I you calloc some memory and then the first thing you do is write to it, can the compiler optimize away the initial write of zeros since they will just be overwritten?
The zero'ing is done significantly for security reasons anyway. If a program could somehow disable that feature and get leftover memory from the kernel it could very easily contain password, secret keys, and other important bits of data that you wouldn't want random programs on your computer to have access to.
If you have a calloc, even along conditional paths, it will understand the value along that path is zero until the store, and that the zero is killed by the store.
What it will do varies, because it does not want to screw up the sparse memory usage.
But, for example, it even understands how to take the zero values + stores and turn them into a memset and kill the stores, etc.
It generally does not do something like split the allocation into a calloc and malloc+memset part, or whatever
On modern server/desktop/mobile CPUs, this won't make much difference anyway because the second write to the same location is essentially 'free' due to the store buffer.
And of course if you're calling calloc() in a tight loop, then the zeroing is the least of your performance concerns!