[bennettnate5]

The simplest example is `memcpy(dst, src, len)` and similar iterative byte copying operations. If the function did not use noalias, the compiler wouldn't be free to optimize individual byte read/writes into register-sized writes, as the destination may overlap with the source. In practice this means 8x more CPU instructions per copy operation on a 64-bit machine.

Note that memcpy specifically may already be implemented this way under the hood because it requires noalias; but I imagine similar iterative copying operations can be optimized in a like manner ad-hoc when aliasing information is baked in like it is with Rust.

[repstosb]

That's not a great example, since memcpy() already has all the information it needs to determine whether the regions overlap (src < dest + len && dst < src + len) and even where and by how much. So pretty much any quality implementation is already performing this test and selecting an appropriate code path, at the cost of a single branch rather than 8x as many memory operations.

The real purpose of restrict is to allow the compiler to cache a value that may be used many times in a loop (in memcpy(), each byte/word is used only once) in a register at the start of the loop, and not have to worry about repeatedly reaching back to memory to re-retrieve it because it might have been modified as a side effect of the loop body.