Stack byte allocation when dealing with control transfer-CodePudding

While reading the book Computer Systems A Programmers Approach I came across the following section, where the book shows some assembly code and the states of the %rsp register while storing addresses to pass control(It starts at the bottom):

<leaf>:
lea    0x2(%rdi), %rax.          //%rsp at 0x7fffffffe810  storing 0x4004e
retq

<top>:
sub    $0x5,%rdi.                //%rsp at 0x7fffffffe818 storing 0x400560
callq  400540 <leaf>.            
add    %rax,%rax
retq

<main>:
callq  400545 <top>              //%rsp at 0x7fffffffe820
mov    %rax,%rax

Where the stored addresses are for the next instruction after each call.

I fail to see the logic behind the bytes allocated to %rsp here, it makes sense to allocate 3 bytes since that's enough to represent the addresses, but it allocates 2 bytes on the first call and then 8 bytes on the next one. Does %rsp point to an unused space and that's why it can just decrement 2 bytes? Then why does it decrement 8 on the call to leaf?

CodePudding user response：

but it allocates 2 bytes on the first call and then 8 bytes on the next one

It always allocates 8 bytes. It does not depend on the small value of the address 0x400560.
The numbers in the comments are in hexadecimal (because of the 0x prefix)!

  0x7fffffffe820
- 0x7fffffffe818
  --------------
               8

  0x7fffffffe818
- 0x7fffffffe810
  --------------
               8