“enter” vs “push ebp; mov ebp, esp; sub esp, imm”

问题:

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.

回答1:

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.

回答2:

There is no real speed advantage using either of them, though the long method will probably run better due to the fact CPU's these days are more 'optimized' to the shorter simpler instructions that are more generic in use (plus it allows saturation of the execution ports if your lucky).

The advantage of LEAVE (which is still used, just see the windows dlls) is that its smaller than manually tearing down a stack frame, this helps a lot when your space is limited.

The Intel instruction manuals (volume 2A to be precise) will have more nitty gritty details on the instructions, so should Dr Agner Fogs Optimization manuals

回答3:

When designing the 80286, Intel's CPU designers decided to add two instructions to help maintain displays.

Here the micro code inside the CPU:

; ENTER Locals, LexLevel

push    bp              ;Save dynamic link.
mov     tempreg, sp     ;Save for later.
cmp     LexLevel, 0     ;Done if this is lex level zero.
je      Lex0

lp:
dec     LexLevel
jz      Done            ;Quit if at last lex level.
sub     bp, 2           ;Index into display in prev act rec
push    [bp]            ; and push each element there.
jmp     lp              ;Repeat for each entry.

Done:
push    tempreg         ;Add entry for current lex level.

Lex0:
mov     bp, tempreg     ;Ptr to current act rec.
sub     sp, Locals      ;Allocate local storage

Alternative to ENTER would be:

; enter n, 0 ;14 cycles on the 486

push    bp              ;1 cycle on the 486
sub     sp, n           ;1 cycle on the 486

; enter n, 1 ;17 cycles on the 486

push    bp              ;1 cycle on the 486
push    [bp-2]          ;4 cycles on the 486
mov     bp, sp          ;1 cycle on the 486
add     bp, 2           ;1 cycle on the 486
sub     sp, n           ;1 cycle on the 486

; enter n, 3 ;23 cycles on the 486

push    bp              ;1 cycle on the 486
push    [bp-2]          ;4 cycles on the 486
push    [bp-4]          ;4 cycles on the 486
push    [bp-6]          ;4 cycles on the 486
mov     bp, sp          ;1 cycle on the 486
add     bp, 6           ;1 cycle on the 486
sub     sp, n           ;1 cycle on the 486

Ect. The long way might increase your file size, but is way quicker.

on last note, programmer don't really use display anymore since that was a very slow work around, making ENTER pretty useless now.

Source: https://courses.engr.illinois.edu/ece390/books/artofasm/CH12/CH12-3.html