On Tue, 27 May 2025 at 15:48, Segher Boessenkool wrote: > > Hi! > > On Tue, May 27, 2025 at 09:52:38AM +0100, Jonathan Wakely via Gcc-help wrote: > > 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. > > Yup. Many (or most?) builtin functions can actually expand to something > calling the actual function with the same name. But they can also > expand to more optimal code, and in all cases the compiler can see the > actual semantics of what the code is doing, perhaps changing what code > the compiler will generate for it. > > GCC always can use memcpy, the C standard requires memcpy to be usable. > The externally callable version of the memcpy function is provided by > some C library, on most configurations (just like many other functions, > on systems that implement the POSIX requirements). > > There are four or five functions not provided by GCC itself (in libgcc > or such) that the compiler needs to be available. The manual lists > them: > > Most of the compiler support routines used by GCC are present in > 'libgcc', but there are a few exceptions. GCC requires the freestanding > environment provide 'memcpy', 'memmove', 'memset' and 'memcmp'. > Contrary to the standards covering 'memcpy' GCC expects the case of an > exact overlap of source and destination to work and not invoke undefined > behavior. Finally, if '__builtin_trap' is used, and the target does not > implement the 'trap' pattern, then GCC emits a call to 'abort'. > > (https://gcc.gnu.org/onlinedocs/gcc-15.1.0/gcc/Standards.html#C-Language, > the one but last paragraph) Thanks! I thought we had that documented somewhere but tried (briefly) to find it, and failed.
