How do you allocate and deallocate local variables on the stack?

Subtract from %rsp to allocate stack space (the stack grows down), and add the same amount back to deallocate before returning.

Because the stack grows toward lower addresses, reserving space means making %rsp smaller, and releasing it means making %rsp larger again.

subq $32, %rsp   # allocate 32 bytes of locals
...
addq $32, %rsp   # release them before ret

Within that region you address locals by offset from %rsp:

movq %rdi, (%rsp)     # local[0]
movq %rsi, 8(%rsp)    # local[1]
movq %rdx, 16(%rsp)   # local[2]
movq %rcx, 24(%rsp)   # local[3]

Alignment constraint: the stack must be 16-byte aligned before a call. Since the function entered with %rsp already off by 8 (the return address call pushed), allocations are typically rounded so that alignment is restored — which is why you'll often see a function reserve, say, 24 bytes when it only needs 20.

Tip: a matched sub $N,%rsp … add $N,%rsp pair bracketing a function body is the no-frame-pointer style of managing locals, common in optimized x86-64.

Go deeper:

• Call stack (Wikipedia) — frames, frame pointers and per-invocation local storage.