Comment by Thorrez

(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.

He's actually wrong on the ABI requiring the top bits to be 0. It only requires that the bottom 32 bits match the parameter, but the top bits of a 32-bit parameter passed in a 64-bit register can be anything (at least on Linux).

You can see that in this godbolt example: https://godbolt.org/z/M1ze74Gh6

The reason the code in his post works is because the upper 32 bits of the parameters going into an addition can't affect the low 32 bits of the result, and he's only storing the low 32 bits.