(Almost) any instruction on x64 that writes to a 32-bit register as destination, writes the lower 32-bits of the value into the lower 32 bits of the full 64-bit register and zeroes out the upper 32 bits of the full register. He touched on it in his previous note "why xor eax, eax".

But the funny thing is, the x64-specific supplement for SysV ABI doesn't actually specify whether the top bits should be zeroes or not (and so, if the compiler could rely on e.g. function returning ints to have upper 32 bits zeroes, or those could be garbage), and historically GCC and Clang diverged in their behaviour.