Add OpenMP overload with stepping (#10891)

lib/system.nim

@@ -2681,6 +2681,24 @@ iterator `||`*[S, T](a: S, b: T, annotation: static string = "parallel for"): T
   ## and GC.
   discard
 
+iterator `||`*[S, T](a: S, b: T, step: Positive, annotation: static string = "parallel for"): T {.
+  inline, magic: "OmpParFor", sideEffect.} =
+  ## OpenMP parallel loop iterator with stepping.
+  ## Same as `countup` but the loop may run in parallel.
+  ##
+  ## `annotation` is an additional annotation for the code generator to use.
+  ## The default annotation is `parallel for`.
+  ## Please refer to the `OpenMP Syntax Reference
+  ## <https://www.openmp.org/wp-content/uploads/OpenMP-4.5-1115-CPP-web.pdf>`_
+  ## for further information.
+  ##
+  ## Note that the compiler maps that to
+  ## the ``#pragma omp parallel for`` construct of `OpenMP`:idx: and as
+  ## such isn't aware of the parallelism in your code! Be careful! Later
+  ## versions of ``||`` will get proper support by Nim's code generator
+  ## and GC.
+  discard
+
 {.push stackTrace:off.}
 proc min*(x, y: int): int {.magic: "MinI", noSideEffect.} =
   if x <= y: x else: y