_bittestandreset, _bittestandreset64
Visual Studio 2010
Microsoft Specific
Generate the btr instruction, which examines bit b of the address a, returns its current value, and resets the bit to 0.
unsigned char _bittestandreset( long *a, long b ); unsigned char _bittestandreset64( __int64 *a, __int64 b );
On the IPF architecture, the btr instruction is not available, so this intrinsic is a custom function that imitates the behavior of btr. This custom function might be slower than a hand-written inline function because it includes overhead, such as handling the case where b is negative, that might be unnecessary in specific cases.
This routine is only available as an intrinsic.
// bittestandreset.cpp
// processor: x86, IPF, x64
#include <stdio.h>
#include <limits.h>
#include <intrin.h>
#pragma intrinsic(_bittestandreset)
// Check the sign bit and reset to 0 (taking the absolute value)
// Returns 0 if the number is positive or zero
// Returns 1 if the number is negative
unsigned char absolute_value(long* p)
{
const int SIGN_BIT = 31;
return _bittestandreset(p, SIGN_BIT);
}
int main()
{
long i = -112;
unsigned char result;
// Check the sign bit and reset to 0 (taking the absolute value)
result = absolute_value(&i);
if (result == 1)
printf_s("The number was negative.\n");
}
The number was negative.
