[LLVMdev] [PATCH] fix outs/ins of MOV16mr instruction (X86)

[Tim Northover]

> What does CodGen track these registers (defined & used) for?

Almost all optimisations need that information. It determines which
instructions depend on the data from others and how the compiler can
move them around safely. Two instructions using (reading) a given
register have to be treated very differently from one defining
(setting) it and the second using it.

Cheers.

Tim.