Compiler generates call to memcpy from std::copy when pointers are of __restrict type?

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The gcc compiler generates call to memcpy when i add __restrict to function parameters. How does compiler/standard library figure out that it can generate calls to memcpy when appropriate?

void call_stdcpy_r(int *__restrict p, int *__restrict q, int sz) {
  std::copy(p, p+sz, q); // generates call to memcpy
}

void call_stdcpy(int *p, int *q, int sz) {
  std::copy(p, p+sz, q); // generates call to memmove
}

As per https://en.cppreference.com/w/cpp/algorithm/copy

The behavior is undefined if the source and the destination ranges overlap.

Shouldn't the compiler, in principle, generate calls to memcpy all the time?

Link to godbolt: https://godbolt.org/z/aKj3Y5K8M

libstdc++ g++ copy gcc c++
1 Answers

Your quote applies to std::copy_if, not to std::copy.

The only requirement for std::copy is that q is not in the range [p,p+sz). The destination range is allowed to overlap and therefore memmove is the only option without additional assumptions, such as introduced by __restrict.

__restrict guarantees the compiler that the ranges will not overlap.

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