c - Assembly x86 - "leave" Instruction

Question

It's said that the "leave" instruction is similar to:

movl %ebp, %esp
popl %ebp

I understand the movl %ebp, %esp part, and that it acts to release stored up memory (as discussed in this question).

But what is the purpose of the popl %ebp code?

Answer 1

LEAVE is the counterpart to ENTER. The ENTER instruction sets up a stack frame by first pushing EBP onto the stack and then copies ESP into EBP, so LEAVE has to do the opposite, i.e. copy EBP to ESP and then restore the old EBP from the stack.

See the section named PROCEDURE CALLS FOR BLOCK-STRUCTURED LANGUAGES in Intel's Software Developer's Manual Vol 1 if you want to read more about how ENTER and LEAVE work.

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enter n,0 is exactly equivalent to (and should be replaced with)

push  %ebp
mov   %esp, %ebp     # ebp = esp,  mov  ebp,esp in Intel syntax
sub   $n, %esp       # allocate space on the stack.  Omit if n=0

leave is exactly equivalent to

mov   %ebp, %esp     # esp = ebp,  mov  esp,ebp in Intel syntax
pop   %ebp

enter is very slow and compilers don't use it, but leave is fine. (http://agner.org/optimize). Compilers do use leave if they make a stack frame at all (at least gcc does). But if esp is already equal to ebp, it's most efficient to just pop ebp.