AMD 250 Manuale Utente

25112

Rev. 3.06

September 2005

The recommended optimization for function epilogues depends on whether the function allocates 
local variables.

This optimization applies to:

•

32-bit software

•

64-bit software

The LEAVE instruction is a single-byte instruction and saves 2 bytes of code space over the 
traditional epilogue. Replacing the traditional sequence with LEAVE also preserves decode 
bandwidth.

Accessing function arguments and local variables directly through ESP frees EBP for use as a 
general-purpose register.

The function arguments and local variables inside a function are referenced through a so-called frame 
pointer. In AMD64 code, the base pointer register (rBP) is customarily used as a frame pointer. You 
set up the frame pointer at the beginning of the function using a function prologue:

push ebp                    ; Save old frame pointer.

mov  ebp, esp               ; Initialize new frame pointer.

sub  esp, 

n                 ; Allocate space for local variables (only if the

                            ; function allocates local variables).

Allocates local variables

Replace the traditional function epilogue with the LEAVE instruction.

Does not allocate local variables

Do no use function prologues or epilogues. Access function 
arguments and local variables through rSP.