How Can I Efficiently Manipulate Floating-Point Rounding Modes in C and Assembly?

Manipulating Floating-Point Rounding Modes: An Efficient Approach

Enhancing floating-point operations often involves controlling their rounding mode, which determines how results are rounded after calculations. This article explores the most efficient techniques for altering the rounding mode, catering to both portable C functions and x86 assembly solutions.

Portable C Function:

The C99 standard provides the fegetround() and fesetround() functions for obtaining and setting the rounding mode. Here's a code snippet showcasing their usage:

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

int main() {
    // Store the initial rounding mode
    int originalRounding = fegetround();
    
    // Set the rounding mode to round towards zero
    fesetround(FE_TOWARDZERO);
    
    // Perform calculations and rounding operations
    
    // Reset the original rounding mode
    fesetround(originalRounding);
    
    return 0;
}

x86 Assembly Solution:

If targeting platforms lacking C99 support, resort to x86 assembly instructions to manipulate the rounding modes. This involves setting for both the x87 unit (via fldcw) and SSE (via ldmxcsr).

MSVC-Specific Solution:

For Microsoft Visual C (MSVC), leverage the non-standard _controlfp() function instead of the assembly approach.

unsigned int originalRounding = _controlfp(0, 0);
_controlfp(_RC_CHOP, _MCW_RC);
// Perform calculations and rounding operations
_controlfp(originalRounding, _MCW_RC);

Rounding Mode Decoder Ring:

For reference, here's a decoder ring for the different rounding modes and their corresponding C and MSVC macros:

Rounding Mode	C Macro	MSVC Macro
Nearest	FE_TONEAREST	_RC_NEAR
Towards Zero	FE_TOWARDZERO	_RC_CHOP
Towards Infinity	FE_UPWARD	_RC_UP
Towards -Infinity	FE_DOWNWARD	_RC_DOWN

These techniques provide efficient means of controlling floating-point rounding modes, enabling optimized calculations and results.

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