Re: Ada and OpenMP

[Georg Bauhaus <rm.dash-bauhaus@futureapps.de>]

On 08.03.13 00:43, Georg Bauhaus wrote:
> "Peter C. Chapin" <PChapin@vtc.vsc.edu> wrote:
>> OpenMP is a different animal than Ada tasks. It provides fine grained
>> parallelism where, for example, it is possible to have the compiler
>> automatically parallelize a loop. In C:
>>
>> #pragma omp parallel for
>> for( i = 0; i < MAX; ++i ) {
>>    array[i]++;
>> }
>
> Fortunately, OpenMP is no longer needed to achieve automatic
> parallelism in either C or Ada at the low level. GCC's vectorizer
> produces code that runs in parallel for a number of loop patterns.
> These are documented, and they work in GNAT GPL or more
> recent FSF GNATs. Later 4.7s IIRC.

For example, adding -ftree-vectorize to the set of options (-O2 ...)
increases the speed of the program below by factors up to 3, depending
to some extent on the value of MAX. (Option -O3 is even easier in this
case, and yields improvements when MAX = 8.)

The assembly listing includes instructions like MOVDQA and PADDD used
with SSE registers.

GNAT will report successful optimizations when -fopt-info-optimized
is among the switches (or -ftree-vectorizer-verbose=2 for older GNATs).

package Fast is

    MAX : constant := 50;

    subtype Number is Integer;
    type Index is new Natural range 0 .. MAX;

    type Vect is array (Index) of Number;

    procedure Inc_Array (V : in out Vect);

end Fast;

package body Fast is

    procedure Inc_Array (V : in out Vect) is
    begin
       for K in Index loop
          V (K) := V (K) + 1;
       end loop;
    end Inc_Array;

end Fast;


with Ada.Real_Time;    use Ada.Real_Time;
with Ada.Text_IO;
with Fast;    use Fast;
procedure Test_Fast is
    Start, Finish : Time;
    Data : Vect;
    Result : Integer := 0;
    pragma Volatile (Result);
begin
    Start := Clock;
    for Run in 1 .. 500_000_000/MAX loop
       Inc_Array (Data);
       if Data (Index(MAX/2 + Run mod MAX/2)) rem 2 = 1 then
          Result := 1;
       end if;
    end loop;
    Finish := Clock;
    Ada.Text_IO.Put_Line
      (Duration'Image (To_Duration (Finish - Start)));
end Test_Fast;