Re: [fpc-pascal] for loop vs while loop

2017-02-02 Thread Lars 
On Wed, February 1, 2017 4:49 am, Graeme Geldenhuys wrote:
> Hi,
>
>
> Just curious (been in a discussion with somebody else). Is there a
> performance difference between a FOR loop and a WHILE loop?
>
> Not sure if it will make a difference, but the usage is with three
> nested loops, iterating over some 300,000 plus data points in total.
>
>
> Regards,
> Graeme
>

Don't forget Repeat Until...

Simply look at the assembler code generated (a big sin)

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Re: [fpc-pascal] for loop vs while loop

2017-02-02 Thread DaWorm 
On Thu, Feb 2, 2017 at 4:01 AM, Mark Morgan Lloyd <
markmll.fpc-pas...@telemetry.co.uk> wrote:

>
> It's interesting that the real (wallclock) and user times are consistently
> in a different sequence. /If/ the user time is to be believed, there's a
> very small advantage to counting down even if the sequence you want is
> ascending (i.e. test3) rather than simply counting up.


Not looking at the assembly either, but I'd assume it's because you can do
a simple test for zero in the count down case, and have to do a compare
against a value and then test the result on count up, which probably takes
an extra instruction or two.  The optimization can sometimes convert
everything to count down as part of the setup of the loop (I know in Delphi
you'd sometimes see loops run the other way in the debugger when there was
no behavioral change in function).  I guess since in your examples the
final contents of the a variable after the loop exits depends on direction,
then it couldn't do this for the count up loops.  Maybe try again with
Inc(a) in the body?

Jeff.
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Re: [fpc-pascal] for loop vs while loop

2017-02-02 Thread Mark Morgan Lloyd 

On 01/02/17 20:30, Nitorami wrote:

Just a note - I learned that for very short "for" loops it may be even a bit
faster to count downwards (to zero) rather than upwards if possible, because
the comparison to zero is very effiicient. Not sure though whether that
still makes a difference on modern processors.


Testing that on fpc 3.0.0 x86, compiled with -O4 but without looking at 
the generated code:



program test1;

const   top = $7fff;

var a, i: longint;

begin
  for i := 0 to top do
a := i;
  writeln(a)
end.

real0m4.353s
user0m3.284s
sys 0m0.000s


program test2;
  for i := top downto 0 do
a := i;

real0m4.449s
user0m3.256s
sys 0m0.000s


program test3;
  for i := top downto 0 do
a := top - i;

real0m4.671s
user0m3.264s
sys 0m0.000s


It's interesting that the real (wallclock) and user times are 
consistently in a different sequence. /If/ the user time is to be 
believed, there's a very small advantage to counting down even if the 
sequence you want is ascending (i.e. test3) rather than simply counting up.


--
Mark Morgan Lloyd
markMLl .AT. telemetry.co .DOT. uk

[Opinions above are the author's, not those of his employers or colleagues]
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Re: [fpc-pascal] for loop vs while loop

2017-02-01 Thread Nitorami 
Just a note - I learned that for very short "for" loops it may be even a bit
faster to count downwards (to zero) rather than upwards if possible, because
the comparison to zero is very effiicient. Not sure though whether that
still makes a difference on modern processors.



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Re: [fpc-pascal] for loop vs while loop

2017-02-01 Thread Jürgen Hestermann 

Am 2017-02-01 um 12:49 schrieb Graeme Geldenhuys:
> Just curious (been in a discussion with somebody else). Is there a
> performance difference between a FOR loop and a WHILE loop?
> Not sure if it will make a difference, but the usage is with three
> nested loops, iterating over some 300,000 plus data points in total.

I think that in general a FOR loop *can* be faster than a WHILE loop
because the iterator variable will be held in a register (if possible)
and its start and end values will be calculated only once.

Also, because the iterator variable will be incremented (or decremented)
by a constant value it helps the compiler to speed it up further.

If the major time of the loop is needed for the loop body it will not make much 
difference of course.

Still it makes sense to always use a FOR loop if possible.

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Re: [fpc-pascal] for loop vs while loop

2017-02-01 Thread Graeme Geldenhuys 
On 2017-02-01 12:12, Mattias Gaertner wrote:
> Yes, for-loop calculates the end value only once.

Ah, thanks for that. Seems so obvious now. :)

Regards,
  Graeme

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Re: [fpc-pascal] for loop vs while loop

2017-02-01 Thread Mattias Gaertner 
On Wed, 1 Feb 2017 11:49:34 +
Graeme Geldenhuys  wrote:

> Hi,
> 
> Just curious (been in a discussion with somebody else). Is there a
> performance difference between a FOR loop and a WHILE loop?

Yes, for-loop calculates the end value only once.

 
> Not sure if it will make a difference, but the usage is with three
> nested loops, iterating over some 300,000 plus data points in total.

Mattias
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