In compiler theory, peephole optimization is a kind of optimization performed over a very small set of instructions in a segment of generated code. The set is called a "peephole" or a "window". It works by recognising sets of instructions that can be replaced by shorter or faster sets of instructions.
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Common techniques applied in peephole optimization:
- Constant folding - Evaluate constant subexpressions in advance.
- Strength reduction - Replace slow operations with faster equivalents.
- Null sequences - Delete useless operations
- Combine Operations: Replace several operations with one equivalent.
- Algebraic Laws: Use algebraic laws to simplify or reorder instructions.
- Special Case Instructions: Use instructions designed for special operand cases.
- Address Mode Operations: Use address modes to simplify code.
There can, of course, be other types of peephole optimizations involving simplifying the target machine instructions, assuming that the target machine is known in advance. Advantages of a given architecture and instruction sets can be exploited in this case.
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Replacing slow instructions with faster ones
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).
Removing redundant code
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
Removing redundant stack instructions
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. 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
Modern architectures typically allow for many hundreds of different kinds of peephole optimizations, and it is therefore often appropriate for compiler programmers to implement them using a pattern matching algorithm. 
- Object code optimizers, discussion in relation to general algorithmic efficiency
- Capex Corporation - produced the COBOL optimizer, an early mainframe object code optimizer for IBM Cobol
- Crafting a Compiler with C++, Fischer/LeBlanc
- Compilers - Principles, Techniques, and Tools 2e, p560
The dictionary definition of peephole optimization at Wiktionary