[Serplat]

For smaller memory allocations that don't go directly to the OS, I suppose it's theoretically possible (though I'm not sure any compilers do it). For the larger allocations that the author mentions that go directly to the OS to fulfill, however, the compiler wouldn't be able to optimize this away. In that case, the zero'ing occurs in the kernel, which is something that the compiler has no control over.

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.