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Unions

Introduction

Introduce C++ Union data types into HLSL.

Motivation

Unions in C++ are used in a wide array of cases.

One common case is when the layout of data matches between two data types and the user wants to be able to access the data interchangeably as the two types. This is frequently used in C++ SIMD code in conjunction with anonymous data structures to implement vector objects. For example:

struct vector {
  union {
    struct {
      float x;
      float y;
      float z;
      float w;
    };
    struct {
      float r;
      float g;
      float b;
      float a;
    };
    float f[4];
  };
};

Additionally, unions are often used to reduce the storage requirements when a data structure may contain one of a exclusive set of objects. The example below is a common tagged union:

struct FloatOrBool {
  enum Format {
    Invalid,
    Float,
    Bool
  } F;
  union {
    float f;
    bool b;
  };
};

Proposed solution

Union data types are defined in *[class.union]*of the ISO C++ language specification. HLSL 202x introduces a compliant implementation with some HLSL specific clarifications.

  • Following C++ Unions are always sized large enough to contain their largest member type.
  • Union members cannot have HLSL semantics applied to them.
  • Union objects in buffer layouts are aligned as elements of their most-restrictive member type.
  • Union objects in struct layouts behave as defined in C++.
  • Union objects cannot have semantics applied to them.
  • Union's cannot have user-defined constructors or destructors until the language supports them for other user defined data types (See: 0007).

Acknowledgments

Special thanks to Dan Brown, and Meghana Thatishetti (Unknown GitHub ID), for their contributions to this implementation.