Use intersect and minus instead of filter

These are asymptotically better and convey the intent
a bit better.

Test Plan: ./validate

Reviewers: simonpj, bgamari, austin, goldfire

Reviewed By: bgamari

Subscribers: rwbarton, thomie, simonmar

Differential Revision: https://phabricator.haskell.org/D3455

5 files changed, 13 insertions, 6 deletions

diff --git a/compiler/basicTypes/VarSet.hs b/compiler/basicTypes/VarSet.hs
index 8877f64080..e4f0d25dfb 100644
--- a/compiler/basicTypes/VarSet.hs
+++ b/compiler/basicTypes/VarSet.hs
@@ -32,7 +32,8 @@ module VarSet (
         extendDVarSet, extendDVarSetList,
         elemDVarSet, dVarSetElems, subDVarSet,
         unionDVarSet, unionDVarSets, mapUnionDVarSet,
-        intersectDVarSet, intersectsDVarSet, disjointDVarSet,
+        intersectDVarSet, dVarSetIntersectVarSet,
+        intersectsDVarSet, disjointDVarSet,
         isEmptyDVarSet, delDVarSet, delDVarSetList,
         minusDVarSet, foldDVarSet, filterDVarSet,
         dVarSetMinusVarSet, anyDVarSet, allDVarSet,
@@ -259,6 +260,9 @@ mapUnionDVarSet get_set xs = foldr (unionDVarSet . get_set) emptyDVarSet xs
 intersectDVarSet :: DVarSet -> DVarSet -> DVarSet
 intersectDVarSet = intersectUniqDSets
 
+dVarSetIntersectVarSet :: DVarSet -> VarSet -> DVarSet
+dVarSetIntersectVarSet = uniqDSetIntersectUniqSet
+
 -- | True if empty intersection
 disjointDVarSet :: DVarSet -> DVarSet -> Bool
 disjointDVarSet s1 s2 = disjointUDFM s1 s2
diff --git a/compiler/typecheck/TcSimplify.hs b/compiler/typecheck/TcSimplify.hs
index e5f3fe97cc..2822985f54 100644
--- a/compiler/typecheck/TcSimplify.hs
+++ b/compiler/typecheck/TcSimplify.hs
@@ -956,7 +956,7 @@ decideQuantifiedTyVars mono_tvs name_taus psigs candidates
        ; let DV {dv_kvs = cand_kvs, dv_tvs = cand_tvs}
                       = candidateQTyVarsOfTypes $
                         psig_tys ++ candidates ++ tau_tys
-             pick     = filterDVarSet (`elemVarSet` grown_tvs)
+             pick     = (`dVarSetIntersectVarSet` grown_tvs)
              dvs_plus = DV { dv_kvs = pick cand_kvs, dv_tvs = pick cand_tvs }
 
        ; mono_tvs <- TcM.zonkTyCoVarsAndFV mono_tvs
diff --git a/compiler/typecheck/TcType.hs b/compiler/typecheck/TcType.hs
index c76647cfa7..ab2f8430d8 100644
--- a/compiler/typecheck/TcType.hs
+++ b/compiler/typecheck/TcType.hs
@@ -1092,7 +1092,7 @@ split_dvs bound dvs ty
 
     kill_bound free
       | isEmptyVarSet bound = free
-      | otherwise           = filterDVarSet (not . (`elemVarSet` bound)) free
+      | otherwise           = free `dVarSetMinusVarSet` bound
 
 -- | Like 'splitDepVarsOfType', but over a list of types
 candidateQTyVarsOfTypes :: [Type] -> CandidatesQTvs
diff --git a/compiler/utils/UniqDFM.hs b/compiler/utils/UniqDFM.hs
index 9f81e4dca6..17f2747f83 100644
--- a/compiler/utils/UniqDFM.hs
+++ b/compiler/utils/UniqDFM.hs
@@ -294,7 +294,7 @@ intersectUDFM (UDFM x i) (UDFM y _j) = UDFM (M.intersection x y) i
   -- M.intersection is left biased, that means the result will only have
   -- a subset of elements from the left set, so `i` is a good upper bound.
 
-udfmIntersectUFM :: UniqDFM elt -> UniqFM elt -> UniqDFM elt
+udfmIntersectUFM :: UniqDFM elt1 -> UniqFM elt2 -> UniqDFM elt1
 udfmIntersectUFM (UDFM x i) y = UDFM (M.intersection x (ufmToIntMap y)) i
   -- M.intersection is left biased, that means the result will only have
   -- a subset of elements from the left set, so `i` is a good upper bound.
diff --git a/compiler/utils/UniqDSet.hs b/compiler/utils/UniqDSet.hs
index 4e8c7ed97f..eef545eedd 100644
--- a/compiler/utils/UniqDSet.hs
+++ b/compiler/utils/UniqDSet.hs
@@ -20,7 +20,7 @@ module UniqDSet (
         addOneToUniqDSet, addListToUniqDSet,
         unionUniqDSets, unionManyUniqDSets,
         minusUniqDSet, uniqDSetMinusUniqSet,
-        intersectUniqDSets,
+        intersectUniqDSets, uniqDSetIntersectUniqSet,
         intersectsUniqDSets,
         foldUniqDSet,
         elementOfUniqDSet,
@@ -69,12 +69,15 @@ unionManyUniqDSets sets = foldr1 unionUniqDSets sets
 minusUniqDSet :: UniqDSet a -> UniqDSet a -> UniqDSet a
 minusUniqDSet = minusUDFM
 
-uniqDSetMinusUniqSet :: UniqDSet a -> UniqSet a -> UniqDSet a
+uniqDSetMinusUniqSet :: UniqDSet a -> UniqSet b -> UniqDSet a
 uniqDSetMinusUniqSet xs ys = udfmMinusUFM xs (getUniqSet ys)
 
 intersectUniqDSets :: UniqDSet a -> UniqDSet a -> UniqDSet a
 intersectUniqDSets = intersectUDFM
 
+uniqDSetIntersectUniqSet :: UniqDSet a -> UniqSet b -> UniqDSet a
+uniqDSetIntersectUniqSet xs ys = xs `udfmIntersectUFM` getUniqSet ys
+
 intersectsUniqDSets :: UniqDSet a -> UniqDSet a -> Bool
 intersectsUniqDSets = intersectsUDFM