4 files changed, 18 insertions, 15 deletions
diff --git a/compiler/GHC/Iface/Ext/Ast.hs b/compiler/GHC/Iface/Ext/Ast.hs
index 24a68e63c4..5af938cbe7 100644
--- a/compiler/GHC/Iface/Ext/Ast.hs
+++ b/compiler/GHC/Iface/Ext/Ast.hs
@@ -303,7 +303,7 @@ mkHieFileWithSource src_file src ms ts rs =
       tcs = tcg_tcs ts
       (asts',arr) = getCompressedAsts tc_binds rs top_ev_binds insts tcs in
   HieFile
-      { hie_hs_file = src_file
+      { hie_hs_file = mkFastString src_file
       , hie_module = ms_mod ms
       , hie_types = arr
       , hie_asts = asts'
diff --git a/compiler/GHC/Iface/Ext/Binary.hs b/compiler/GHC/Iface/Ext/Binary.hs
index 6474fbeb8e..e537d2ecd8 100644
--- a/compiler/GHC/Iface/Ext/Binary.hs
+++ b/compiler/GHC/Iface/Ext/Binary.hs
@@ -32,6 +32,7 @@ import GHC.Types.SrcLoc as SrcLoc
 import GHC.Types.Unique
 import GHC.Types.Unique.FM
 
+import Data.Bifunctor             (first)
 import qualified Data.Array        as A
 import qualified Data.Array.IO     as A
 import qualified Data.Array.Unsafe as A
@@ -344,7 +345,7 @@ putHieName bh (LocalName occName span) = do
   put_ bh (occName, BinSrcSpan span)
 putHieName bh (KnownKeyName uniq) = do
   putByte bh 2
-  put_ bh $ unpkUnique uniq
+  put_ bh $ (first SerialisableChar $ unpkUnique uniq)
 
 getHieName :: BinHandle -> IO HieName
 getHieName bh = do
@@ -358,5 +359,5 @@ getHieName bh = do
       return $ LocalName occ $ unBinSrcSpan span
     2 -> do
       (c,i) <- get bh
-      return $ KnownKeyName $ mkUnique c i
+      return $ KnownKeyName $ mkUnique (getSerialisedChar c) i
     _ -> panic "GHC.Iface.Ext.Binary.getHieName: invalid tag"
diff --git a/compiler/GHC/Iface/Ext/Fields.hs b/compiler/GHC/Iface/Ext/Fields.hs
index 37322303d8..76cd936bdc 100644
--- a/compiler/GHC/Iface/Ext/Fields.hs
+++ b/compiler/GHC/Iface/Ext/Fields.hs
@@ -15,23 +15,25 @@ where
 
 import GHC.Prelude
 import GHC.Utils.Binary
+import GHC.Data.FastString
+import GHC.Types.Unique.Map
 
+import Data.Function (on)
+import Data.List (sortBy)
 import Control.Monad
-import Data.Map         ( Map )
-import qualified Data.Map as Map
 import Control.DeepSeq
 
-type FieldName = String
+type FieldName = FastString
 
-newtype ExtensibleFields = ExtensibleFields { getExtensibleFields :: (Map FieldName BinData) }
+newtype ExtensibleFields = ExtensibleFields { getExtensibleFields :: (UniqMap FastString BinData) }
 
 instance Binary ExtensibleFields where
   put_ bh (ExtensibleFields fs) = do
-    put_ bh (Map.size fs :: Int)
+    put_ bh (sizeUniqMap fs :: Int)
 
     -- Put the names of each field, and reserve a space
     -- for a payload pointer after each name:
-    header_entries <- forM (Map.toList fs) $ \(name, dat) -> do
+    header_entries <- forM (sortBy (lexicalCompareFS `on` fst) $ nonDetUniqMapToList fs) $ \(name, dat) -> do
       put_ bh name
       field_p_p <- tellBin bh
       put_ bh field_p_p
@@ -58,13 +60,13 @@ instance Binary ExtensibleFields where
       dat <- get bh
       return (name, dat)
 
-    return . ExtensibleFields . Map.fromList $ fields
+    return . ExtensibleFields . listToUniqMap $ fields
 
 instance NFData ExtensibleFields where
   rnf (ExtensibleFields fs) = rnf fs
 
 emptyExtensibleFields :: ExtensibleFields
-emptyExtensibleFields = ExtensibleFields Map.empty
+emptyExtensibleFields = ExtensibleFields emptyUniqMap
 
 --------------------------------------------------------------------------------
 -- | Reading
@@ -74,7 +76,7 @@ readField name = readFieldWith name get
 
 readFieldWith :: FieldName -> (BinHandle -> IO a) -> ExtensibleFields -> IO (Maybe a)
 readFieldWith name read fields = sequence $ ((read =<<) . dataHandle) <$>
-  Map.lookup name (getExtensibleFields fields)
+  lookupUniqMap (getExtensibleFields fields) name
 
 --------------------------------------------------------------------------------
 -- | Writing
@@ -88,7 +90,7 @@ writeFieldWith name write fields = do
   write bh
   --
   bd <- handleData bh
-  return $ ExtensibleFields (Map.insert name bd $ getExtensibleFields fields)
+  return $ ExtensibleFields (addToUniqMap (getExtensibleFields fields) name bd)
 
 deleteField :: FieldName -> ExtensibleFields -> ExtensibleFields
-deleteField name (ExtensibleFields fs) = ExtensibleFields $ Map.delete name fs
+deleteField name (ExtensibleFields fs) = ExtensibleFields $ delFromUniqMap fs name
diff --git a/compiler/GHC/Iface/Ext/Types.hs b/compiler/GHC/Iface/Ext/Types.hs
index b8a398465c..6fe5a9f7af 100644
--- a/compiler/GHC/Iface/Ext/Types.hs
+++ b/compiler/GHC/Iface/Ext/Types.hs
@@ -65,7 +65,7 @@ Besides saving compilation cycles, @.hie@ files also offer a more stable
 interface than the GHC API.
 -}
 data HieFile = HieFile
-    { hie_hs_file :: FilePath
+    { hie_hs_file :: FastString
     -- ^ Initial Haskell source file path
 
     , hie_module :: Module