As others have alluded, __builtin_memcpy doesn't resolve to a runtime implementation. GCC and clang treat functions like memcpy specially. Because they're defined by the standard and are reserved names, compilers can assume the exact semantics specified by the standard and elide library calls altogether with optimized inline code. But if the compiler can't do the optimization (can't prove alignment, indeterminate length, etc), it just emits a library call, even if your source has some other local function definition named "memcpy". Explicit use of __builtin_memcpy is treated identically to calls to memcpy, unless the compiler is invoked with -ffreestanding, in which case it only optimizes __builtin_memcpy and skips special treatment of calls to memcpy, but __builtin_memcpy could still expand into a call to memcpy. If you're writing a C library you want to use -ffreestanding. (I think. There may be more nuance. More info at https://gcc.gnu.org/bugzilla/show_bug.cgi?id=56888)