Branch Prediction in C (using GCC)

Branch prediction is a technique used in computer architecture to improve the flow in instruction pipelines. Pipelines are a key feature of modern processors, allowing multiple instructions to be processed simultaneously, each at a different stage of execution. However, this efficiency can be disrupted by branches (like if statements or loops), where the path of execution depends on a condition that might not yet be resolved.

Understanding Branch Prediction

Branch prediction tries to guess the outcome of a branch before it's known for sure, allowing the pipeline to continue filling with instructions as if the guess were correct. If the prediction is right, the pipeline operates smoothly. If it's wrong, the pipeline must be flushed and refilled, which causes a performance penalty.

Types of Branch Predictors:

1. Static Predictors: Make a fixed prediction (always taken or not taken).

2. Dynamic Predictors: Use historical information to make predictions.

Compiler Hints in GCC

In GCC (GNU Compiler Collection), programmers can provide hints to the compiler about the expected behavior of branches. This is done using built-in functions that inform the compiler about the likely direction of branches, enabling more efficient branch prediction.

GCC Branch Prediction Hints:

• __builtin_expect(expr, value): This tells the compiler that expr is expected to equal value. It's often used with conditionals to indicate which branch is more likely.

Consider a function that searches for an element in a large, sorted array. The function is called frequently, but it's more likely that the searched element isn't present.

#include <stdbool.h>
#include <stddef.h>

#define LIKELY(x) __builtin_expect(!!(x), 1)
#define UNLIKELY(x) __builtin_expect(!!(x), 0)

bool search_in_array(int *array, size_t size, int key) {
    for (size_t i = 0; i < size; i++) {
        if (UNLIKELY(array[i] == key)) {
            return true;
        }
    }
    return false;
}

int main() {
    int large_array[] = {/* large sorted array */};
    size_t size = sizeof(large_array) / sizeof(large_array[0]);
    int key_to_search = 10;

    if (search_in_array(large_array, size, key_to_search)) {
        // handle the case where the element is found
    } else {
        // handle the case where the element is not found
    }

    return 0;
}

Explanation:

• LIKELY and UNLIKELY macros use __builtin_expect to give hints to the compiler.

• In search_in_array, the condition array[i] == key is marked as UNLIKELY, informing the compiler that it's more likely that this condition will be false.

• This hint helps the compiler optimize the code for the common case (element not found), potentially improving the performance of this function in scenarios where the function is called frequently with elements not present in the array.

Remember, the actual performance impact of such hints can vary based on the specific processor and workload, and overusing them can sometimes lead to poorer performance. It's often best to use them judiciously and based on profiling data.