Peephole optimization

Peephole optimization is an optimization technique performed on a small set of compiler-generated instructions; the small set is known as the peephole or window.

Peephole optimization involves changing the small set of instructions to an equivalent set that has better performance.

For example:

• instead of pushing register A onto the stack and then immediately popping the value back into register A, a peephole optimization would remove both instructions;
• instead of adding A to A, a peephole optimization might do an arithmetic shift left;
• instead of multiplying a floating point register by 8, a peephole optimization might add 3 to the floating point register's exponent; and
• instead of multiplying an index by 4, adding the result to a base address to get a pointer value, and then dereferencing the pointer, a peephole optimization might use a hardware addressing mode that accomplishes the same result with one instruction.

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