It's not just about inlining, the compiler may decide to entirely remove memory accesses when memcpy is used to copy one struct to another, like in this case:

It's not just about inlining, the compiler may decide to entirely remove memory accesses when memcpy is used to copy one struct to another, like in this case: https://www.godbolt.org/z/Ezb7hh6eq https://www.godbolt.org/z/Ezb7hh6eq ...and for the same reason, AFAIK the compiler isn't required to preserve struct padding bytes either in such a case, so even though memcpy() is used for copying one struct into another in the original source code, the compiler may actually generate code which behaves differently from a straight memcpy() - but in this case I'm not 100% sure, I just wouldn't depend on it, because I've been bitten in the ass multiple times when it comes to assuming the content of padding bytes.