Whilst the C standard doesn't guarantee it, both LLVM and GCC _do_. They have implementation-defined that it will work, so are not free to optimise it away.

[0] https://llvm.org/docs/LangRef.html#llvm-memset-intrinsics

[1] https://gitweb.git.savannah.gnu.org/gitweb/?p=gnulib.git;a=b...