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/Utils/Backpack.hs b/compiler/GHC/Tc/Utils/Backpack.hs
index 20b81f8b3c..d553ec4fad 100644
--- a/compiler/GHC/Tc/Utils/Backpack.hs
+++ b/compiler/GHC/Tc/Utils/Backpack.hs
@@ -146,7 +146,7 @@ checkHsigIface tcg_env gr sig_iface
                         tcg_fam_inst_env = emptyFamInstEnv,
                         tcg_insts = [],
                         tcg_fam_insts = [] } $ do
-    mapM_ check_inst sig_insts
+    mapM_ check_inst (instEnvElts sig_insts)
     failIfErrsM
   where
     -- NB: the Names in sig_type_env are bogus.  Let's say we have H.hsig
@@ -156,7 +156,7 @@ checkHsigIface tcg_env gr sig_iface
     sig_type_occ_env = mkOccEnv
                      . map (\t -> (nameOccName (getName t), t))
                      $ nonDetNameEnvElts sig_type_env
-    dfun_names = map getName sig_insts
+    dfun_names = map getName (instEnvElts sig_insts)
     check_export name
       -- Skip instances, we'll check them later
       -- TODO: Actually this should never happen, because DFuns are
@@ -865,7 +865,7 @@ mergeSignatures
                 = (inst:insts, extendInstEnv inst_env inst)
             (insts, inst_env) = foldl' merge_inst
                                     (tcg_insts tcg_env, tcg_inst_env tcg_env)
-                                    (md_insts details)
+                                    (instEnvElts $ md_insts details)
             -- This is a HACK to prevent calculateAvails from including imp_mod
             -- in the listing.  We don't want it because a module is NOT
             -- supposed to include itself in its dep_orphs/dep_finsts.  See #13214