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`NestedPatternMatch` pass desugared complex patterns in a very inefficient way resulting in an exponential generation of the number of `case` IR (and Truffle) nodes. Every failed nested pattern would copy all the remaining patterns of the original case expression, in a desugared form. While the execution itself of such deeply nested `case` expression might not have many problems, the time spent in compilation phases certainly was a blocker. This change desugars deeply nested into individual cases with a fallthrough logic. However the fallthrough logic is implemented directly in Truffle nodes, rather than via IR. That way we can generate much simpler IR for nested patterns. Consider a simple case of ``` case x of Cons (Cons a b) Nil -> a + b Cons a Nil -> a _ -> 0 ``` Before the change, the compiler would generate rather large IR even for those two patterns: ``` case x of Cons w y -> case w of Cons a b -> case y of Nil -> a + b _ -> case x of Cons a z -> case z of Nil -> a _ -> case x of _ -> 0 _ -> 0 _ -> case x of Cons a z -> case z of Nil -> a _ -> case x of _ -> 0 _ -> 0 Cons a z -> case z of Nil -> a _ -> case x of _ -> 0 _ -> 0 ``` Now we generate simple patterns with fallthrough semantics and no catch-all branches: ``` case x of Cons w y -> case w of Cons a b -> case y of ## fallthrough on failed match ## Nil -> a + b ## fallthrough on failed match ## Cons a z -> case z of Nil -> a ## fallthrough on failed match ## _ -> 0 ``` # Important Notes If you wonder how much does it improve, then @radeusgd's example in https://www.pivotaltracker.com/story/show/183971366/comments/234688327 used to take at least 8 minutes to compile and run. Now it takes 5 seconds from cold start. Also, the example in the benchmark includes compilation time on purpose (that was the main culprit of the slowdown). For the old implementation I had to kill it after 15 minutes as it still wouldn't finish a single compilation. Now it runs 2 seconds or less. Bonus points: This PR will also fix problem reported in https://www.pivotaltracker.com/story/show/184071954 (duplicate errors for nested patterns) |
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