Use top-level instances to solve superclasses where possible

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

1 files changed, 9 insertions, 0 deletions

diff --git a/docs/users_guide/8.2.1-notes.rst b/docs/users_guide/8.2.1-notes.rst
index 9a38299739..d934418588 100644
--- a/docs/users_guide/8.2.1-notes.rst
+++ b/docs/users_guide/8.2.1-notes.rst
@@ -129,6 +129,15 @@ Compiler
   output as JSON documents. It is experimental and will be refined depending
   on feedback from tooling authors for the next release.
 
+- GHC is now able to better optimize polymorphic expressions by using known
+  superclass dictionaries where possible. Some examples:
+
+    -- uses of `Monad IO` or `Applicative IO` here are improved
+    foo :: MonadBaseControl IO m => ...
+
+    -- uses of `Monoid MyMonoid` here are improved
+    bar :: MonadWriter MyMonoid m => ...
+
 GHCi
 ~~~~