Float to int (more intel asm)

References : Posted by Laurent de Soras (via flipcode)

Notes :
[Found this on flipcode, seemed worth posting here too, hopefully Laurent will approve :) -- Ross]

Here is the code I often use. It is not a _ftol replacement, but it provides useful functions. The current processor rounding mode should be "to nearest" ; it is the default setting for most of the compilers.

The [fadd st, st (0) / sar i,1] trick fixes the "round to nearest even number" behaviour. Thus, round_int (N+0.5) always returns N+1 and floor_int function is appropriate to convert floating point numbers into fixed point numbers.

---------------------

Laurent de Soras
Audio DSP engineer & software designer
Ohm Force - Digital audio software
http://www.ohmforce.com

Code :
inline int round_int (double x)
{
   int      i;
   static const float round_to_nearest = 0.5f;
   __asm
   {
      fld      x
      fadd     st, st (0)
      fadd     round_to_nearest
      fistp    i
      sar      i, 1
   }

   return (i);
}

inline int floor_int (double x)
{
   int      i;
   static const float round_toward_m_i = -0.5f;
   __asm
    {
      fld      x
      fadd     st, st (0)
      fadd     round_toward_m_i
      fistp    i
      sar      i, 1
   }

   return (i);
}

inline int ceil_int (double x)
{
   int      i;
   static const float round_toward_p_i = -0.5f;
   __asm
   {
      fld      x
      fadd     st, st (0)
      fsubr    round_toward_p_i
      fistp    i
      sar      i, 1
   }

   return (-i);
}

Comments
from : daniel[DOT]schaack[-dot-]basementarts[DOT]de
comment : cool trick, but using round to zero FPU mode is still the best methode (2 lines): __asm { fld x fistp y }

from : ejd[AT]dork[DOT]com
comment : If anyone is using NASM, here is how I implemented the first function, if anyone is interested. I did this after sitting down for a while with the intel manuals. I've not done any x86-FPU stuff before, so this may not be the *best* code. The other functions are easily implemented by modifying this one. bits 32 global dsp_round HALF dq 0.5 align 4 dsp_round: fld qword[HALF] fld qword[esp+4] fadd st0 fadd st1 fistp qword[eax] mov eax, [eax] sar eax, 1 ret

from : ejd[AT]dork[DOT]com
comment : Whoops. I've really gone and embarassed myself this time. The code I posted is actually broken -- I don't know what I was thinking. I'll post the fixed version a bit later today. My appologies.

from : hammer[AT]utcnetwork[DOT]jp
comment : Will this method also work for other processor types like AMD and CELERON?

from : scoofy[AT]inf[DOT]elte[DOT]hu
comment : It works with AMD and Celeron too (and as I know, probably with all x87 FPUs)