How to Enhance Branch Prediction Optimization in GCC?

Branch Prediction Optimization in GCC

The Intel architecture provides a mechanism to influence branch prediction in generated code. With the __builtin_expect() function, GCC can force branch prediction to go a certain way.

The syntax for __builtin_expect() is:

long __builtin_expect (long exp, long c)

where:

• exp is the condition being evaluated.
• c is the expected value of the condition.

For example, to force branch prediction to always take the "true" branch in the following C code:

if (normal) {
  doSomethingNormal();
} else {
  exceptionalCase();
}

You would use the following statement:

if (__builtin_expect(normal, 1))

To simplify the usage, it's common to define macros:

#define likely(x)    __builtin_expect (!!(x), 1)
#define unlikely(x)  __builtin_expect (!!(x), 0)

However, it's important to note that:

• This technique is non-standard, so it may not be supported by all compilers or architectures.
• Modern compilers and hardware are typically very good at branch prediction, so premature micro-optimizations like this should be handled with caution.

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