Ada Reference Manual (Ada 2022)

5.6.1 Parallel Block Statements

A parallel_block_statement comprises two or more sequence_of_statements separated by and where each represents an independent activity that is intended to proceed concurrently with the others.

Syntax

parallel_block_statement ::=
parallel [(chunk_specification)] [aspect_specification] do
sequence_of_statements
and
sequence_of_statements
{and
sequence_of_statements}
end do;

The chunk_specification, if any, of a parallel_block_statement shall be an integer_simple_expression.

Dynamic Semantics

For the execution of a parallel_block_statement, the chunk_specification and the aspect_specification, if any, are elaborated in an arbitrary order. After elaborating the chunk_specification, if any, a check is made that the determined maximum number of chunks is greater than zero. If this check fails, Program_Error is raised.

Then, the various sequence_of_statements are grouped into one or more chunks, each with its own logical thread of control (see Clause 9), up to the maximum number of chunks specified by the chunk_specification, if any. Within each chunk every sequence_of_statements of the chunk is executed in turn, in an arbitrary order. The parallel_block_statement is complete once every one of the sequence_of_statements has completed, either by reaching the end of its execution, or due to a transfer of control out of the construct by one of the sequence_of_statements (see 5.1).

Examples

Example of a parallel block used to walk a binary tree in parallel:

procedure Traverse (T : Expr_Ptr) is -- see 3.9.1
begin
if T /= null and then
T.all in Binary_Operation'Class -- see 3.9.1
then -- recurse down the binary tree
parallel do
Traverse (T.Left);
and
Traverse (T.Right);
and
Ada.Text_IO.Put_Line
("Processing " & Ada.Tags.Expanded_Name (T'Tag));
end do;
end if;
end Traverse;

Example of a parallel block used to search two halves of a string in parallel:

function Search (S : String; Char : Character) return Boolean is
begin
if S'Length <= 1000 then
-- Sequential scan
return (for some C of S => C = Char);
else
-- Parallel divide and conquer
declare
Mid : constant Positive := S'First + S'Length/2 - 1;
begin
parallel do
for C of S(S'First .. Mid) loop
if C = Char then
return True; -- Terminates enclosing do
end if;
end loop;
and
for C of S(Mid + 1 .. S'Last) loop
if C = Char then
return True; -- Terminates enclosing do
end if;
end loop;
end do;
-- Not found
return False;
end;
end if;
end Search;