Shading Language Support

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

Contents

• Environment variables
• GLSL 1.40 support
• Unsupported Features
• Implementation Notes
• Programming Hints
• 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.
• 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

The GLSL compiler currently supports version 1.40 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 fully supported in Mesa:

• 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

All other major features of the shading language should function.

Implementation Notes

• Shading language programs are compiled into low-level programs very similar to those of GL_ARB_vertex/fragment_program.
• All vector types (vec2, vec3, vec4, bvec2, etc) currently occupy full float[4] registers.
• Float constants and variables are packed so that up to four floats can occupy one program parameter/register.
• All function calls are inlined.
• Shaders which use too many registers will not compile.
• 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.
• The ftransform() function doesn't necessarily match the results of fixed-function transformation.

These issues will be addressed/resolved in the future.

Programming Hints

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

          float x = 1.0 / sqrt(y);
  

  Write this:

          float x = inversesqrt(y);
  

Stand-alone GLSL Compiler

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
• Debugging the GLSL compiler itself

After building Mesa, the compiler can be found at src/glsl/glsl_compiler

Here's an example of using the compiler to compile a vertex shader and emit GL_ARB_vertex_program-style instructions:

    src/glsl/glsl_compiler --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
• --link - ???

Compiler Implementation

The source code for Mesa's shading language compiler is in the src/glsl/ directory.

XXX provide some info about the compiler....

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).

Code Generation Options

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:

struct gl_shader_state
{
   ...
   /** Driver-selectable options: */
   GLboolean EmitHighLevelInstructions;
   GLboolean EmitCondCodes;
   GLboolean EmitComments;
};

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

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. Extra NOP instructions will also be inserted.

Compiler Validation

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

The Piglit project has many GLSL tests.

The Mesa demos repository also has some good GLSL tests.