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Using intrinsics

Compiler intrinsics are functions provided by the compiler. They enable you to easily incorporate domain-specific operations in C and C++ source code without resorting to complex implementations in assembly language.

The C and C++ languages are suited to a wide variety of tasks but they do not provide in-built 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 have to be performed frequently. However, often 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. The following example shows a C implementation of a saturated add operation:

#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;

Using compiler intrinsics, you can achieve more complete coverage of target architecture instructions than you would from the instruction selection of the compiler.

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. The following example shows how to access the __qadd saturated add intrinsic:

#include <arm_acle.h>    /* Include ACLE intrinsics */

int foo(int a, int b)
  return __qadd(a, b);  /* Saturated add of a and b */

The use of compiler intrinsics offers a number of performance benefits:

  • The low-level instructions substituted for an intrinsic might be more efficient than corresponding implementations in C or C++, resulting in both reduced instruction and cycle counts. To implement the intrinsic, the compiler automatically generates the best sequence of instructions for the specified target architecture. For example, the __qadd intrinsic maps directly to the A32 assembly language instruction qadd:

    QADD r0, r0, r1    /* Assuming r0 = a, r1 = b on entry */
  • More information is given to the compiler than the underlying C and C++ language is able to convey. This enables the compiler to perform optimizations and to generate instruction sequences that it could not otherwise have performed.

These performance benefits can be significant for real-time processing applications. However, care is required because the use of intrinsics can decrease code portability.