"enter" vs "push ebp; mov ebp, esp; sub esp, imm" and "leave" vs "mov esp, ebp; pop ebp"

assembly stack
小太郎 picture 小太郎 · May 11, 2011 · Viewed 11.3k times · Source

What is the difference between the enter and

push ebp
mov  ebp, esp
sub  esp, imm

instructions? Is there a performence difference? If so, which is faster and why do compilers always use the latter?

Similarily with the leave and

mov  esp, ebp
pop  ebp

instructions.

Answer

Gunther Piez picture Gunther Piez · May 11, 2011

There is a performance difference, especially for enter. On modern processors this decodes to some 10 to 20 µops, while the three instruction sequence is about 4 to 6, depending on the architecture. For details consult Agner Fog's instruction tables.

Additionally the enter instruction usually has a quite high latency, for example 8 clocks on a core2, compared to the 3 clocks dependency chain of the three instruction sequence.

Furthermore the three instruction sequence may be spread out by the compiler for scheduling purposes, depending on the surrounding code of course, to allow more parallel execution of instructions.

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