compiler: Introduce and use RoughMap for instance environments

Here we introduce a new data structure, RoughMap, inspired by the
previous `RoughTc` matching mechanism for checking instance matches.
This allows [Fam]InstEnv to be implemented as a trie indexed by these
RoughTc signatures, reducing the complexity of instance lookup and
FamInstEnv merging (done during the family instance conflict test)
from O(n) to O(log n).

The critical performance improvement currently realised by this patch is
in instance matching. In particular the RoughMap mechanism allows us to
discount many potential instances which will never match for constraints
involving type variables (see Note [Matching a RoughMap]). In realistic
code bases matchInstEnv was accounting for 50% of typechecker time due
to redundant work checking instances when simplifying instance contexts
when deriving instances. With this patch the cost is significantly
reduced.

The larger constants in InstEnv creation do mean that a few small
tests regress in allocations slightly. However, the runtime of T19703 is
reduced by a factor of 4. Moreover, the compilation time of the Cabal
library is slightly improved.

A couple of test cases are included which demonstrate significant
improvements in compile time with this patch.

This unfortunately does not fix the testcase provided in #19703 but does
fix #20933

-------------------------
Metric Decrease:
    T12425
Metric Increase:
    T13719
    T9872a
    T9872d
    hard_hole_fits
-------------------------

Co-authored-by: Matthew Pickering <matthewtpickering@gmail.com>

1 files changed, 10 insertions, 11 deletions

diff --git a/compiler/GHC/Tc/Module.hs b/compiler/GHC/Tc/Module.hs
index 505f0dd627..c8f65e2453 100644
--- a/compiler/GHC/Tc/Module.hs
+++ b/compiler/GHC/Tc/Module.hs
@@ -401,7 +401,7 @@ tcRnImports hsc_env import_decls
               tcg_rdr_env      = tcg_rdr_env gbl `plusGlobalRdrEnv` rdr_env,
               tcg_imports      = tcg_imports gbl `plusImportAvails` imports,
               tcg_rn_imports   = rn_imports,
-              tcg_inst_env     = extendInstEnvList (tcg_inst_env gbl) home_insts,
+              tcg_inst_env     = tcg_inst_env gbl `unionInstEnv` home_insts,
               tcg_fam_inst_env = extendFamInstEnvList (tcg_fam_inst_env gbl)
                                                       home_fam_insts,
               tcg_hpc          = hpc_info
@@ -1721,7 +1721,7 @@ tcMissingParentClassWarn warnFlag isName shouldName
            -- "<location>: Warning: <type> is an instance of <is> but not
            -- <should>" e.g. "Foo is an instance of Monad but not Applicative"
            ; let instLoc = srcLocSpan . nameSrcLoc $ getName isInst
-                 warnMsg (KnownTc name:_) =
+                 warnMsg (RM_KnownTc name:_) =
                       addDiagnosticAt instLoc $
                         TcRnUnknownMessage $ mkPlainDiagnostic (WarningWithFlag warnFlag) noHints $
                            hsep [ (quotes . ppr . nameOccName) name
@@ -1734,7 +1734,7 @@ tcMissingParentClassWarn warnFlag isName shouldName
                            hsep [ text "This will become an error in"
                                 , text "a future release." ]
                  warnMsg _ = pure ()
-           ; when (null shouldInsts && null instanceMatches) $
+           ; when (nullUnifiers shouldInsts && null instanceMatches) $
                   warnMsg (is_tcs isInst)
            }
 
@@ -2041,7 +2041,7 @@ runTcInteractive hsc_env thing_inside
     withDefaultingPlugins hsc_env $ withHoleFitPlugins hsc_env $
     do { traceTc "setInteractiveContext" $
             vcat [ text "ic_tythings:" <+> vcat (map ppr (ic_tythings icxt))
-                 , text "ic_insts:" <+> vcat (map (pprBndr LetBind . instanceDFunId) ic_insts)
+                 , text "ic_insts:" <+> vcat (map (pprBndr LetBind . instanceDFunId) (instEnvElts ic_insts))
                  , text "icReaderEnv (LocalDef)" <+>
                       vcat (map ppr [ local_gres | gres <- nonDetOccEnvElts (icReaderEnv icxt)
                                                  , let local_gres = filter isLocalGRE gres
@@ -2064,13 +2064,12 @@ runTcInteractive hsc_env thing_inside
                where
                  gbl_env' = gbl_env { tcg_rdr_env      = icReaderEnv icxt
                                     , tcg_type_env     = type_env
-                                    , tcg_inst_env     = extendInstEnvList
-                                                          (extendInstEnvList (tcg_inst_env gbl_env) ic_insts)
-                                                          home_insts
+
+                                    , tcg_inst_env     = tcg_inst_env gbl_env `unionInstEnv` ic_insts `unionInstEnv` home_insts
                                     , tcg_fam_inst_env = extendFamInstEnvList
-                                                          (extendFamInstEnvList (tcg_fam_inst_env gbl_env)
-                                                                                ic_finsts)
-                                                          home_fam_insts
+                                               (extendFamInstEnvList (tcg_fam_inst_env gbl_env)
+                                                                     ic_finsts)
+                                               home_fam_insts
                                     , tcg_field_env    = mkNameEnv con_fields
                                          -- setting tcg_field_env is necessary
                                          -- to make RecordWildCards work (test: ghci049)
@@ -2103,7 +2102,7 @@ runTcInteractive hsc_env thing_inside
       = Right thing
 
     type_env1 = mkTypeEnvWithImplicits top_ty_things
-    type_env  = extendTypeEnvWithIds type_env1 (map instanceDFunId ic_insts)
+    type_env  = extendTypeEnvWithIds type_env1 (map instanceDFunId (instEnvElts ic_insts))
                 -- Putting the dfuns in the type_env
                 -- is just to keep Core Lint happy