Cure exponential behaviour in the simplifier
This patch nails a Bad Bug exposed in Trac #13379. Roughly, a deeply-nested application like f (f (f ....) ) ) could make the simplifier go exponential -- without producing an exponential-sized result! The reason was that we - simplified a (big) function argument - then decided to inline the function - then preInilneUnconditionally the argument - and then re-simplified the big argument And if the "big argument" itself had a similar structure things could get very bad. Once I'd understood, it was easy to fix: * See Note Note [Avoiding exponential behaviour] for an overview * The key change is that Simplify.simplLam now as a case for (isSimplified dup). This is what removes the perf bug. * But I also made simplCast more parsimonious about simplifying, avoiding doing so when the coercion is Refl * And similarly I now try to avoid simplifying arguments where possible before applying rules. See Note [Trying rewrite rules] The latter two points tackle common cases, and in those cases make the simplifier take fewer iterations.
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