Float to int

  • Author or source: Ross Bencina
  • Created: 2002-01-17 03:15:14
notes
intel only
code
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int truncate(float flt)
{
  int i;
  static const double half = 0.5f;
  _asm
  {
     fld flt
     fsub half
     fistp i
  }
  return i
}

Comments

Note: Round nearest doesn't work, because the Intel FPU uses Even-Odd rounding in order to conform to the IEEE floating-point standard: when the FPU is set to use the round-nearest rule, values whose fractional part is exactly 0.5
round toward the nearest *even* integer.  Thus, 1.5 rounds to 2, 2.5 rounds to 2, 3.5 rounds to 4. This is quite disastrous for the FLOOR/CEIL
functions if you use the strategy you describe.

This version below seems to be more accurate on my Win32/P4.  Doesn't deal with the Intel FPU issue.  A faster solution than c-style casts though.  But you're not always going to get the most accurate conversion.  Like the previous comment; 2.5 will convert to 2, but 2.501 will convert to 3.

int truncate(float flt)
{
    int i;
    _asm
    {
        fld flt
        fistp i
    }
    return i
}

if you use msvc, just use the /QIfist compile-switch