Reduce Intents¶

Overview¶

Reduce intents are a kind of forall intent - see Section 26.3 "Forall Intents" of the Chapel Language Specification.

As with any forall intent, a reduce intent can be specified on any outer variable - that is, a variable used within the body of a forall loop and declared outside that loop. References to such a variable within the loop implicitly refer to the corresponding formal argument of the task function created by the parallel iterator. If/when the parallel iterator executes a yield outside any parallel constructs, the reference is implicitly to the corresponding formal argument of the parallel iterator itself. In both cases, these formals are added implicitly by the compiler.

Reduce intents are distinct:

• The references, within the loop body, to a reduce-intented outer variable implicitly refer to the reduction state corresponding to the task function's formal. At the beginning of the task, this reduction state is initialized to the identity value for the corresponding reduction.
• The value of the outer variable immediately after the forall loop is a reduction of the values of the corresponding formals at the end of their tasks and the value of the outer variable immediately before the forall/coforall loop.

Reduce intents are currently available for forall and coforall statements.

Syntax¶

The syntax of task-intent-list is extended to allow reduce-intent:

task-intent-list:
  // no change with these
  formal-intent identifier
  formal-intent identifier, task-intent-list
  // added for reduce intents:
  reduce-intent
  reduce-intent, task-intent-list

reduce-intent:
  reduce-operator 'reduce' identifier
  reduce-class    'reduce' identifier
  reduce-expr     'reduce' identifier

reduce-operator: one of
   // these have the same meaning as in a reduction expression
   +  *  &&  ||  &  |  ^  min  max

reduce-class:
   // the name of the class that implements a user-defined reduction
   identifier

reduce-expr:
   // an expression producing an instance of a user-defined reduction class
   expr

Examples¶

Increment x in the loop -- counts the number of iterations:

var x = 0;
forall myIterator() with (+ reduce x) {
  x += 1;
}
writeln("The number of loop iterations is: ", x);

Set x in the loop -- counts the number of tasks:

var x = 0;
forall myIterator() with (+ reduce x) {
  x = 1;
}
writeln("The number of tasks is: ", x);

For a user-defined reduction, there is a task-private instance of the reduction class for each task created for the forall or coforall loop. Here is an example of such a class:

/* Implements + reduction over numeric data. */
class PlusReduceOp: ReduceScanOp {

  /* the type of the elements to be reduced */
  type eltType;

  /* task-private accumulator/reduction state */
  var value: eltType;

  /* identity w.r.t. the reduction operation */
  proc identity         return 0: eltType;

  /* accumulate a single element onto the accumulator */
  proc accumulate(elm)  { value = value + elm; }

  /* accumulate a single element onto the state */
  proc accumulateOntoState(ref state, elm)  { state = state + elm; }

  /* accumulate the value of the outer variable at the entry to the loop */
  // Note: this method is optional. If it is not provided,
  // accumulate(outerVar) is used instead.
  proc initialAccumulate(outerVar) { value = value + outerVar: eltType; }

  // Note: 'this' can be accessed by multiple calls to combine()
  // concurrently. The Chapel implementation serializes such calls
  // with a lock on 'this'.
  // 'other' will not be accessed concurrently.
  /* combine the accumulations in 'this' and 'other' */
  proc combine(other)   { value = value + other.value; }

  /* Convert the accumulation into the value of the reduction
     that is reported to the user. This is trivial in our case. */
  proc generate()       return value;

  /* produce a new instance of this class */
  proc clone()          return new PlusReduceOp(eltType=eltType);
}

// Use the above class.
var A = [1000, 200, 30, 4];
var sum: int;
forall elm in A with (PlusReduceOp reduce sum) {
  sum += elm;  // equivalently:  sum reduce= elm;
}
writeln(sum);

// To have different input/accumulator/result types of the reduction,
// specify the input type explicitly, e.g. PlusReduceOp(int) below:
var A = [false, false, true, false, true];
var sum: real;
forall elm in A with (PlusReduceOp(int) reduce sum) {
  sum reduce= elm;   // bools are implicitly coerced to 'int' input type
  writeln(sum);      // accumulation state: int
}
writeln(sum);        // result: real

Future Work¶

• Implement reduce intents for cobegin statements.

• Provide the other predefined reduction operators as reduce intents:

  minloc maxloc
  

• We are working on a new interface for user-defined reductions, addressing the need for user-defined synchronization choices and the ability to provide reduction state without the overhead of synchronization support for partial reductions.

• We are also considering replacing classes with records for user-defined reductions. The goal is to eliminate the required malloc+free, which is possible because the lifetime of a reduction class instance matches the forall or coforall statement.