Avoiding Array Element Initialization with noinit


Chapel 1.23 includes an early implementation of noinit for arrays. As of 1.23, noinit cannot be used with types other than arrays and it only works for types that are trivially copyable.


In some cases, one would like to allocate memory for array elements without initializing the elements. This might aid in optimization. The elements might be initialized later and then the default initialization might be redundant. Additionally, the default initialization might impact the memory affinity of the array on a NUMA system.


To use the noinit feature, declare an array and initialize it with the keyword noinit as this example shows:

config const n = 10;
var D = {1..n};
var A: [D] real = noinit;  // A's elements won't be initialized here

// Reading one of A's elements here is a memory error -
// it will read uninitialized memory.

// A's elements can be initialized using the assignment operator
forall a in A do
  a = 0.0;

Future Work

We are hoping to extend this feature to:

  • provide a way to indicate when the array is fully initialized (to support memory registration for communication, e.g.)

  • provide an explicit way to move-initialize elements (so that the feature can be extended to types that are not trivially copyable)

  • adjust the implementation to avoid initializing new elements when a noinit ed array is resized through its domain.