Halide (programming language)

Halide is a computer programming language designed for writing digital image processing code that takes advantage of memory locality, vectorized computation and multi-core central processing units (CPU) and graphics processing units (GPU).^[1] Halide is implemented as an internal domain-specific language (DSL) in C++.

Halide

Paradigms	functional, parallel
Designed by	Jonathan Ragan-Kelley Andrew Adams
Developer	MIT, (with help from Stanford, Google, Adobe)
First appeared	2012; 13 years ago (2012)
Typing discipline	static
Implementation language	C++
OS	macOS, mainstream Linux distributions, Windows
License	MIT License
Website	halide-lang.org

Language

The main innovation Halide brings is the separation of the algorithm being implemented from its execution schedule, i.e. code specifying the loop nesting, parallelization, loop unrolling and vector instruction. These two are usually interleaved together and experimenting with changing the schedule requires the programmer to rewrite large portions of the algorithm with every change. With Halide, changing the schedule does not require any changes to the algorithm and this allows the programmer to experiment with scheduling and finding the most efficient one.

Sample source code

The following function defines and sets the schedule for a 3×3 box filter defined as a series of two 3×1 passes:

Func blur_3x3(Func input) {
  Func blur_x, blur_y;
  Var x, y, xi, yi;

  // The algorithm - no storage or order
  blur_x(x, y) = (input(x-1, y) + input(x, y) + input(x+1, y))/3;
  blur_y(x, y) = (blur_x(x, y-1) + blur_x(x, y) + blur_x(x, y+1))/3;

  // The schedule - defines order, locality; implies storage
  blur_y.tile(x, y, xi, yi, 256, 32)
        .vectorize(xi, 8).parallel(y);
  blur_x.compute_at(blur_y, x).vectorize(x, 8);

  return blur_y;
}

Use

Google used Halide and TensorFlow for its Pixel 2 Pixel Visual Core.^[2] Adobe Photoshop also uses Halide.^[3] Both Google and Adobe have been involved in Halide research.^[4]

See also

• Cuneiform (programming language)
• Algorithmic skeleton
• Parallel programming model