[fpc-pascal] fpc 32 / 64 bit / c++/ floating point question

[Helmut Hartl]

While stabilizing my bullet physics port i saw some differences in
floating point behaviour between c++ and fpc32 and fpc 64 bit.

In question is a simple dot product.

The function :
result := a*d+b*e+c*f;

Gives "different" results( 6.3846106530 vs 6.3846111610 ) depending
on the usage of intermediate variables for the final result and the fpc 
version
32 bit / 64 bit, while the C++ version does not show this behaviour and 
always
gives the same result. (XCode 3.2.1/ GCC on OSX)

I only show the (extracted) compilable fpc exampe:

program fpc_floatmul;
var a,b,c,d,e,f      : single;
     i,j,k                   : single;
     result_same_as_c : single;
begin
   a:= 1;
   b:= -5.65984446;
   c:= -3.4953573;
   d:= 8.48976051;
   e:= 1.5;
   f:= -1.82659933;
   i := a * d;
   j := b * e;
   k := c * f;
   result_same_as_c := i + j + k;
   writeln(result_same_as_c:10:10);
   writeln(a*d+b*e+c*f:10:10); // different result
end.

Running the above program gives
1) C++ / FPC 64 Bit
  6.3846106530
  6.3846106530
2) FPC32 Bit
  6.3846106530
  6.3846111610

Is this explainable or wrong behaviour ?

thanks,
   helmut