When is __builtin_expect() Useful for Controlling Branch Prediction in GCC?

Efficient Branch Prediction with GCC

In optimizing code performance, controlling branch prediction can significantly enhance execution speed. For Intel architectures, GCC provides a mechanism to guide branch prediction behavior in a desired direction, maximizing performance for scenarios where a particular case is frequently encountered.

GCC's __builtin_expect() function empowers developers with the ability to provide hints to the compiler regarding expected branch outcomes. By leveraging this function, you can instruct the compiler to generate code that consistently predicts a specific branch path, even when the branch has recently taken an alternate route.

The syntax of __builtin_expect() is as follows:

__builtin_expect(long exp, long c)

In the code sample you provided, you want branch prediction to always prioritize the "normal" case. Using __builtin_expect(), you can achieve this by wrapping the "if" condition as shown below:

if (__builtin_expect(normal, 1))

However, due to the cumbersome syntax of __builtin_expect(), custom macros like "likely" and "unlikely" are often employed for convenience. These macros encapsulate the expected value argument, simplifying code readability.

It's important to note that:

• GCC's __builtin_expect() is non-standard.
• Modern compilers and CPUs have sophisticated branch predictor mechanisms that may already perform optimized predictions. Premature micro-optimizations like forced branch prediction may not yield significant benefits.

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