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This patch introduces a new flag `-fsolve-constant-dicts` which makes the constraint solver solve super class constraints with available dictionaries if possible. The flag is enabled by `-O1`. The motivation of this patch is that the compiler can produce more efficient code if the constraint solver used top-level instance declarations to solve constraints that are currently solved givens and their superclasses. In particular, as it currently stands, the compiler imposes a performance penalty on the common use-case where superclasses are bundled together for user convenience. The performance penalty applies to constraint synonyms as well. This example illustrates the issue: ``` {-# LANGUAGE ConstraintKinds, MultiParamTypeClasses, FlexibleContexts #-} module B where class M a b where m :: a -> b type C a b = (Num a, M a b) f :: C Int b => b -> Int -> Int f _ x = x + 1 ``` Output without the patch, notice that we get the instance for `Num Int` by using the class selector `p1`. ``` f :: forall b_arz. C Int b_arz => b_arz -> Int -> Int f = \ (@ b_a1EB) ($d(%,%)_a1EC :: C Int b_a1EB) _ (eta1_B1 :: Int) -> + @ Int (GHC.Classes.$p1(%,%) @ (Num Int) @ (M Int b_a1EB) $d(%,%)_a1EC) eta1_B1 B.f1 ``` Output with the patch, nicely optimised code! ``` f :: forall b. C Int b => b -> Int -> Int f = \ (@ b) _ _ (x_azg :: Int) -> case x_azg of { GHC.Types.I# x1_a1DP -> GHC.Types.I# (GHC.Prim.+# x1_a1DP 1#) } ``` Reviewers: simonpj, bgamari, austin Reviewed By: simonpj Subscribers: mpickering, rwbarton, thomie Differential Revision: https://phabricator.haskell.org/D2714 GHC Trac Issues: #12791, #5835
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