Re: Timing code blocks

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On Thu, 4 Mar 2010 22:34:47 -0800 (PST), deadlyhead wrote:

> I've been trying to determine if there is any significant performance
> difference between two functionally equivalent pieces of code. I've
> done the standard thing and, using the Ada.Real_Time package, I'm
> saving the time when the code starts, then the time when the code
> ends, then examining the difference between them after the code runs.
> 
> The problem I'm having, though, is that the timing just isn't
> happening.  This code will run for 15 seconds and when I examine the
> time span, it tells me that no time passed.
> 
> Here's my actual code

This is an ancient bug, which managed to survive a number of GNAT compiler
versions.

As a workaround, add delay 0.0 at the beginning of your program:

> ----------------------------------------------------------------------
> 
> --  A test of the cost of conditionals
> 
> with Ada.Text_IO;  use Ada.Text_IO;
> with Ada.Real_Time; use Ada.Real_Time;
> procedure Conditional_Test is
> 
>    Test_Dur : constant := 100_000;

Test_Dur : constant := 10_000; -- 100_000 overflows on a 32-bit machine

>    -- We require the input/output for Duration
>    package Duration_IO is new Fixed_IO (Duration);
>    use Duration_IO;
> 
> 
>    Start_Time : Time;
>    End_Time   : Time;
>    Is_Even    : Boolean;
>    Is_Odd     : Boolean;
>    Junk       : Positive;
>    Junk_In    : Natural := 0;
> 
> begin   -- Conditional_test

delay 0.0; -- Wake up that dozing Ada RTL!

>    Put_Line ("---Starting per-branch assignment test---");
> 
>    Start_Time := Clock;
>    for I in 1 .. Test_Dur loop
>       if I rem 2 = 1 then
>          Is_Odd  := True;
>       else
>          Is_Even := True;
>       end if;
> 
>       if Is_Even then
>          for J in reverse 1 .. Test_Dur loop
>             Junk_In := Junk_In + 1;
>          end loop;
>          Junk := I;
>       elsif Is_Odd then
>          for J in reverse 1 .. Test_Dur loop
>             Junk_In := Junk_In + 1;
>          end loop;
>          Junk := I;
>       end if;
> 
>       Is_Even := False;
>       Is_Odd  := False;
>    end loop;
>    End_Time := Clock;
> 
>    Put ("Assignment within each branch took ");
>    Put (To_Duration (End_Time - Start_Time), 1, 12, 0);
>    New_Line (2);
> 
> 
>    Put_Line ("---Starting combined-branch assignment test---");
> 
>    Start_Time := Clock;
>    for I in 1 .. Test_Dur loop
>       if I rem 2 = 1 then
>          Is_Odd  := True;
>       else
>          Is_Even := True;
>       end if;
> 
>       if Is_Even then
>          for J in reverse 1 .. Test_Dur loop
>             Junk_In := Junk_In + 1;
>          end loop;
>       elsif Is_Odd then
>          for J in reverse 1 .. Test_Dur loop
>             Junk_In := Junk_In + 1;
>          end loop;
>       end if;
> 
>       if Is_Even or Is_Odd then
>          Junk := I;
>       end if;
> 
>       Is_Even := False;
>       Is_Odd  := False;
>    end loop;
>    End_Time := Clock;
> 
>    Put ("Assignment outside of the branching took ");
>    Put (To_Duration (End_Time - Start_Time), 1, 12, 0);
>    New_Line (2);
> 
> end Conditional_Test;

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de