About intrinsics

C and C++ are suited to a wide variety of tasks but do not provide inbuilt support for specific areas of application, for example, Digital Signal Processing (DSP).

Within a given application domain, there is usually a range of domain-specific operations that need to be performed frequently. Often, however, these operations cannot be efficiently implemented in C or C++. A typical example is the saturated add of two 32-bit signed two’s complement integers, commonly used in DSP programming. Example 3.1 shows its implementation in C.

#include <limits.h>

int L_add(const int a, const int b)
{
    int c;

    c = a + b;
    if (((a ^ b) & INT_MIN) == 0)
    {
        if ((c ^ a) & INT_MIN)
        {
            c = (a < 0) ? INT_MIN : INT_MAX;
        }
    }
    return c;
}

Intrinsic functions provide a way of easily incorporating domain-specific operations in C and C++ source code without resorting to complex implementations, for example, in embedded assembler or inline assembler. An intrinsic function has the appearance of a function call in C or C++, but is replaced during compilation by a specific sequence of low-level instructions. When implemented using an intrinsic, for example, the saturated add function of Example 3.1 has the form:

#include <dspfns.h>    /* Include ETSI intrinsics */
...
int a, b, result;
...
result = L_add(a, b);  /* Saturated add of a and b */

The use of intrinsics offers several performance benefits: