How to Use GCC Compiler Hint for Forceful Branch Prediction?

Compiler Hint for Forceful Branch Prediction in GCC

In C code, there may be situations where a specific branch should always exhibit predictable behavior for optimal performance. Can GCC provide a compiler hint to accomplish this?

GCC supports the __builtin_expect() function for this purpose. It takes two parameters: exp (the condition) and c (the expected value). To force the branch prediction in a specific direction, use the following syntax:

<code class="c++">if (__builtin_expect(normal, 1)) {
    // code for predicted branch
} else {
    // code for unpredicted branch
}</code>

where normal is the condition and 1 is the expected value.

Alternatively, you can define custom macros for convenience:

<code class="c++">#define likely(x)    __builtin_expect (!!(x), 1)
#define unlikely(x)  __builtin_expect (!!(x), 0)</code>

This enables more concise usage:

<code class="c++">if (likely(normal)) {
    // code for predicted branch
} else {
    // code for unpredicted branch
}</code>

It's important to note that this is a non-standard feature and may not be supported by all compilers or hardware architectures. Additionally, modern compilers and CPUs are highly sophisticated and may make more optimal branch prediction decisions than manual hints. Therefore, premature micro-optimizations should be avoided.

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