This is controlled via following environment variables:

mat2x3, mat2x4, etc. types and functions -
transpose(), outerProduct(), matrixCompMult() functions -(but untested) -
precision qualifiers (lowp, mediump, highp) -
invariant qualifier -
array.length() method -
float[5] a; array syntax -
centroid qualifier -
unsized array constructors -
initializers for uniforms -
const initializers calling built-in functions +
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. +

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

+ +

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:

Linking of multiple shaders is not supported -
gl_ClipVertex +
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

@@ -75,8 +116,7 @@ All other major features of the shading language should function.

- -

Implementation Notes

Shading language programs are compiled into low-level programs @@ -99,43 +139,9 @@ These issues will be addressed/resolved in the future.
-
-
Programming Hints
+
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:
```
@@ -145,28 +151,20 @@ These issues will be addressed/resolved in the future.
          float x = inversesqrt(y);
 
-
-   Use ++i when possible as it's more efficient than i++
 
 
```
-
-
Stand-alone GLSL Compiler
- -
-A unique stand-alone GLSL compiler driver has been added to Mesa. -
+
Stand-alone GLSL Compiler

-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 @@ -174,97 +172,36 @@ This tool is useful for:
-After building Mesa, the glslcompiler can be built by manually running: +After building Mesa, the compiler can be found at src/glsl/glsl_compiler
-
```
-    cd src/mesa/drivers/glslcompiler
-    make
-
```
-
Here's an example of using the compiler to compile a vertex shader and emit GL_ARB_vertex_program-style instructions:
```
-    bin/glslcompiler --debug --numbers --fs progs/glsl/CH06-brick.frag.txt
+    src/glsl/glsl_compiler --dump-ast myshader.vert
 
```
-
-results in: -
-
```
-# Fragment Program/Shader
-  0: RCP TEMP[4].x, UNIFORM[2].xxxx;
-  1: RCP TEMP[4].y, UNIFORM[2].yyyy;
-  2: MUL TEMP[3].xy, VARYING[0], TEMP[4];
-  3: MOV TEMP[1], TEMP[3];
-  4: MUL TEMP[0].w, TEMP[1].yyyy, CONST[4].xxxx;
-  5: FRC TEMP[1].z, TEMP[0].wwww;
-  6: SGT.C TEMP[0].w, TEMP[1].zzzz, CONST[4].xxxx;
-  7: IF (NE.wwww); # (if false, goto 9);
-  8:    ADD TEMP[1].x, TEMP[1].xxxx, CONST[4].xxxx;
-  9: ENDIF;
- 10: FRC TEMP[1].xy, TEMP[1];
- 11: SGT TEMP[2].xy, UNIFORM[3], TEMP[1];
- 12: MUL TEMP[1].z, TEMP[2].xxxx, TEMP[2].yyyy;
- 13: LRP TEMP[0], TEMP[1].zzzz, UNIFORM[0], UNIFORM[1];
- 14: MUL TEMP[0].xyz, TEMP[0], VARYING[1].xxxx;
- 15: MOV OUTPUT[0].xyz, TEMP[0];
- 16: MOV OUTPUT[0].w, CONST[4].yyyy;
- 17: 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. -
-
-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. -
+Options include +
- --dump-ast - dump GPU code +
- --dump-hir - dump high-level IR code +
- --dump-lir - dump low-level IR code +
- --link - ??? +
-
-
Compiler Implementation
+
Compiler Implementation

The source code for Mesa's shading language compiler is in the -src/mesa/shader/slang/ directory. +src/glsl/ directory.

-The compiler follows a fairly standard design and basically works as follows: +XXX provide some info about the compiler....
-
- 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. -
- 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. -
- 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. -
- 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. -
- The emitted GPU instructions (see prog_instruction.h) are stored in a -gl_program object (see mtypes.h). -
- 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). -
The final vertex and fragment programs may be interpreted in software @@ -291,51 +228,48 @@ struct gl_shader_state }; -
- EmitHighLevelInstructions -
  +
  +
  EmitHighLevelInstructions
  +
  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. -
- EmitCondCodes -
  +
  EmitCondCodes
  +
  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. -
- EmitComments -
  -If set, instructions will be annoted with comments to help with debugging. +
  EmitComments
  +
  +If set, instructions will be annotated with comments to help with debugging. Extra NOP instructions will also be inserted. -
  - -
+ + -
-
Compiler Validation
+
Compiler Validation

-A Glean test has -been create to exercise the GLSL compiler. -
-
-The glsl1 test runs over 170 sub-tests to check that the language -features and built-in functions work properly. -This test should be run frequently while working on the compiler to catch +Developers working on the GLSL compiler should test frequently to avoid regressions.
+
-The test coverage is reasonably broad and complete but additional tests -should be added. +The Piglit project +has many GLSL tests.
+
+The Mesa demos repository also has some good GLSL tests. +
- - +

The Mesa 3D Graphics Library

Shading Language Support

Shading Language Support

Environment Variables

GLSL 1.20 support

GLSL Version

Unsupported Features

Unsupported Features

Implementation Notes

Implementation Notes

Programming Hints

Programming Hints

Stand-alone GLSL Compiler

Stand-alone GLSL Compiler

Compiler Implementation

Compiler Implementation

Compiler Validation

Compiler Validation