Re: [Caml-list] Comparing floats

["Mr. Herr" <misterherr@freenet.de>]

On 23.07.2015 11:01, Xavier Leroy wrote:
> On 23/07/2015 10:35, Sébastien Hinderer wrote:
>> What's the most efficient way to compare floats, please?
>> Is it the polymorphic compare function, or is there a more specialized
>> version of it?
> You'll get good performance by type-specializing Pervasives.compare:
>
> let compare_float (x: float) (y: float) = compare x y
>
> If you're absolutely sure your floats are not NaN, you can shave a few
> CPU cycles:
>
> let compare_float (x: float) (y: float) =
>    if x < y then -1 else if x > y then 1 else 0
>
The assembler code says compare_float is directly compiled to a function that 
compares the 2 values
in xmm0 and xmm1 registers, while Pervasives.compare is a library function written in 
C doing the same thing.

The assembler code looks very very good.

But I doubt that you could measure a difference. The type system will always
yield the float_compare function, doesn't it? So far my quickcheck...

--- Assembler code of your suggested function:
camlTest_comparefl3__compare_float_1008:
     .cfi_startproc
.L102:
     movsd    (%rbx), %xmm0
     movsd    (%rax), %xmm1
     comisd    %xmm1, %xmm0
     jbe    .L101
     movq    $-1, %rax
     ret
     .align    4
.L101:
     comisd    %xmm0, %xmm1
     jbe    .L100
     movq    $3, %rax
     ret
     .align    4
.L100:
     movq    $1, %rax
     ret
     .cfi_endproc

--- C code of library function:

CAMLprim value caml_float_compare(value vf, value vg)
{
   double f = Double_val(vf);
   double g = Double_val(vg);
   if (f == g) return Val_int(0);
   if (f < g) return Val_int(-1);
   if (f > g) return Val_int(1);
   /* One or both of f and g is NaN.  Order according to the
      convention NaN = NaN and NaN < x for all other floats x. */
   if (f == f) return Val_int(1);  /* f is not NaN, g is NaN */
   if (g == g) return Val_int(-1); /* g is not NaN, f is NaN */
   return Val_int(0);              /* both f and g are NaN */
}