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Closes#1914
Peepmatic is a DSL for peephole optimizations and compiler for generating
peephole optimizers from them. The user writes a set of optimizations in the
DSL, and then `peepmatic` compiles the set of optimizations into an efficient
peephole optimizer:
```
DSL ----peepmatic----> Peephole Optimizer
```
The generated peephole optimizer has all of its optimizations' left-hand sides
collapsed into a compact automata that makes matching candidate instruction
sequences fast.
The DSL's optimizations may be written by hand or discovered mechanically with a
superoptimizer like [Souper][]. Eventually, `peepmatic` should have a verifier
that ensures that the DSL's optimizations are sound, similar to what [Alive][]
does for LLVM optimizations.
[Souper]: https://github.com/google/souper
[Alive]: https://github.com/AliveToolkit/alive2