An implementation of green threads-style register saving cannot be as memory-efficient as async/await, because it's more dynamic. Green threads must save all (

An implementation of green threads-style register saving cannot be as memory-efficient as async/await, because it's more dynamic. Green threads must save all (callee-save) registers, support arbitrary dynamic stack growth, and work with unsuspecting frames in the middle of the stack. Async/await has the "thread" itself do all the state saving, so it can save only what's actually live. It computes a fixed stack size up front, so can be done with zero allocation. And because all intermediate stack frames are from async functions, they can also be smaller- transient state can be moved to the actual thread stack, potentially shrinking the persistent task state. (This also has benefits for cancellation- there is no need to use the exception/stack unwinding ABI to tear down a "stack," because it is instead a normal struct with normal drop glue.)