[fpc-pascal] Why is Random(255) some 529x slower compared to Delphi 7?

[Graeme Geldenhuys]

Hi,

I'm busy working on some of the remaining unit tests for the tiOPF
project that doesn't run 100% to my satisfaction yet under FPC
(compared to Delphi 7). One of the tests is for a Simple Encryption
algorithm implemented in tiOPF, that runs extremely slow under FPC
2.4.x (and 2.6.0-rc), and near instant under Delphi 7. I'm trying to
figure out why.

Below is a snippet of code I tracked down which contains the slowdown.
The problem under FPC lies with line 08, and more specifically the
call to Random(255). Simply replacing the Random(255) call with a
variable speeds up the loop by some 529 times!

I did a simple GetTickCount() timing around this loop. Delphi executes
the loop in 20 ticks. FPC 2.6.0-rc2 takes 10585 ticks!!!! The outer
loop runs 200400 iterations. The types for BitValue, ByteValue and
RandSeed is of type Byte.

01  for Index := 1 to Length(Source) do
02  begin
03    OrdValue := Ord(Source[Index]);
04    for BitCount := 0 to 7 do
05    begin
06      BitValue := Byte(OrdValue and (1 shl BitCount) = 1 shl BitCount);
07      RandSeed := ByteValue;
08      ByteValue := (((Random(255) + 1) div 2) * 2) + BitValue;
09      Result[(Index - 1) * 8 + BitCount + 1] := AnsiChar(ByteValue);
10    end;
11  end;

I'm using 64bit Ubuntu Linux 10.04.3 with 64-bit FPC 2.6.0-rc2.
Anybody have any ideas why the code under FPC runs so slowly? Is it
maybe some type conversion problem in FPC? I noticed that Random()
returns a Int64 on my Linux system. I'm not sure what it returns under
Delphi 7 (the docs don't say, and I can see the implementation of it).


If you wanted to test this, I attached a fully working program code
(snippets from the original tiOPF code) to demonstrate the problem.


-- 
Regards,
  - Graeme -


_______________________________________________
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