You can, but gcc may replace it with an equivalent set of instructions as a compiler optimization, so you would have no guarantee it is used unless you hack the compiler.
On a related note, GCC optimizing away things is a problem for memset when zeroing buffers containing sensitive data, as GCC can often tell that the buffers are going to be freed and thus the write is deemed unnecessary. That is a security issue and has to be resolved by breaking the compiler’s optimization through a clever trick:
Similarly, GCC may delete a memcpy to a buffer about to be freed, although I have never observed that as you generally don’t do that in production code.
> Similarly, GCC may delete a memcpy to a buffer about to be freed, although I have never observed that as you generally don’t do that in production code.
It's not that crazy. You could have a refcounted object that poisons itself when the refcount drops to zero, but doesn't immediately free itself because many malloc implementations can have bad lock contention on free(). So you poison the object to detect bugs, possibly only in certain configurations, and then queue the pointer for deferred freeing on a single thread at a better time.
(Ok, this doesn't quite do it: poisoning is much more likely to use memset than memcpy, but I assume gcc would optimize out a doomed memset too?)
Yes, it potentially could be optimised out, which is why platforms provide functions like SecureZeroMemory() for cases where you want to be sure that memory is zeroed out.
That would be why I introduced an explicit_memset() into the OpenZFS encryption module in the commit that I linked. It uses two different techniques to guard against the compiler deleting it.
On a related note, GCC optimizing away things is a problem for memset when zeroing buffers containing sensitive data, as GCC can often tell that the buffers are going to be freed and thus the write is deemed unnecessary. That is a security issue and has to be resolved by breaking the compiler’s optimization through a clever trick:
https://github.com/openzfs/zfs/commit/d634d20d1be31dfa8cf06e... 12352
Similarly, GCC may delete a memcpy to a buffer about to be freed, although I have never observed that as you generally don’t do that in production code.