How much Data Does __builtin_prefetch Read from a Given Address?

__builtin_prefetch: Understanding its Range of Read

prefetch is a powerful intrinsic in C that allows developers to optimize code by preloading data from memory into the cache. Understanding how __builtin_prefetch operates is crucial for effective optimization.

The original question explores how much data __builtin_prefetch reads from a given address. The correct answer is that it typically fetches a single cache line, which is processor-specific in size. This means that it will not load the entire structure pointed to by the address.

In the context of the code snippet provided in the question:

<code class="cpp">for (int i = from; i < to; i++)
{
    ...
    __builtin_prefetch(con[i++].Pfrom);
    __builtin_prefetch(con[i].Pto);
    ...
}</code>

The developer intends to prefetch the values located at con[i ].Pfrom and con[i].Pto, ensuring that they are loaded into the cache before they are accessed. It's recommended to prefetch multiple elements in advance for optimal results.

However, it's important to use __builtin_prefetch sparingly and measure its impact on performance. Excessive use can hinder performance. Additionally, modern processors and compilers have significantly improved cache handling, reducing the need for manual prefetching. Always benchmark before using __builtin_prefetch to determine its effectiveness.

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