Shading Language Support

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

Last updated on 20 Jan 2007.

Unsupported Features

The following features of the shading language are not yet 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).

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.
• There are massive memory leaks in the compiler.

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:

          float x = 1.0 / sqrt(y);
  

  Write this:

          float x = inversesqrt(y);