[fpc-devel] CompareMem slower in FPC 2.4.4

[Sven Barth]

On 01.06.2011 22:07, Michalis Kamburelis wrote:
> Any thoughts? Maybe something can be improved?
>
> 1. Why CompareMem got slower in FPC 2.4.4? Maybe something can be fixed?
>

Let's see...

CompareMem in 2.4.2:

=== source begin ===

function CompareMem(P1, P2: Pointer; Length: cardinal): Boolean;
var
   i: cardinal;
begin
   Result:=True;
   I:=0;
   If (P1)<>(P2) then
     While Result and (i<Length) do
       begin
       Result:=PByte(P1)^=PByte(P2)^;
       Inc(I);
       Inc(pchar(P1));
       Inc(pchar(P2));
       end;
end;

=== source end ===

CompareMem in 2.4.4:

=== source begin ===

function CompareMem(P1, P2: Pointer; Length: cardinal): Boolean;
begin
   Result:=CompareByte(P1^,P2^,Length)=0;
end;

=== source end ===

This at least explains why CompareMem behaves similar to CompareByte in 
your test :D


> 2. The simple comparison "(V1[0] = V2[0]) and..." is much faster than
> any CompareXxx. Any chance of improving it? In this case, size is known
> at compile time, so maybe CompareXxx could be "magic" and (for
> reasonably small sizes) the compiler could just generate a proper code
> to compare them just like "=" operator? Just an idea of course, I don't
> know how easy it would be to actually implement.

The CompareMem function is in unit SysUtils and I don't think it is a 
good idea to "magicalize" that unit.

Maybe it's better if you or someone else would try to improve the 
performance of the i386 assembler code that makes up the 
Compare(Byte/Word/DWord) functions (it's located in rtl/i386/i386.inc btw).

Regards,
Sven