Peephole optimization involves changing the small set of instructions to an equivalent set that has better performance.
The term peephole optimization was introduced by William Marshall McKeeman in 1965.
Common techniques applied in peephole optimization:
There can be other types of peephole optimizations.
This section does not cite any sources.(March 2013)
The following Java bytecode
... aload 1 aload 1 mul ...
can be replaced by
... aload 1 dup mul ...
This kind of optimization, like most peephole optimizations, makes certain assumptions about the efficiency of instructions. For instance, in this case, it is assumed that the
dup operation (which duplicates and pushes the top of the stack) is more efficient than the
aload X operation (which loads a local variable identified as
X and pushes it on the stack).
Another example is to eliminate redundant load stores.
a = b + c; d = a + e;
is straightforwardly implemented as
MOV b, R0 ; Copy b to the register ADD c, R0 ; Add c to the register, the register is now b+c MOV R0, a ; Copy the register to a MOV a, R0 ; Copy a to the register ADD e, R0 ; Add e to the register, the register is now a+e [(b+c)+e] MOV R0, d ; Copy the register to d
but can be optimised to
MOV b, R0 ; Copy b to the register ADD c, R0 ; Add c to the register, which is now b+c (a) MOV R0, a ; Copy the register to a ADD e, R0 ; Add e to the register, which is now b+c+e [(a)+e] MOV R0, d ; Copy the register to d
If the compiler saves registers on the stack before calling a subroutine and restores them when returning, consecutive calls to subroutines may have redundant stack instructions.
Suppose the compiler generates the following Z80 instructions for each procedure call:
PUSH AF PUSH BC PUSH DE PUSH HL CALL _ADDR POP HL POP DE POP BC POP AF
If there were two consecutive subroutine calls, they would look like this:
PUSH AF PUSH BC PUSH DE PUSH HL CALL _ADDR1 POP HL POP DE POP BC POP AF PUSH AF PUSH BC PUSH DE PUSH HL CALL _ADDR2 POP HL POP DE POP BC POP AF
The sequence POP regs followed by PUSH for the same registers is generally redundant. In cases where it is redundant, a peephole optimization would remove these instructions. In the example, this would cause another redundant POP/PUSH pair to appear in the peephole, and these would be removed in turn. Assuming that subroutine _ADDR2 does not depend on previous register values, removing all of the redundant code in the example above would eventually leave the following code:
PUSH AF PUSH BC PUSH DE PUSH HL CALL _ADDR1 CALL _ADDR2 POP HL POP DE POP BC POP AF
Edited: 2021-06-18 14:30:13