A peephole optimizer is a simple optimizer in a Compiler. A peephole optimizer tries to optimize specific code patterns on speed or code size. After recognizing these specific patterns, they are replaced by other optimized patterns.
After code is generated by the backend of an optimizing Compiler, it is still possible that code patterns may result that are still capable of being optimized. The optimizations of the peephole optimizer are highly backend-dependent because the peephole optimizer was implemented with characteristic code patterns of the backend in mind.
Certain peephole optimizations only make sense in conjunction with other optimizations, or together with some code patterns. These patterns may have been generated by doing other optimizations. There are optimizations (e.g., removing of a branch to the next instructions) that are removed by the peephole optimizer, though they could have been removed by the branch optimizer as well. Such simple branch optimizations are performed in the peephole optimizer to reach new optimizable states.