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# Peephole optimisation

## Basic idea

Slide a small window over the instruction stream and replace short patterns with cheaper equivalents (e.g. `mul x, 2` → `shl x, 1`; `mov a, b; mov b, a` → `mov a, b`). Cheap, local, and surprisingly effective at the end of the pipeline.

## Key facts

* Window typically 2-5 instructions.
* Rules are target-specific (instruction selection's afterburn).
* Modern compilers often implement these as e-graph rewrites or table-driven patterns.