From 7f017fe07ebe2184193d43bd9c1a39fa7e63959e Mon Sep 17 00:00:00 2001
From: Simon Jakobi <simon.jakobi@gmail.com>
Date: Thu, 12 May 2022 00:06:06 +0200
Subject: [PATCH 1/3] Try something else for avoiding allocations on no-ops

---
 Data/HashMap/Internal.hs | 50 +++++++++++++++++++++++++---------------
 1 file changed, 32 insertions(+), 18 deletions(-)

diff --git a/Data/HashMap/Internal.hs b/Data/HashMap/Internal.hs
index 79c6babb..2a9ababf 100644
--- a/Data/HashMap/Internal.hs
+++ b/Data/HashMap/Internal.hs
@@ -4,6 +4,7 @@
 {-# LANGUAGE LambdaCase            #-}
 {-# LANGUAGE MagicHash             #-}
 {-# LANGUAGE PatternGuards         #-}
+{-# LANGUAGE PatternSynonyms       #-}
 {-# LANGUAGE RoleAnnotations       #-}
 {-# LANGUAGE ScopedTypeVariables   #-}
 {-# LANGUAGE StandaloneDeriving    #-}
@@ -803,37 +804,38 @@ insert k v m = insert' (hash k) k v m
 {-# INLINABLE insert #-}
 
 insert' :: Eq k => Hash -> k -> v -> HashMap k v -> HashMap k v
-insert' h0 k0 v0 m0 = go h0 k0 v0 0 m0
+insert' h0 k0 v0 m0 =
+    case go h0 k0 v0 0 m0 of
+      NoChange  -> m0
+      Changed m -> m
   where
-    go !h !k x !_ Empty = Leaf h (L k x)
+    go !h !k x !_ Empty = Changed (Leaf h (L k x))
     go h k x s t@(Leaf hy l@(L ky y))
         | hy == h = if ky == k
                     then if x `ptrEq` y
-                         then t
-                         else Leaf h (L k x)
-                    else collision h l (L k x)
-        | otherwise = runST (two s h k x hy t)
-    go h k x s t@(BitmapIndexed b ary)
+                         then NoChange
+                         else Changed (Leaf h (L k x))
+                    else Changed $ collision h l (L k x)
+        | otherwise = Changed $ 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)
-            in bitmapIndexedOrFull (b .|. m) ary'
+            in Changed $ bitmapIndexedOrFull (b .|. m) ary'
         | otherwise =
             let !st  = A.index ary i
-                !st' = go h k x (nextShift s) st
-            in if st' `ptrEq` st
-               then t
-               else BitmapIndexed b (A.update ary i st')
+            in case go h k x (nextShift s) st of
+                 NoChange -> NoChange
+                 Changed st' -> Changed $ BitmapIndexed b (A.update ary i st')
       where m = mask h s
             i = sparseIndex b m
-    go h k x s t@(Full ary) =
+    go h k x s (Full ary) =
         let !st  = A.index ary i
-            !st' = go h k x (nextShift s) st
-        in if st' `ptrEq` st
-            then t
-            else Full (update32 ary i st')
+        in case go h k x (nextShift s) st of
+             NoChange -> NoChange
+             Changed st' -> Changed $ Full (update32 ary i st')
       where i = index h s
     go h k x s t@(Collision hy v)
-        | h == hy   = Collision h (updateOrSnocWith (\a _ -> (# a #)) k x v)
+        | h == hy   = Changed $ Collision h (updateOrSnocWith (\a _ -> (# a #)) k x v) -- TODO: Improve
         | otherwise = go h k x s $ BitmapIndexed (mask hy s) (A.singleton t)
 {-# INLINABLE insert' #-}
 
@@ -2502,6 +2504,18 @@ ptrEq :: a -> a -> Bool
 ptrEq x y = Exts.isTrue# (Exts.reallyUnsafePtrEquality# x y ==# 1#)
 {-# INLINE ptrEq #-}
 
+type Change a = (# (# #) | a #)
+
+pattern NoChange :: Change a
+pattern NoChange <- (# (# #) | #)
+  where NoChange = (# (# #) | #)
+
+pattern Changed :: a -> Change a
+pattern Changed a <- (# | a #) where
+  Changed a = (# | a #)
+
+{-# COMPLETE NoChange, Changed #-}
+
 ------------------------------------------------------------------------
 -- IsList instance
 instance (Eq k, Hashable k) => Exts.IsList (HashMap k v) where

From 2e0014e7be62071f478f995060910c95cf028218 Mon Sep 17 00:00:00 2001
From: Simon Jakobi <simon.jakobi@gmail.com>
Date: Thu, 12 May 2022 00:19:59 +0200
Subject: [PATCH 2/3] Avoid re-allocating NoChanges

---
 Data/HashMap/Internal.hs | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/Data/HashMap/Internal.hs b/Data/HashMap/Internal.hs
index 2a9ababf..69795650 100644
--- a/Data/HashMap/Internal.hs
+++ b/Data/HashMap/Internal.hs
@@ -824,15 +824,15 @@ insert' h0 k0 v0 m0 =
         | otherwise =
             let !st  = A.index ary i
             in case go h k x (nextShift s) st of
-                 NoChange -> NoChange
                  Changed st' -> Changed $ BitmapIndexed b (A.update ary i st')
+                 noChange    -> noChange
       where m = mask h s
             i = sparseIndex b m
     go h k x s (Full ary) =
         let !st  = A.index ary i
         in case go h k x (nextShift s) st of
-             NoChange -> NoChange
              Changed st' -> Changed $ Full (update32 ary i st')
+             noChange    -> noChange
       where i = index h s
     go h k x s t@(Collision hy v)
         | h == hy   = Changed $ Collision h (updateOrSnocWith (\a _ -> (# a #)) k x v) -- TODO: Improve

From 2cc5d07078c339b7f14fd3e13c32c958ff541f36 Mon Sep 17 00:00:00 2001
From: Simon Jakobi <simon.jakobi@gmail.com>
Date: Thu, 12 May 2022 00:26:53 +0200
Subject: [PATCH 3/3] Changed is strict now

---
 Data/HashMap/Internal.hs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Data/HashMap/Internal.hs b/Data/HashMap/Internal.hs
index 69795650..10b37bd0 100644
--- a/Data/HashMap/Internal.hs
+++ b/Data/HashMap/Internal.hs
@@ -2512,7 +2512,7 @@ pattern NoChange <- (# (# #) | #)
 
 pattern Changed :: a -> Change a
 pattern Changed a <- (# | a #) where
-  Changed a = (# | a #)
+  Changed !a = (# | a #)
 
 {-# COMPLETE NoChange, Changed #-}