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I think it is hypothetically possible that the GCC authors might create an optimization pass that uses prefetchw in a useful way at some point in the future. However, I have no example code. If any such code existed, I imagine that Intel would use it to improve the design of their next chip. That is the fate of all such microarchitecture-specific optimizations.
That code already exists. Remember AMD chips has had a prefetch instruction since K6, and so has most non-x86 architectures. This is not new stuff, it is only new for Intel CPU.