3 files changed, 25 insertions, 17 deletions
diff --git a/compiler/utils/Encoding.hs b/compiler/utils/Encoding.hs
index 712de6ca82..f2b0979995 100644
--- a/compiler/utils/Encoding.hs
+++ b/compiler/utils/Encoding.hs
@@ -17,7 +17,8 @@ module Encoding (
         utf8PrevChar,
         utf8CharStart,
         utf8DecodeChar,
-        utf8DecodeString,
+        utf8DecodeByteString,
+        utf8DecodeStringLazy,
         utf8EncodeChar,
         utf8EncodeString,
         utf8EncodedLength,
@@ -33,9 +34,15 @@ module Encoding (
   ) where
 
 import Foreign
+import Foreign.ForeignPtr.Unsafe
 import Data.Char
 import qualified Data.Char as Char
 import Numeric
+import GHC.IO
+
+import Data.ByteString (ByteString)
+import qualified Data.ByteString.Internal as BS
+
 import GHC.Exts
 
 -- -----------------------------------------------------------------------------
@@ -115,19 +122,24 @@ utf8CharStart p = go p
                         then go (p `plusPtr` (-1))
                         else return p
 
-utf8DecodeString :: Ptr Word8 -> Int -> IO [Char]
-utf8DecodeString ptr len
-  = unpack ptr
+utf8DecodeByteString :: ByteString -> [Char]
+utf8DecodeByteString (BS.PS ptr offset len)
+  = utf8DecodeStringLazy ptr offset len
+
+utf8DecodeStringLazy :: ForeignPtr Word8 -> Int -> Int -> [Char]
+utf8DecodeStringLazy fptr offset len
+  = unsafeDupablePerformIO $ unpack start
   where
-    !end = ptr `plusPtr` len
+    !start = unsafeForeignPtrToPtr fptr `plusPtr` offset
+    !end = start `plusPtr` len
 
     unpack p
-        | p >= end = return []
-        | otherwise  =
-        case utf8DecodeChar# (unPtr p) of
-           (# c#, nBytes# #) -> do
-                chs <- unpack (p `plusPtr#` nBytes#)
-                return (C# c# : chs)
+        | p >= end  = touchForeignPtr fptr >> return []
+        | otherwise =
+            case utf8DecodeChar# (unPtr p) of
+                (# c#, nBytes# #) -> do
+                  rest <- unsafeDupableInterleaveIO $ unpack (p `plusPtr#` nBytes#)
+                  return (C# c# : rest)
 
 countUTF8Chars :: Ptr Word8 -> Int -> IO Int
 countUTF8Chars ptr len = go ptr 0
diff --git a/compiler/utils/FastString.hs b/compiler/utils/FastString.hs
index 8d1bbb5c67..8653485e0c 100644
--- a/compiler/utils/FastString.hs
+++ b/compiler/utils/FastString.hs
@@ -485,9 +485,7 @@ nullFS f = BS.null (fs_bs f)
 
 -- | Unpacks and decodes the FastString
 unpackFS :: FastString -> String
-unpackFS (FastString _ _ bs _) =
-  inlinePerformIO $ BS.unsafeUseAsCStringLen bs $ \(ptr, len) ->
-        utf8DecodeString (castPtr ptr) len
+unpackFS (FastString _ _ bs _) = utf8DecodeByteString bs
 
 -- | Gives the UTF-8 encoded bytes corresponding to a 'FastString'
 bytesFS :: FastString -> [Word8]
diff --git a/compiler/utils/StringBuffer.hs b/compiler/utils/StringBuffer.hs
index ec5184a1c2..fcc344554b 100644
--- a/compiler/utils/StringBuffer.hs
+++ b/compiler/utils/StringBuffer.hs
@@ -251,9 +251,7 @@ lexemeToString :: StringBuffer
                -> String
 lexemeToString _ 0 = ""
 lexemeToString (StringBuffer buf _ cur) bytes =
-  inlinePerformIO $
-    withForeignPtr buf $ \ptr ->
-      utf8DecodeString (ptr `plusPtr` cur) bytes
+  utf8DecodeStringLazy buf cur bytes
 
 lexemeToFastString :: StringBuffer
                    -> Int               -- ^ @n@, the number of bytes