How Can I Efficiently Change IEEE 754 Floating Point Rounding Modes in C and x86 Assembly?

Manipulating IEEE 754 Floating Point Rounding Mode

Floating point arithmetic often involves rounding of numbers to approximate the true mathematical result. The IEEE 754 standard defines several rounding modes that determine how floating point numbers are rounded. This article explores efficient methods to change the rounding mode in a portable C or x86 assembly environment.

Standard C Solution

For portable C code, the solution involves using the C99 function fesetround(), which allows you to set the rounding mode. However, you should remember to restore the original rounding mode after modifying it.

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

int main() {
    int originalRounding = fegetround();
    fesetround(FE_TOWARDZERO);
    // Perform operations

    fesetround(originalRounding);
    return 0;
}

x86 Assembly Solution

If you're working on older platforms without C99 support, you may need to use x86 assembly to manipulate the rounding mode. This involves setting the rounding mode in both the x87 unit (using the fldcw instruction) and SSE (using the ldmxcsr instruction).

MSVC-Specific Solution

For MS Visual C (MSVC), there's an alternative to assembly. You can use the non-standard _controlfp() function to change the rounding mode.

unsigned int originalRounding = _controlfp(0, 0);
_controlfp(_RC_CHOP, _MCW_RC);
// Do some operations

_controlfp(originalRounding, _MCW_RC);

Rounding Mode Constants

To specify the rounding mode, you need to use the appropriate macro constant. Here's a summary of the standard C and MSVC names for rounding modes:

Rounding Mode	C	MSVC
Nearest	FE_TONEAREST	_RC_NEAR
Toward Zero	FE_TOWARDZERO	_RC_CHOP
Positive Infinity	FE_UPWARD	_RC_UP
Negative Infinity	FE_DOWNWARD	_RC_DOWN

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