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, 3 insertions, 3 deletions

diff --git a/compiler/GHC/Tc/Errors.hs b/compiler/GHC/Tc/Errors.hs
index 70f5d0ddd7..82aa8fcc6a 100644
--- a/compiler/GHC/Tc/Errors.hs
+++ b/compiler/GHC/Tc/Errors.hs
@@ -1897,7 +1897,7 @@ mkDictErr ctxt cts
     is_no_inst (ct, (matches, unifiers, _))
       =  no_givens
       && null matches
-      && (null unifiers || all (not . isAmbiguousTyVar) (tyCoVarsOfCtList ct))
+      && (nullUnifiers unifiers || all (not . isAmbiguousTyVar) (tyCoVarsOfCtList ct))
 
     lookup_cls_inst inst_envs ct
       = (ct, lookupInstEnv True inst_envs clas tys)
@@ -1988,13 +1988,13 @@ mk_dict_err ctxt (ct, (matches, unifiers, unsafe_overlapped))
 
     cannot_resolve_msg :: Ct -> [ClsInst] -> RelevantBindings -> [ImportError] -> [GhcHint] -> TcReportMsg
     cannot_resolve_msg ct candidate_insts binds imp_errs field_suggestions
-      = CannotResolveInstance ct unifiers candidate_insts imp_errs field_suggestions binds
+      = CannotResolveInstance ct (getPotentialUnifiers unifiers) candidate_insts imp_errs field_suggestions binds
 
     -- Overlap errors.
     overlap_msg, safe_haskell_msg :: TcReportMsg
     -- Normal overlap error
     overlap_msg
-      = assert (not (null matches)) $ OverlappingInstances ct ispecs unifiers
+      = assert (not (null matches)) $ OverlappingInstances ct ispecs (getPotentialUnifiers unifiers)
 
     -- Overlap error because of Safe Haskell (first
     -- match should be the most specific match)