diff --git a/GHC/Data/SizedSeq.hs b/GHC/Data/SizedSeq.hs
--- a/GHC/Data/SizedSeq.hs
+++ b/GHC/Data/SizedSeq.hs
@@ -1,4 +1,4 @@
-{-# LANGUAGE StandaloneDeriving, DeriveGeneric #-}
+{-# LANGUAGE StandaloneDeriving, DeriveGeneric, CPP #-}
 module GHC.Data.SizedSeq
   ( SizedSeq(..)
   , emptySS
@@ -11,9 +11,12 @@
 import Prelude -- See note [Why do we import Prelude here?]
 import Control.DeepSeq
 import Data.Binary
-import Data.List (genericLength)
 import GHC.Generics
 
+#if ! MIN_VERSION_base(4,20,0)
+import Data.List (foldl')
+#endif
+
 data SizedSeq a = SizedSeq {-# UNPACK #-} !Word [a]
   deriving (Generic, Show)
 
@@ -37,9 +40,9 @@
 addToSS :: SizedSeq a -> a -> SizedSeq a
 addToSS (SizedSeq n r_xs) x = SizedSeq (n+1) (x:r_xs)
 
+-- NB, important this is eta-expand so that foldl' is inlined.
 addListToSS :: SizedSeq a -> [a] -> SizedSeq a
-addListToSS (SizedSeq n r_xs) xs
-  = SizedSeq (n + genericLength xs) (reverse xs ++ r_xs)
+addListToSS s xs = foldl' addToSS s xs
 
 ssElts :: SizedSeq a -> [a]
 ssElts (SizedSeq _ r_xs) = reverse r_xs
diff --git a/ghc-boot.cabal b/ghc-boot.cabal
--- a/ghc-boot.cabal
+++ b/ghc-boot.cabal
@@ -5,7 +5,7 @@
 -- ghc-boot.cabal.
 
 name:           ghc-boot
-version:        9.10.2
+version:        9.10.3
 license:        BSD-3-Clause
 license-file:   LICENSE
 category:       GHC
@@ -81,7 +81,7 @@
                    filepath   >= 1.3 && < 1.6,
                    deepseq    >= 1.4 && < 1.6,
                    ghc-platform >= 0.1,
-                   ghc-boot-th == 9.10.2
+                   ghc-boot-th == 9.10.3
     if !os(windows)
         build-depends:
                    unix       >= 2.7 && < 2.9