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A closely related technique for debugging optimization passes is that of "optimization fuel". Each rewrite decreases the fuel by one, and when the fuel is gone no more rewrites happen. You can then perform binary search on the optimization fuel to find a specific rewrite instance that breaks things.

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Yes, I believe LLVM has a flag for "run only the first N optimization passes" that gets used with binary search this way.

A global optimization fuel that worked at finer granularity would be even more precise but you'd have to have the compiler run single-threaded to make sure the numbering always matches. At least in the Go compiler, we compile different functions in different threads, so that fouls up any global numbering.

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