Via https://reviews.llvm.org/D27855, LLVM is likely to gain the ability to delete null checks in callers based on attribute((nonnull)) on the callee. This interacts badly with glibc’s choice to mark the pointer parameters of memcpy, memmove, etc. as attribute((nonnull)) – it is relatively common for programs to pass a null pointer and a zero size to these functions, and in practice libc implementations accept such usage. Indeed, LLVM’s lowering of @llvm.memcpy intrinsics relies on these calls working.
Deleting a null pointer check on p after a memcpy(p, q, 0) call seems extremely user-hostile, and very unlikely to result in a valuable improvement to the program, so I propose that we stop lowering attribute((nonnull)) on these builtin library functions to the llvm nonnull attribute.
(Chandler is working on a paper for the C++ committee proposing to give these functions defined behavior when given a null pointer and a zero size, but optimizing on the basis of these particular nonnull attributes seems like a bad idea regardless of the C or C++ committees’ decisions.)