What is the fastest way to convert float to int on x86

Question

What is the fastest way you know to convert a floating-point number to an int on an x86 CPU. Preferrably in C or assembly (that can be in-lined in C) for any combination of

Answer 1

The Lua code base has the following snippet to do this (check in src/luaconf.h from www.lua.org). If you find (SO finds) a faster way, I'm sure they'd be thrilled.

Oh, lua_Number means double. :)

/*
@@ lua_number2int is a macro to convert lua_Number to int.
@@ lua_number2integer is a macro to convert lua_Number to lua_Integer.
** CHANGE them if you know a faster way to convert a lua_Number to
** int (with any rounding method and without throwing errors) in your
** system. In Pentium machines, a naive typecast from double to int
** in C is extremely slow, so any alternative is worth trying.
*/

/* On a Pentium, resort to a trick */
#if defined(LUA_NUMBER_DOUBLE) && !defined(LUA_ANSI) && !defined(__SSE2__) && \
    (defined(__i386) || defined (_M_IX86) || defined(__i386__))

/* On a Microsoft compiler, use assembler */
#if defined(_MSC_VER)

#define lua_number2int(i,d)   __asm fld d   __asm fistp i
#define lua_number2integer(i,n)     lua_number2int(i, n)

/* the next trick should work on any Pentium, but sometimes clashes
   with a DirectX idiosyncrasy */
#else

union luai_Cast { double l_d; long l_l; };
#define lua_number2int(i,d) \
  { volatile union luai_Cast u; u.l_d = (d) + 6755399441055744.0; (i) = u.l_l; }
#define lua_number2integer(i,n)     lua_number2int(i, n)

#endif

/* this option always works, but may be slow */
#else
#define lua_number2int(i,d) ((i)=(int)(d))
#define lua_number2integer(i,d) ((i)=(lua_Integer)(d))

#endif