Peephole optimization

Peephole optimization is an optimization technique performed on a small set of compiler-generated instructions, known as a peephole or window, that involves replacing the instructions with a logically equivalent set that has better performance.

For example:


 * Instead of pushing a register onto the stack and then immediately popping the value back into the register, remove both instructions
 * Instead of multiplying x by 2, do x + x
 * Instead of multiplying a floating point register by 8, add 3 to the floating point register's exponent

The term peephole optimization was introduced by William Marshall McKeeman in 1965.

Replacements
Peephole optimization replacements include but are not limited to:
 * 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

Implementation
Modern compilers often implement peephole optimizations with a pattern matching algorithm.

Replacing slow instructions with faster ones
The following Java bytecode:

aload 1 aload 1 mul

can be replaced with the following which executes faster:

aload 1 dup mul

As for most peephole optimizations, this is based on the relative efficiency of different instructions. In this case,  (which duplicates and pushes the top of the stack) is known/assumed to be more efficient than   (which loads a local variable and pushes it onto the stack).

Removing redundant code
The following source code:

a = b + c; d = a + e;

is straightforwardly compiled to:

but can be optimized to:

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:

If there were two consecutive subroutine calls, they would look like this:

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: