[zAy0LfpBZLC8mAC]

You are mixing up the C level buffer abstraction and some potentially underlying RAM. C doesn't deal with RAM, only with the abstraction, so you can't look at the RAM in C, you only can look at the abstract buffer, and the only thing that the compiler has to guarantee is that the abstraction holds - namely, that after you write zeroes into the abstract buffer, a subsequent conditional that checks whether the buffer is zeroed will branch accordingly, which is a fact that is trivial to evaluate during compile time, and as soon as the compiler has determined that the conditional is statically determined, it can eliminate any alternative branches as dead code and translate the abstract buffer write into a NOOP at the machine code level.