Modify two to enhance sharing

Previously, we passed `two` two unpacked key-value pairs:
`hx`, `kx`, `vx`, `hy`, `ky`, `vy`. But at every call site,
we already have `Leaf hy (L ky vy)`. So instead of building a
new copy of the leaf, we can just pass the one we have to
`two`.

Author: treeowl

Data/HashMap/Base.hs

@@ -675,7 +675,7 @@ insert' h0 k0 v0 m0 = go h0 k0 v0 0 m0
                          then t
                          else Leaf h (L k x)
                     else collision h l (L k x)
-        | otherwise = runST (two s h k x hy ky y)
+        | otherwise = runST (two s h k x hy t)
     go h k x s t@(BitmapIndexed b ary)
         | b .&. m == 0 =
             let !ary' = A.insert ary i $! Leaf h (L k x)
@@ -711,9 +711,9 @@ insertNewKey :: Hash -> k -> v -> HashMap k v -> HashMap k v
 insertNewKey !h0 !k0 x0 !m0 = go h0 k0 x0 0 m0
   where
     go !h !k x !_ Empty = Leaf h (L k x)
-    go h k x s (Leaf hy l@(L ky y))
+    go h k x s t@(Leaf hy l)
       | hy == h = collision h l (L k x)
-      | otherwise = runST (two s h k x hy ky y)
+      | otherwise = runST (two s h k x hy t)
     go h k x s (BitmapIndexed b ary)
         | b .&. m == 0 =
             let !ary' = A.insert ary i $! Leaf h (L k x)
@@ -800,7 +800,7 @@ unsafeInsert k0 v0 m0 = runST (go h0 k0 v0 0 m0)
                          then return t
                          else return $! Leaf h (L k x)
                     else return $! collision h l (L k x)
-        | otherwise = two s h k x hy ky y
+        | otherwise = two s h k x hy t
     go h k x s t@(BitmapIndexed b ary)
         | b .&. m == 0 = do
             ary' <- A.insertM ary i $! Leaf h (L k x)
@@ -823,18 +823,20 @@ unsafeInsert k0 v0 m0 = runST (go h0 k0 v0 0 m0)
         | otherwise = go h k x s $ BitmapIndexed (mask hy s) (A.singleton t)
 {-# INLINABLE unsafeInsert #-}
 
--- | Create a map from two key-value pairs which hashes don't collide.
-two :: Shift -> Hash -> k -> v -> Hash -> k -> v -> ST s (HashMap k v)
+-- | Create a map from two key-value pairs which hashes don't collide. To
+-- enhance sharing, the second key-value pair is represented by the hash of its
+-- key and a singleton HashMap pairing its key with its value.
+two :: Shift -> Hash -> k -> v -> Hash -> HashMap k v -> ST s (HashMap k v)
 two = go
   where
-    go s h1 k1 v1 h2 k2 v2
+    go s h1 k1 v1 h2 t2
         | bp1 == bp2 = do
-            st <- go (s+bitsPerSubkey) h1 k1 v1 h2 k2 v2
+            st <- go (s+bitsPerSubkey) h1 k1 v1 h2 t2
             ary <- A.singletonM st
             return $ BitmapIndexed bp1 ary
         | otherwise  = do
             mary <- A.new 2 $! Leaf h1 (L k1 v1)
-            A.write mary idx2 $! Leaf h2 (L k2 v2)
+            A.write mary idx2 $! t2
             ary <- A.unsafeFreeze mary
             return $! BitmapIndexed (bp1 .|. bp2) ary
       where
@@ -875,7 +877,7 @@ insertModifying x f k0 m0 = go h0 k0 0 m0
                       (# v' #) | ptrEq y v' -> t
                                | otherwise -> Leaf h (L k (v'))
                     else collision h l (L k x)
-        | otherwise = runST (two s h k x hy ky y)
+        | otherwise = runST (two s h k x hy t)
     go h k s t@(BitmapIndexed b ary)
         | b .&. m == 0 =
             let ary' = A.insert ary i $! Leaf h (L k x)
@@ -935,11 +937,11 @@ unsafeInsertWith f k0 v0 m0 = runST (go h0 k0 v0 0 m0)
     h0 = hash k0
     go :: Hash -> k -> v -> Shift -> HashMap k v -> ST s (HashMap k v)
     go !h !k x !_ Empty = return $! Leaf h (L k x)
-    go h k x s (Leaf hy l@(L ky y))
+    go h k x s t@(Leaf hy l@(L ky y))
         | hy == h = if ky == k
                     then return $! Leaf h (L k (f x y))
                     else return $! collision h l (L k x)
-        | otherwise = two s h k x hy ky y
+        | otherwise = two s h k x hy t
     go h k x s t@(BitmapIndexed b ary)
         | b .&. m == 0 = do
             ary' <- A.insertM ary i $! Leaf h (L k x)

Data/HashMap/Strict/Base.hs

@@ -152,11 +152,11 @@ insertWith f k0 v0 m0 = go h0 k0 v0 0 m0
   where
     h0 = hash k0
     go !h !k x !_ Empty = leaf h k x
-    go h k x s (Leaf hy l@(L ky y))
+    go h k x s t@(Leaf hy l@(L ky y))
         | hy == h = if ky == k
                     then leaf h k (f x y)
                     else x `seq` (collision h l (L k x))
-        | otherwise = x `seq` runST (two s h k x hy ky y)
+        | otherwise = x `seq` runST (two s h k x hy t)
     go h k x s (BitmapIndexed b ary)
         | b .&. m == 0 =
             let ary' = A.insert ary i $! leaf h k x
@@ -186,13 +186,13 @@ unsafeInsertWith f k0 v0 m0 = runST (go h0 k0 v0 0 m0)
   where
     h0 = hash k0
     go !h !k x !_ Empty = return $! leaf h k x
-    go h k x s (Leaf hy l@(L ky y))
+    go h k x s t@(Leaf hy l@(L ky y))
         | hy == h = if ky == k
                     then return $! leaf h k (f x y)
                     else do
                         let l' = x `seq` (L k x)
                         return $! collision h l l'
-        | otherwise = x `seq` two s h k x hy ky y
+        | otherwise = x `seq` two s h k x hy t
     go h k x s t@(BitmapIndexed b ary)
         | b .&. m == 0 = do
             ary' <- A.insertM ary i $! leaf h k x