compiler: Introduce and use RoughMap for instance environmentswip/roughmap-mp

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, 8 insertions, 10 deletions

diff --git a/compiler/GHC/Tc/Instance/Family.hs b/compiler/GHC/Tc/Instance/Family.hs
index 6ce522385b..8b76c9f4cd 100644
--- a/compiler/GHC/Tc/Instance/Family.hs
+++ b/compiler/GHC/Tc/Instance/Family.hs
@@ -700,7 +700,7 @@ checkForConflicts :: FamInstEnvs -> FamInst -> TcM ()
 checkForConflicts inst_envs fam_inst
   = do { let conflicts = lookupFamInstEnvConflicts inst_envs fam_inst
        ; traceTc "checkForConflicts" $
-         vcat [ ppr (map fim_instance conflicts)
+         vcat [ ppr conflicts
               , ppr fam_inst
               -- , ppr inst_envs
          ]
@@ -991,20 +991,18 @@ buildInjectivityError :: (TyCon -> NonEmpty CoAxBranch -> TcRnMessage)
 buildInjectivityError mkErr fam_tc branches
   = ( coAxBranchSpan (NE.head branches), mkErr fam_tc branches )
 
-reportConflictInstErr :: FamInst -> [FamInstMatch] -> TcRn ()
+reportConflictInstErr :: FamInst -> [FamInst] -> TcRn ()
 reportConflictInstErr _ []
   = return ()  -- No conflicts
-reportConflictInstErr fam_inst (match1 : _)
-  | FamInstMatch { fim_instance = conf_inst } <- match1
-  , let sorted  = NE.sortBy (SrcLoc.leftmost_smallest `on` getSpan) (fam_inst NE.:| [conf_inst])
-        fi1     = NE.head sorted
-        span    = coAxBranchSpan (coAxiomSingleBranch (famInstAxiom fi1))
-  = setSrcSpan span $ addErr $ TcRnConflictingFamInstDecls sorted
- where
-   getSpan = getSrcSpan . famInstAxiom
+reportConflictInstErr fam_inst (conf_inst : _) =
    -- The sortBy just arranges that instances are displayed in order
    -- of source location, which reduced wobbling in error messages,
    -- and is better for users
+  let   sorted  = NE.sortBy (SrcLoc.leftmost_smallest `on` getSpan) (fam_inst NE.:| [conf_inst])
+        fi1     = NE.head sorted
+        span    = coAxBranchSpan (coAxiomSingleBranch (famInstAxiom fi1))
+        getSpan = getSrcSpan . famInstAxiom
+  in setSrcSpan span $ addErr $ TcRnConflictingFamInstDecls sorted
 
 tcGetFamInstEnvs :: TcM FamInstEnvs
 -- Gets both the external-package inst-env