Index Tasks

Regent provides a number of optimizations to ensure that tasks execute in parallel and with maximum efficiency. One of the most important of these is index task optimization. An index task launch encapsulates a parallel loop and ensures that the runtime is able to amortize the cost of analysis of the contained tasks.

Optimizing Index Tasks

Loops are a common pattern in parallel codes. Regent optimizes loops to ensure that they execute efficiently on the Legion runtime. Recall that tasks execute in parallel only if they do not interfere. Legion must expend effort analyzing each task to determine non-interference with respect to surrounding tasks. Index tasks ensure that this analysis is amortized over a loop of task calls, increasing the efficiency of the runtime.

Regent generates index task launches on loops containing tasks calls. For example, the following code calls a task double_of in a loop, and accumulates the results into the variable total.

var total = 0
__demand(__parallel)
for i = 0, num_points do
  total += double_of(i, i + 10)
end

The line __demand(__parallel) marks the loop as an index launch. This annotation is not required, and is shown mostly for pedagogical purposes. Regent will optimize this loop regardless.

Generally speaking, annotations are recommended only as a means of defensive programming against bugs (in the application or compiler). The __parallel annotation guarantees that the compiler will produce an error if it is unable to optimize the loop in question. Most often, this happens because of a loop-carried dependence between the iterations of the loop. (This is in contrast to an OpenMP-style #pragma, which applies the optimization even if it is unsound.)

Final Code

import "regent"

local c = terralib.includec("stdio.h")

task double_of(i : int, x : int)
  c.printf("Hello world from task %d!\n", i)
  return 2*x
end

task main()
  var num_points = 4

  var total = 0
  __demand(__parallel)
  for i = 0, num_points do
    total += double_of(i, i + 10)
  end
  regentlib.assert(total == 92, "check failed")
end
regentlib.start(main)