Link-time optimization is not a very descriptive term. It's usually called whole-program or interprocedural optimization. Link-time optimization is just how i

Link-time optimization is not a very descriptive term. It's usually called whole-program or interprocedural optimization. Link-time optimization is just how it has been implemented. It uses a lot of memory because it requires keeping a representation of more or less the entire program in memory at one time, in a format which is amenable to analysis. It is not out of the ordinary for an optimizer's internal representation to be on the order of 1000x the size of the source code.