utils/FastString.hs

    -- (c) The University of Glasgow, 1997-2006

    {-# LANGUAGE BangPatterns, CPP, MagicHash, UnboxedTuples,
        GeneralizedNewtypeDeriving #-}
    {-# OPTIONS_GHC -O2 -funbox-strict-fields #-}
    -- We always optimise this, otherwise performance of a non-optimised
    -- compiler is severely affected

    -- |
    -- There are two principal string types used internally by GHC:
    --
    -- ['FastString']
    --
    --   * A compact, hash-consed, representation of character strings.
    --   * Comparison is O(1), and you can get a 'Unique.Unique' from them.
    --   * Generated by 'fsLit'.
    --   * Turn into 'Outputable.SDoc' with 'Outputable.ftext'.
    --
    -- ['PtrString']
    --
    --   * Pointer and size of a Latin-1 encoded string.
    --   * Practically no operations.
    --   * Outputing them is fast.
    --   * Generated by 'sLit'.
    --   * Turn into 'Outputable.SDoc' with 'Outputable.ptext'
    --   * Requires manual memory management.
    --     Improper use may lead to memory leaks or dangling pointers.
    --   * It assumes Latin-1 as the encoding, therefore it cannot represent
    --     arbitrary Unicode strings.
    --
    -- Use 'PtrString' unless you want the facilities of 'FastString'.
    module FastString
           (
            -- * ByteString
            fastStringToByteString,
            mkFastStringByteString,
            fastZStringToByteString,
            unsafeMkByteString,

            -- * FastZString
            FastZString,
            hPutFZS,
            zString,
            lengthFZS,

            -- * FastStrings
            FastString(..),     -- not abstract, for now.

            -- ** Construction
            fsLit,
            mkFastString,
            mkFastStringBytes,
            mkFastStringByteList,
            mkFastStringForeignPtr,
            mkFastString#,

            -- ** Deconstruction
            unpackFS,           -- :: FastString -> String
            bytesFS,            -- :: FastString -> [Word8]

            -- ** Encoding
            zEncodeFS,

            -- ** Operations
            uniqueOfFS,
            lengthFS,
            nullFS,
            appendFS,
            headFS,
            tailFS,
            concatFS,
            consFS,
            nilFS,
            isUnderscoreFS,

            -- ** Outputing
            hPutFS,

            -- ** Internal
            getFastStringTable,
            hasZEncoding,

            -- * PtrStrings
            PtrString (..),

            -- ** Construction
            sLit,
            mkPtrString#,
            mkPtrString,

            -- ** Deconstruction
            unpackPtrString,

            -- ** Operations
            lengthPS
           ) where

    #include "HsVersions.h"

    import GhcPrelude as Prelude

    import Encoding
    import FastFunctions
    import Panic
    import Util

    import Control.Concurrent.MVar
    import Control.DeepSeq
    import Control.Monad
    import Data.ByteString (ByteString)
    import qualified Data.ByteString          as BS
    import qualified Data.ByteString.Char8    as BSC
    import qualified Data.ByteString.Internal as BS
    import qualified Data.ByteString.Unsafe   as BS
    import Foreign.C
    import GHC.Exts
    import System.IO
    import Data.Data
    import Data.IORef
    import Data.Maybe       ( isJust )
    import Data.Char
    import Data.Semigroup as Semi

    import GHC.IO

    import Foreign

    #if STAGE >= 2
    import GHC.Conc.Sync    (sharedCAF)
    #endif

    import GHC.Base         ( unpackCString#, unpackNBytes# )

    fastStringToByteString :: FastString -> ByteString
    fastStringToByteString f = fs_bs f

    fastZStringToByteString :: FastZString -> ByteString
    fastZStringToByteString (FastZString bs) = bs

    -- This will drop information if any character > '\xFF'
    unsafeMkByteString :: String -> ByteString
    unsafeMkByteString = BSC.pack

    hashFastString :: FastString -> Int
    hashFastString (FastString _ _ bs _)
        = inlinePerformIO $ BS.unsafeUseAsCStringLen bs $ \(ptr, len) ->
          return $ hashStr (castPtr ptr) len

    -- -----------------------------------------------------------------------------

    newtype FastZString = FastZString ByteString
      deriving NFData

    hPutFZS :: Handle -> FastZString -> IO ()
    hPutFZS handle (FastZString bs) = BS.hPut handle bs

    zString :: FastZString -> String
    zString (FastZString bs) =
        inlinePerformIO $ BS.unsafeUseAsCStringLen bs peekCAStringLen

    lengthFZS :: FastZString -> Int
    lengthFZS (FastZString bs) = BS.length bs

    mkFastZStringString :: String -> FastZString
    mkFastZStringString str = FastZString (BSC.pack str)

    -- -----------------------------------------------------------------------------

    {-|
    A 'FastString' is an array of bytes, hashed to support fast O(1)
    comparison.  It is also associated with a character encoding, so that
    we know how to convert a 'FastString' to the local encoding, or to the
    Z-encoding used by the compiler internally.

    'FastString's support a memoized conversion to the Z-encoding via zEncodeFS.
    -}

    data FastString = FastString {
          uniq    :: {-# UNPACK #-} !Int, -- unique id
          n_chars :: {-# UNPACK #-} !Int, -- number of chars
          fs_bs   :: {-# UNPACK #-} !ByteString,
          fs_ref  :: {-# UNPACK #-} !(IORef (Maybe FastZString))
      }

    instance Eq FastString where
      f1 == f2  =  uniq f1 == uniq f2

    instance Ord FastString where
        -- Compares lexicographically, not by unique
        a <= b = case cmpFS a b of { LT -> True;  EQ -> True;  GT -> False }
        a <  b = case cmpFS a b of { LT -> True;  EQ -> False; GT -> False }
        a >= b = case cmpFS a b of { LT -> False; EQ -> True;  GT -> True  }
        a >  b = case cmpFS a b of { LT -> False; EQ -> False; GT -> True  }
        max x y | x >= y    =  x
                | otherwise =  y
        min x y | x <= y    =  x
                | otherwise =  y
        compare a b = cmpFS a b

    instance IsString FastString where
        fromString = fsLit

    instance Semi.Semigroup FastString where
        (<>) = appendFS

    instance Monoid FastString where
        mempty = nilFS
        mappend = (Semi.<>)
        mconcat = concatFS

    instance Show FastString where
       show fs = show (unpackFS fs)

    instance Data FastString where
      -- don't traverse?
      toConstr _   = abstractConstr "FastString"
      gunfold _ _  = error "gunfold"
      dataTypeOf _ = mkNoRepType "FastString"

    cmpFS :: FastString -> FastString -> Ordering
    cmpFS f1@(FastString u1 _ _ _) f2@(FastString u2 _ _ _) =
      if u1 == u2 then EQ else
      compare (fastStringToByteString f1) (fastStringToByteString f2)

    foreign import ccall unsafe "memcmp"
      memcmp :: Ptr a -> Ptr b -> Int -> IO Int

    -- -----------------------------------------------------------------------------
    -- Construction

    {-
    Internally, the compiler will maintain a fast string symbol table, providing
    sharing and fast comparison. Creation of new @FastString@s then covertly does a
    lookup, re-using the @FastString@ if there was a hit.

    The design of the FastString hash table allows for lockless concurrent reads
    and updates to multiple buckets with low synchronization overhead.

    See Note [Updating the FastString table] on how it's updated.
    -}
    data FastStringTable = FastStringTable
      {-# UNPACK #-} !(IORef Int) -- the unique ID counter shared with all buckets
      (Array# (IORef FastStringTableSegment)) -- concurrent segments

    data FastStringTableSegment = FastStringTableSegment
      {-# UNPACK #-} !(MVar ()) -- the lock for write in each segment
      {-# UNPACK #-} !(IORef Int) -- the number of elements
      (MutableArray# RealWorld [FastString]) -- buckets in this segment

    {-
    Following parameters are determined based on:

    * Benchmark based on testsuite/tests/utils/should_run/T14854.hs
    * Stats of @echo :browse | ghc --interactive -dfaststring-stats >/dev/null@:
      on 2018-10-24, we have 13920 entries.
    -}
    segmentBits, numSegments, segmentMask, initialNumBuckets :: Int
    segmentBits = 8
    numSegments = 256   -- bit segmentBits
    segmentMask = 0xff  -- bit segmentBits - 1
    initialNumBuckets = 64

    hashToSegment# :: Int# -> Int#
    hashToSegment# hash# = hash# `andI#` segmentMask#
      where
        !(I# segmentMask#) = segmentMask

    hashToIndex# :: MutableArray# RealWorld [FastString] -> Int# -> Int#
    hashToIndex# buckets# hash# =
      (hash# `uncheckedIShiftRL#` segmentBits#) `remInt#` size#
      where
        !(I# segmentBits#) = segmentBits
        size# = sizeofMutableArray# buckets#

    maybeResizeSegment :: IORef FastStringTableSegment -> IO FastStringTableSegment
    maybeResizeSegment segmentRef = do
      segment@(FastStringTableSegment lock counter old#) <- readIORef segmentRef
      let oldSize# = sizeofMutableArray# old#
          newSize# = oldSize# *# 2#
      (I# n#) <- readIORef counter
      if isTrue# (n# <# newSize#) -- maximum load of 1
      then return segment
      else do
        resizedSegment@(FastStringTableSegment _ _ new#) <- IO $ \s1# ->
          case newArray# newSize# [] s1# of
            (# s2#, arr# #) -> (# s2#, FastStringTableSegment lock counter arr# #)
        forM_ [0 .. (I# oldSize#) - 1] $ \(I# i#) -> do
          fsList <- IO $ readArray# old# i#
          forM_ fsList $ \fs -> do
            let -- Shall we store in hash value in FastString instead?
                !(I# hash#) = hashFastString fs
                idx# = hashToIndex# new# hash#
            IO $ \s1# ->
              case readArray# new# idx# s1# of
                (# s2#, bucket #) -> case writeArray# new# idx# (fs: bucket) s2# of
                  s3# -> (# s3#, () #)
        writeIORef segmentRef resizedSegment
        return resizedSegment

    {-# NOINLINE stringTable #-}
    stringTable :: FastStringTable
    stringTable = unsafePerformIO $ do
      let !(I# numSegments#) = numSegments
          !(I# initialNumBuckets#) = initialNumBuckets
          loop a# i# s1#
            | isTrue# (i# ==# numSegments#) = s1#
            | otherwise = case newMVar () `unIO` s1# of
                (# s2#, lock #) -> case newIORef 0 `unIO` s2# of
                  (# s3#, counter #) -> case newArray# initialNumBuckets# [] s3# of
                    (# s4#, buckets# #) -> case newIORef
                        (FastStringTableSegment lock counter buckets#) `unIO` s4# of
                      (# s5#, segment #) -> case writeArray# a# i# segment s5# of
                        s6# -> loop a# (i# +# 1#) s6#
      uid <- newIORef 603979776 -- ord '$' * 0x01000000
      tab <- IO $ \s1# ->
        case newArray# numSegments# (panic "string_table") s1# of
          (# s2#, arr# #) -> case loop arr# 0# s2# of
            s3# -> case unsafeFreezeArray# arr# s3# of
              (# s4#, segments# #) -> (# s4#, FastStringTable uid segments# #)

      -- use the support wired into the RTS to share this CAF among all images of
      -- libHSghc
    #if STAGE < 2
      return tab
    #else
      sharedCAF tab getOrSetLibHSghcFastStringTable

    -- from the RTS; thus we cannot use this mechanism when STAGE<2; the previous
    -- RTS might not have this symbol
    foreign import ccall unsafe "getOrSetLibHSghcFastStringTable"
      getOrSetLibHSghcFastStringTable :: Ptr a -> IO (Ptr a)
    #endif

    {-

    We include the FastString table in the `sharedCAF` mechanism because we'd like
    FastStrings created by a Core plugin to have the same uniques as corresponding
    strings created by the host compiler itself.  For example, this allows plugins
    to lookup known names (eg `mkTcOcc "MySpecialType"`) in the GlobalRdrEnv or
    even re-invoke the parser.

    In particular, the following little sanity test was failing in a plugin
    prototyping safe newtype-coercions: GHC.NT.Type.NT was imported, but could not
    be looked up /by the plugin/.

       let rdrName = mkModuleName "GHC.NT.Type" `mkRdrQual` mkTcOcc "NT"
       putMsgS $ showSDoc dflags $ ppr $ lookupGRE_RdrName rdrName $ mg_rdr_env guts

    `mkTcOcc` involves the lookup (or creation) of a FastString.  Since the
    plugin's FastString.string_table is empty, constructing the RdrName also
    allocates new uniques for the FastStrings "GHC.NT.Type" and "NT".  These
    uniques are almost certainly unequal to the ones that the host compiler
    originally assigned to those FastStrings.  Thus the lookup fails since the
    domain of the GlobalRdrEnv is affected by the RdrName's OccName's FastString's
    unique.

    Maintaining synchronization of the two instances of this global is rather
    difficult because of the uses of `unsafePerformIO` in this module.  Not
    synchronizing them risks breaking the rather major invariant that two
    FastStrings with the same unique have the same string. Thus we use the
    lower-level `sharedCAF` mechanism that relies on Globals.c.

    -}

    mkFastString# :: Addr# -> FastString
    mkFastString# a# = mkFastStringBytes ptr (ptrStrLength ptr)
      where ptr = Ptr a#

    {- Note [Updating the FastString table]

    We use a concurrent hashtable which contains multiple segments, each hash value
    always maps to the same segment. Read is lock-free, write to the a segment
    should acquire a lock for that segment to avoid race condition, writes to
    different segments are independent.

    The procedure goes like this:

    1. Find out which segment to operate on based on the hash value
    2. Read the relevant bucket and perform a look up of the string.
    3. If it exists, return it.
    4. Otherwise grab a unique ID, create a new FastString and atomically attempt
       to update the relevant segment with this FastString:

       * Resize the segment by doubling the number of buckets when the number of
         FastStrings in this segment grows beyond the threshold.
       * Double check that the string is not in the bucket. Another thread may have
         inserted it while we were creating our string.
       * Return the existing FastString if it exists. The one we preemptively
         created will get GCed.
       * Otherwise, insert and return the string we created.
    -}

    mkFastStringWith :: (Int -> IO FastString) -> Ptr Word8 -> Int -> IO FastString
    mkFastStringWith mk_fs !ptr !len = do
      FastStringTableSegment lock _ buckets# <- readIORef segmentRef
      let idx# = hashToIndex# buckets# hash#
      bucket <- IO $ readArray# buckets# idx#
      res <- bucket_match bucket len ptr
      case res of
        Just found -> return found
        Nothing -> do
          -- The withMVar below is not dupable. It can lead to deadlock if it is
          -- only run partially and putMVar is not called after takeMVar.
          noDuplicate
          n <- get_uid
          new_fs <- mk_fs n
          withMVar lock $ \_ -> insert new_fs
      where
        !(FastStringTable uid segments#) = stringTable
        get_uid = atomicModifyIORef' uid $ \n -> (n+1,n)

        !(I# hash#) = hashStr ptr len
        (# segmentRef #) = indexArray# segments# (hashToSegment# hash#)
        insert fs = do
          FastStringTableSegment _ counter buckets# <- maybeResizeSegment segmentRef
          let idx# = hashToIndex# buckets# hash#
          bucket <- IO $ readArray# buckets# idx#
          res <- bucket_match bucket len ptr
          case res of
            -- The FastString was added by another thread after previous read and
            -- before we acquired the write lock.
            Just found -> return found
            Nothing -> do
              IO $ \s1# ->
                case writeArray# buckets# idx# (fs: bucket) s1# of
                  s2# -> (# s2#, () #)
              modifyIORef' counter succ
              return fs

    bucket_match :: [FastString] -> Int -> Ptr Word8 -> IO (Maybe FastString)
    bucket_match [] _ _ = return Nothing
    bucket_match (v@(FastString _ _ bs _):ls) len ptr
          | len == BS.length bs = do
             b <- BS.unsafeUseAsCString bs $ \buf ->
                 cmpStringPrefix ptr (castPtr buf) len
             if b then return (Just v)
                  else bucket_match ls len ptr
          | otherwise =
             bucket_match ls len ptr

    mkFastStringBytes :: Ptr Word8 -> Int -> FastString
    mkFastStringBytes !ptr !len =
        -- NB: Might as well use unsafeDupablePerformIO, since mkFastStringWith is
        -- idempotent.
        unsafeDupablePerformIO $
            mkFastStringWith (copyNewFastString ptr len) ptr len

    -- | Create a 'FastString' from an existing 'ForeignPtr'; the difference
    -- between this and 'mkFastStringBytes' is that we don't have to copy
    -- the bytes if the string is new to the table.
    mkFastStringForeignPtr :: Ptr Word8 -> ForeignPtr Word8 -> Int -> IO FastString
    mkFastStringForeignPtr ptr !fp len
        = mkFastStringWith (mkNewFastString fp ptr len) ptr len

    -- | Create a 'FastString' from an existing 'ForeignPtr'; the difference
    -- between this and 'mkFastStringBytes' is that we don't have to copy
    -- the bytes if the string is new to the table.
    mkFastStringByteString :: ByteString -> FastString
    mkFastStringByteString bs =
        inlinePerformIO $
          BS.unsafeUseAsCStringLen bs $ \(ptr, len) -> do
            let ptr' = castPtr ptr
            mkFastStringWith (mkNewFastStringByteString bs ptr' len) ptr' len

    -- | Creates a UTF-8 encoded 'FastString' from a 'String'
    mkFastString :: String -> FastString
    mkFastString str =
      inlinePerformIO $ do
        let l = utf8EncodedLength str
        buf <- mallocForeignPtrBytes l
        withForeignPtr buf $ \ptr -> do
          utf8EncodeString ptr str
          mkFastStringForeignPtr ptr buf l

    -- | Creates a 'FastString' from a UTF-8 encoded @[Word8]@
    mkFastStringByteList :: [Word8] -> FastString
    mkFastStringByteList str =
      inlinePerformIO $ do
        let l = Prelude.length str
        buf <- mallocForeignPtrBytes l
        withForeignPtr buf $ \ptr -> do
          pokeArray (castPtr ptr) str
          mkFastStringForeignPtr ptr buf l

    -- | Creates a Z-encoded 'FastString' from a 'String'
    mkZFastString :: String -> FastZString
    mkZFastString = mkFastZStringString

    mkNewFastString :: ForeignPtr Word8 -> Ptr Word8 -> Int -> Int
                    -> IO FastString
    mkNewFastString fp ptr len uid = do
      ref <- newIORef Nothing
      n_chars <- countUTF8Chars ptr len
      return (FastString uid n_chars (BS.fromForeignPtr fp 0 len) ref)

    mkNewFastStringByteString :: ByteString -> Ptr Word8 -> Int -> Int
                              -> IO FastString
    mkNewFastStringByteString bs ptr len uid = do
      ref <- newIORef Nothing
      n_chars <- countUTF8Chars ptr len
      return (FastString uid n_chars bs ref)

    copyNewFastString :: Ptr Word8 -> Int -> Int -> IO FastString
    copyNewFastString ptr len uid = do
      fp <- copyBytesToForeignPtr ptr len
      ref <- newIORef Nothing
      n_chars <- countUTF8Chars ptr len
      return (FastString uid n_chars (BS.fromForeignPtr fp 0 len) ref)

    copyBytesToForeignPtr :: Ptr Word8 -> Int -> IO (ForeignPtr Word8)
    copyBytesToForeignPtr ptr len = do
      fp <- mallocForeignPtrBytes len
      withForeignPtr fp $ \ptr' -> copyBytes ptr' ptr len
      return fp

    cmpStringPrefix :: Ptr Word8 -> Ptr Word8 -> Int -> IO Bool
    cmpStringPrefix ptr1 ptr2 len =
     do r <- memcmp ptr1 ptr2 len
        return (r == 0)

    hashStr  :: Ptr Word8 -> Int -> Int
     -- use the Addr to produce a hash value between 0 & m (inclusive)
    hashStr (Ptr a#) (I# len#) = loop 0# 0#
       where
        loop h n | isTrue# (n ==# len#) = I# h
                 | otherwise  = loop h2 (n +# 1#)
              where
                !c = ord# (indexCharOffAddr# a# n)
                !h2 = (h *# 16777619#) `xorI#` c

    -- -----------------------------------------------------------------------------
    -- Operations

    -- | Returns the length of the 'FastString' in characters
    lengthFS :: FastString -> Int
    lengthFS f = n_chars f

    -- | Returns @True@ if this 'FastString' is not Z-encoded but already has
    -- a Z-encoding cached (used in producing stats).
    hasZEncoding :: FastString -> Bool
    hasZEncoding (FastString _ _ _ ref) =
          inlinePerformIO $ do
            m <- readIORef ref
            return (isJust m)

    -- | Returns @True@ if the 'FastString' is empty
    nullFS :: FastString -> Bool
    nullFS f = BS.null (fs_bs f)

    -- | Unpacks and decodes the FastString
    unpackFS :: FastString -> String
    unpackFS (FastString _ _ bs _) = utf8DecodeByteString bs

    -- | Gives the UTF-8 encoded bytes corresponding to a 'FastString'
    bytesFS :: FastString -> [Word8]
    bytesFS fs = BS.unpack $ fastStringToByteString fs

    -- | Returns a Z-encoded version of a 'FastString'.  This might be the
    -- original, if it was already Z-encoded.  The first time this
    -- function is applied to a particular 'FastString', the results are
    -- memoized.
    --
    zEncodeFS :: FastString -> FastZString
    zEncodeFS fs@(FastString _ _ _ ref) =
          inlinePerformIO $ do
            m <- readIORef ref
            case m of
              Just zfs -> return zfs
              Nothing -> do
                atomicModifyIORef' ref $ \m' -> case m' of
                  Nothing  -> let zfs = mkZFastString (zEncodeString (unpackFS fs))
                              in (Just zfs, zfs)
                  Just zfs -> (m', zfs)

    appendFS :: FastString -> FastString -> FastString
    appendFS fs1 fs2 = mkFastStringByteString
                     $ BS.append (fastStringToByteString fs1)
                                 (fastStringToByteString fs2)

    concatFS :: [FastString] -> FastString
    concatFS = mkFastStringByteString . BS.concat . map fs_bs

    headFS :: FastString -> Char
    headFS (FastString _ 0 _ _) = panic "headFS: Empty FastString"
    headFS (FastString _ _ bs _) =
      inlinePerformIO $ BS.unsafeUseAsCString bs $ \ptr ->
             return (fst (utf8DecodeChar (castPtr ptr)))

    tailFS :: FastString -> FastString
    tailFS (FastString _ 0 _ _) = panic "tailFS: Empty FastString"
    tailFS (FastString _ _ bs _) =
        inlinePerformIO $ BS.unsafeUseAsCString bs $ \ptr ->
        do let (_, n) = utf8DecodeChar (castPtr ptr)
           return $! mkFastStringByteString (BS.drop n bs)

    consFS :: Char -> FastString -> FastString
    consFS c fs = mkFastString (c : unpackFS fs)

    uniqueOfFS :: FastString -> Int
    uniqueOfFS (FastString u _ _ _) = u

    nilFS :: FastString
    nilFS = mkFastString ""

    isUnderscoreFS :: FastString -> Bool
    isUnderscoreFS fs = fs == fsLit "_"

    -- -----------------------------------------------------------------------------
    -- Stats

    getFastStringTable :: IO [[[FastString]]]
    getFastStringTable =
      forM [0 .. numSegments - 1] $ \(I# i#) -> do
        let (# segmentRef #) = indexArray# segments# i#
        FastStringTableSegment _ _ buckets# <- readIORef segmentRef
        let bucketSize = I# (sizeofMutableArray# buckets#)
        forM [0 .. bucketSize - 1] $ \(I# j#) ->
          IO $ readArray# buckets# j#
      where
        !(FastStringTable _ segments#) = stringTable

    -- -----------------------------------------------------------------------------
    -- Outputting 'FastString's

    -- |Outputs a 'FastString' with /no decoding at all/, that is, you
    -- get the actual bytes in the 'FastString' written to the 'Handle'.
    hPutFS :: Handle -> FastString -> IO ()
    hPutFS handle fs = BS.hPut handle $ fastStringToByteString fs

    -- ToDo: we'll probably want an hPutFSLocal, or something, to output
    -- in the current locale's encoding (for error messages and suchlike).

    -- -----------------------------------------------------------------------------
    -- PtrStrings, here for convenience only.

    -- | A 'PtrString' is a pointer to some array of Latin-1 encoded chars.
    data PtrString = PtrString !(Ptr Word8) !Int

    -- | Wrap an unboxed address into a 'PtrString'.
    mkPtrString# :: Addr# -> PtrString
    mkPtrString# a# = PtrString (Ptr a#) (ptrStrLength (Ptr a#))

    -- | Encode a 'String' into a newly allocated 'PtrString' using Latin-1
    -- encoding.  The original string must not contain non-Latin-1 characters
    -- (above codepoint @0xff@).
    {-# INLINE mkPtrString #-}
    mkPtrString :: String -> PtrString
    mkPtrString s =
     -- we don't use `unsafeDupablePerformIO` here to avoid potential memory leaks
     -- and because someone might be using `eqAddr#` to check for string equality.
     unsafePerformIO (do
       let len = length s
       p <- mallocBytes len
       let
         loop :: Int -> String -> IO ()
         loop !_ []    = return ()
         loop n (c:cs) = do
            pokeByteOff p n (fromIntegral (ord c) :: Word8)
            loop (1+n) cs
       loop 0 s
       return (PtrString p len)
     )

    -- | Decode a 'PtrString' back into a 'String' using Latin-1 encoding.
    -- This does not free the memory associated with 'PtrString'.
    unpackPtrString :: PtrString -> String
    unpackPtrString (PtrString (Ptr p#) (I# n#)) = unpackNBytes# p# n#

    -- | Return the length of a 'PtrString'
    lengthPS :: PtrString -> Int
    lengthPS (PtrString _ n) = n

    -- -----------------------------------------------------------------------------
    -- under the carpet

    foreign import ccall unsafe "strlen"
      ptrStrLength :: Ptr Word8 -> Int

    {-# NOINLINE sLit #-}
    sLit :: String -> PtrString
    sLit x  = mkPtrString x

    {-# NOINLINE fsLit #-}
    fsLit :: String -> FastString
    fsLit x = mkFastString x

    {-# RULES "slit"
        forall x . sLit  (unpackCString# x) = mkPtrString#  x #-}
    {-# RULES "fslit"
        forall x . fsLit (unpackCString# x) = mkFastString# x #-}