How Much Data Does \"__builtin_prefetch\" Actually Load?

"__builtin_prefetch: Deciphering Its Reading Range"

"__builtin_prefetch" is a C/C compiler intrinsic used to instruct the compiler to preload data into the cache memory, thereby reducing the latency of accessing that data. However, its exact behavior, particularly with regard to prefetching the contents of a structure, can be confusing.

The documentation provided about "__builtin_prefetch" states that it prefetches a line cache, but the size of this line cache can vary depending on the processor. In the case of prefetching an entire structure, "__builtin_prefetch" will typically only prefetch the first few cache lines of the structure.

For example, let's consider the loop in the given code:

for (int i = from; i < to; i++)
{
    particle* from = con[i].Pfrom;
    particle* to = con[i].Pto;
    // Assume particle has 8 double values
}

In this loop, "__builtin_prefetch (con[i].Pfrom)" will only prefetch the first 8 doubles of the particle structure. If the particle structure contains more than 8 doubles, the remaining values will not be prefetched.

To prefetch additional values, you can use multiple "__builtin_prefetch" intrinsics. For instance, "__builtin_prefetch (con[i 3].Pfrom)" would prefetch the first 8 doubles of the particle structure located at con[i 3].Pfrom.

However, it's important to use "__builtin_prefetch" judiciously. Excessive use can actually degrade performance. It's recommended to measure the impact of "__builtin_prefetch" on your code and only use it where it provides a significant benefit.

Modern processors and compilers have improved cache handling significantly. As a result, the effectiveness of "__builtin_prefetch" has diminished. However, it can still be useful in certain situations, especially for complex data structures or large arrays accessed in a predictable pattern. Benchmarks are always recommended to determine if "__builtin_prefetch" is worthwhile.

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