[ridiculous_fish]

Don't think of %eax as a real register. Think of it as a tag in a compressed dataflow graph. The compression is performed by the compiler's register allocator, and the decompression is performed by the CPU's register renaming.

A compiler's IR is a directed graph, where nodes are instructions and tagged by an assigned register. It would be pleasant to assign each node a distinct register, but then machine instructions would be unacceptably large. So the compiler's register allocator compresses the graph, by finding nodes that do not interfere and assigning them the same register.

The CPU's register renamer then reinflates this graph, by inspecting the dataflow between instructions. If two instructions share a register tag, but the second instruction has no dependence on the first, then they may be assigned different physical registers.

`xor eax, eax` has no dependence on any instruction, so it can be specially recognized as allocating a new physical register. In this way of thinking, `xor eax, eax` doesn't zero anything, but is like malloc: it produces a fresh place to read/write, that doesn't alias anything else.