It turns out that sometime ago #gcc has added the #clang attribute #may_alias to C (and C++). After breaking C semantics, despite clear objections from Dennis Ritchie, to enable unquantified and feeble "optimizations" based on aliasing, the compiler bosses have had to smuggle back a version. Looks like these changes went into some of libc about a year ago.
#wg14 #Cstandard