[fpc-pascal] for loop vs while loop

Mark Morgan Lloyd markMLl.fpc-pascal at telemetry.co.uk
Thu Feb 2 10:01:52 CET 2017


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 = $7fffffff;

var     a, i: longint;

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

real    0m4.353s
user    0m3.284s
sys     0m0.000s


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

real    0m4.449s
user    0m3.256s
sys     0m0.000s


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

real    0m4.671s
user    0m3.264s
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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