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Computer programming language designed for digital image processing
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). Halide is implemented as an internal domain-specific language (DSL) in C++. Halide was announced by MIT in 2012[1] and released in 2013.[2]
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.[3] 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.[4] With Halide, changing the schedule does not require any changes to the algorithm, allowing the programmer to experiment with scheduling.[5][6]
Scheduled blur function[edit]The following function defines and sets the schedule for a 3×3 box filter defined as a series of two 3×1 passes, allowing the blur algorithm to remain independent of the execution schedule.[7]
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;
}
Uses and development[edit]
Halide was developed primarily at MIT's CSAIL lab. Both Google and Adobe have been involved in Halide research.[5] Google uses Halide in Pixel 2's Pixel Visual Core.[8][7] Adobe Photoshop also uses Halide.[9]
Ragan-Kelley notes that programmers can opt for "very painstaking, very unproductive, and very unsafe low-level code," which could introduce bugs, or "more safe, more productive, higher-level programming interfaces," that lack the ability to make fine adjustments in a compiler about how the program is run, and usually deliver lower performance. So, his team is trying to find a middle ground. "We're trying to figure out how to provide control for the key issues that human performance engineers want to be able to control," says Ragan-Kelley, "so, we're trying to build a new class of languages that we call user-schedulable languages that give safer and higher-level handles to control what the compiler does or control how the program is optimized."
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