On Tue, 27 May 2025 at 09:38, Basile Starynkevitch wrote:
>
> On Tue, 2025-05-27 at 09:33 +0100, Jonathan Wakely wrote:
> > GCC assumes certain functions are always present, e.g. memcpy is needed for
> > copying large structs on the stack. The question is which functions does it
> > need. Your answer isn't really relevant to the question.
>
>
> memcpy is actually a builtin inside GCC. and has to be one because many
> processors (including x86-64 and ARM) has specific machine instructions to
> implement it efficiently.
It *is* a built-in, but that doesn't mean it's implemented inside GCC.
You can verify this easily with the following code:
#include <string.h>
void f(void* to, const void* from, unsigned long n)
{
memcpy(to, from, n);
}
Even at -O3 this will be compiled to a call to the memcpy function in
libc. GCC doesn't expand it to machine instructions unless the size is
known (and small).
