Change floating point rounding mode

Question

What is the most efficient way to change the rounding mode* of IEEE 754 floating point numbers? A portable C function would be nice, but a solution that uses x86 assembly is ok too.

*I am referring to the standard rounding modes of towards nearest, towards zero, and towards positive/negative infinity

Answer 1

This is the standard C solution:

#include <fenv.h>
#pragma STDC FENV_ACCESS ON

// store the original rounding mode
const int originalRounding = fegetround( );
// establish the desired rounding mode
fesetround(FE_TOWARDZERO);
// do whatever you need to do ...

// ... and restore the original mode afterwards
fesetround(originalRounding);

On backwards platforms lacking C99 support, you may need to resort to assembly. In this case, you may want to set the rounding for both the x87 unit (via the fldcw instruction) and SSE (via the ldmxcsr instruction).

Edit You don't need to resort to assembly for MSVC. You can use the (totally non-standard) _controlfp( ) instead:

unsigned int originalRounding = _controlfp(0, 0);
_controlfp(_RC_CHOP, _MCW_RC);
// do something ...
_controlfp(originalRounding, _MCW_RC);

You can read more about _controlfp( ) on MSDN.

And, just for completeness, a decoder ring for the macro names for rounding modes:

rounding mode    C name         MSVC name
-----------------------------------------
to nearest       FE_TONEAREST   _RC_NEAR
toward zero      FE_TOWARDZERO  _RC_CHOP
to +infinity     FE_UPWARD      _RC_UP
to -infinity     FE_DOWNWARD    _RC_DOWN