GL_EXT_null_initializer.txt

Name

  EXT_null_initializer

Name Strings

  GL_EXT_null_initializer

Contact

  Alan Baker, Google (alanbaker 'at' google.com)

Contributors

  Alan Baker, Google
  Jeff Bolz, NVIDIA
  Jason Ekstrand, Intel
  John Kessenich, Google
  Daniel Koch, NVIDIA

Status

  Complete

Version

  Last Modified Date:      October 25, 2022
  Revision:                4

Depenendencies

  This extension can be applied to OpenGL GLSL versions 1.40
  (#version 140) and higher.

  This extension can be applied to OpenGL ES ESSL versions 3.10
  (#version 310) and higher.

  This extension is written against the OpenGL Shading Language
  Specification, version 4.60 (revision 7), dated July 10, 2019

  This extension requires GL_KHR_vulkan_glsl

Overview

  This extension adds a null initializer to GLSL and allows shared variables in
  compute shaders to be null initialized. Null initializers are specified using
  a new syntax of empty braces.

New Procedures and Functions

  None.

New Tokens

  None.

Modifications to GL_KHR_vulkan_glsl

  Add to the "Mapping to SPIR-V" section

  Mapping shared variable initializers:

    Null initializers should be generated as OpConstantNull of the variable's type.

Modifications to the OpenGL Shading Language Specification, Version 4.60

  Including the following line in a shader can be used to control the language
  features described in this extension:

    #extension GL_EXT_null_initializer : <behavior>

  where <behavior> is as specified in section 3.3.

  New preprocessor #defines are added to the OpenGL Shading Language:

    #define GL_EXT_null_initializer    1

  Add a new section 4.1.11.1 Null Initializers:

    A null initializer is defined for the following types:

    - bool: false
    - Scalar integers: 0
    - Scalar floating point: +0.0 (all bits 0)
    - Composites: each member is recursively defined to be null according to their
      constituent types

   It is a compile-time error to null initialize any other type, or to null initialize an
   unsized array type.
   
   Null initialization can be achieved by using an empty brace expression.

   For example,

    vec4 x = { };

  In section 4.3.8 Shared Variables:

  Replace the paragraph:

    Variables declared as shared may not have initializers and their contents are
    undefined at the beginning of shader execution. Any data written to shared
    variables will be visible to other work items (executing the same shader)
    within the same workgroup.

  With:

    Variables declared as shared may only have null initializers (see section 4.1.11.1).
    Uninitialized shared variables' contents are undefined at the beginning of shader
    execution. Any data written to shared variables will be visible to other work items
    (executing the same shader) within the same workgroup.

  In section 9 Shading Language Grammar:

  Add the following rule to "initializer: ...":

    initializer :
      LEFT_BRACE RIGHT_BRACE

Issues

  1. How should the null initializer be specified?

  Discussion: Common idioms include "{ }", "{ 0 }" and default constructors.
  "{ 0 }" is problematic when considering structs nested inside structs.

  RESOLVED. Only allow empty braces as a null initializer.

  2. To what extent should null initializers be allowed?

  Discussion: The tooling requirements to support null initializers is error
  prone if they are accepted as a general initializer. Additionally, they are
  useful beyond just shared variables.

  RESOLVED. Allow null initializers generally.

Revision History

  Revision 1 (Alan Baker)
    - Internal revision

  Revision 2 (Alan Baker)
    - Clarify null initializers

  Revision 3 (Alan Baker)
    - Rename extension from GL_EXT_null_initialize_shared_memory
    - Allow null initializers generally
    - Restrict null initializers to sized types

  Revision 4 (Alan Baker)
    - Editorial changes