GHC/CString.hs

    {-# LANGUAGE MagicHash, NoImplicitPrelude, BangPatterns, UnliftedFFITypes #-}
    -----------------------------------------------------------------------------
    -- |
    -- Module      :  GHC.CString
    -- Copyright   :  (c) The University of Glasgow 2011
    -- License     :  see libraries/ghc-prim/LICENSE
    --
    -- Maintainer  :  cvs-ghc@haskell.org
    -- Stability   :  internal
    -- Portability :  non-portable (GHC Extensions)
    --
    -- GHC C strings definitions (previously in GHC.Base).
    -- Use GHC.Exts from the base package instead of importing this
    -- module directly.
    --
    -----------------------------------------------------------------------------

    module GHC.CString (
            -- * Ascii variants
            unpackCString#, unpackAppendCString#, unpackFoldrCString#,
            cstringLength#,

            -- * Utf variants
            unpackCStringUtf8#, unpackAppendCStringUtf8#, unpackFoldrCStringUtf8#,

            -- * Other
            unpackNBytes#,
        ) where

    import GHC.Types hiding (One)
    import GHC.Prim

    {-
    Note [String literals in GHC]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    String literals get quite a bit of special handling in GHC.  This Note
    summarises the moving parts.

    * Desugaring: see GHC.HsToCore.Match.Literal.dsLit, which in
      turn calls GHC.Core.Make.mkStringExprFS.

      The desugarer desugars the Haskell literal "foo" into Core
         GHC.CString.unpackCString# "foo"#
      where "foo"# is primitive string literal (of type Addr#).

      When the string cannot be encoded as a C string, we use UTF8:
         GHC.CString.unpackCStringUtf8# "foo"#

    * The library module ghc-prim:GHC.CString has a bunch of functions that
      work over primitive strings, including GHC.CString.unpackCString#

    * GHC.Core.Op.ConstantFold has some RULES that optimise certain string
      operations on literal strings. For example:

        + Constant folding the desugared form of ("foo" ++ "bar")
          into ("foobar")
        + Comparing strings
        + and more

    * GHC.Base has a number of regular rules for String literals.

      + a rule "eqString": (==) @String = eqString
        where GHC.Base.eqString :: String -> String -> Bool

        ConstantFold has a RULE for eqString on literals:
         eqString (Lit "foo"#) (Lit "bar"#) --> False

        This allows compile time evaluation of things like "foo" == "bar"

      + A bunch of rules to promote fusion:

        "unpack"       [~1] forall a   . unpackCString# a             = build (unpackFoldrCString# a)
        "unpack-list"  [1]  forall a   . unpackFoldrCString# a (:) [] = unpackCString# a
        "unpack-append"     forall a n . unpackFoldrCString# a (:) n  = unpackAppendCString# a n

        And UTF8 variants of these rules.

    * We allow primitive (unlifted) literal strings to be top-level
      bindings, breaking out usual rule.  See GHC.Core
      Note [Core top-level string literals]

    * TODO: There is work on a special code-gen path for top-level boxed strings
         str :: [Char]
         str = unpackCString# "foo"#
      so that they can all share a common code pointer

      There is a WIP MR on gitlab for this: !3012

    -}

    -----------------------------------------------------------------------------
    -- Unpacking C strings
    -----------------------------------------------------------------------------

    -- This code is needed for virtually all programs, since it's used for
    -- unpacking the strings of error messages.

    -- Used to be in GHC.Base, but was moved to ghc-prim because the new generics
    -- stuff uses Strings in the representation, so to give representations for
    -- ghc-prim types we need unpackCString#

    {- Note [Inlining unpackCString#]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    There's really no point in ever inlining things like unpackCString# as the loop
    doesn't specialise in an interesting way and we can't deforest the list
    constructors (we'd want to use unpackFoldrCString# for this). Moreover, it's
    pretty small, so there's a danger that it'll be inlined at every literal, which
    is a waste.

    Moreover, inlining early may interfere with a variety of rules that are supposed
    to match unpackCString#,

     * BuiltInRules in GHC.Core.Opt.ConstantFold; e.g.
           eqString (unpackCString# (Lit s1)) (unpackCString# (Lit s2)
              = s1 == s2

     * unpacking rules; e.g. in GHC.Base,
           unpackCString# a
              = build (unpackFoldrCString# a)

     * stream fusion rules; e.g. in the `text` library,
           unstream (S.map safe (S.streamList (GHC.unpackCString# a)))
              = unpackCString# a

    Moreover, we want to make it CONLIKE, so that:

    * the rules in GHC.Core.Opt.ConstantFold will fire when the string is let-bound.
      E.g. the eqString rule in GHC.Core.Opt.ConstantFold
       eqString (unpackCString# (Lit s1)) (unpackCString# (Lit s2) = s1==s2

    * exprIsConApp_maybe will see the string when we have
         let x = unpackCString# "foo"#
         ...(case x of algs)...

    All of this goes for unpackCStringUtf8# too.
    -}

    {- Note [Inlining of unpackFoldrCString]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    Usually the unpack-list rule turns unpackFoldrCString# into unpackCString#
    It also has a BuiltInRule in PrelRules.hs:
         unpackFoldrCString# "foo" c (unpackFoldrCString# "baz" c n)
           =  unpackFoldrCString# "foobaz" c n

    We use NOINLINE [0] on the grounds that, unlike
    unpackCString#, there *is* some point in inlining
    unpackFoldrCString#, because we get better code for the
    higher-order function call.

    This can cause a code size increase but it was minimal
    when looking at nofib.

    This is especially important for elem which then results in an
    allocation free loop.

      Note [unpackCString# iterating over addr]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    When unpacking unpackCString# and friends repeatedly return a cons cell
    containing:
    * The current character we just unpacked.
    * A thunk to unpack the rest of the string.

    In order to minimize the size of the thunk we do not index of
    the start of the string, offsetting into it, but instead increment
    the addr and always use offset 0#.

    This works since these two expressions will read from the same address.
    * `indexCharOffAddr# a i`
    * `indexCharOffAddr (a `plusAddr#` i) 0#`

    This way we avoid the need for the thunks to close over both the start of
    the string and the current offset, saving a word for each character unpacked.

    This has the additional advantage the we can guarantee that only the
    increment will happen in the loop.

    -}

    unpackCString# :: Addr# -> [Char]
    {-# NOINLINE CONLIKE unpackCString# #-}
    unpackCString# addr
        | isTrue# (ch `eqChar#` '\0'#) = []
        | True                         = C# ch : unpackCString# (addr `plusAddr#` 1#)
          where
            -- See Note [unpackCString# iterating over addr]
            !ch = indexCharOffAddr# addr 0#

    unpackAppendCString# :: Addr# -> [Char] -> [Char]
    {-# NOINLINE unpackAppendCString# #-}
         -- See the NOINLINE note on unpackCString#
    unpackAppendCString# addr rest
        | isTrue# (ch `eqChar#` '\0'#) = rest
        | True                         = C# ch : unpackAppendCString# (addr `plusAddr#` 1#) rest
          where
            -- See Note [unpackCString# iterating over addr]
            !ch = indexCharOffAddr# addr 0#

    -- Usually the unpack-list rule turns unpackFoldrCString# into unpackCString#.
    -- See Note [String literals in GHC] for more details.
    -- See [Inlining of unpackFoldrCString]
    {-# NOINLINE[0] unpackFoldrCString# #-}
    unpackFoldrCString# :: Addr# -> (Char -> a -> a) -> a -> a
    unpackFoldrCString# str f z_init = go str z_init
      where
        go addr z
          | isTrue# (ch `eqChar#` '\0'#) = z
          | True                         = C# ch `f` go (addr `plusAddr#` 1#) z
          where
            -- See Note [unpackCString# iterating over addr]
            !ch = indexCharOffAddr# addr 0#

    -- There's really no point in inlining this for the same reasons as
    -- unpackCString. See Note [Inlining unpackCString#] above for details.
    unpackCStringUtf8# :: Addr# -> [Char]
    {-# NOINLINE CONLIKE unpackCStringUtf8# #-}
    unpackCStringUtf8# addr
        | isTrue# (ch `eqChar#` '\0'#  ) = []
        | True =
            let !byte_count = getByteCount ch
                !utf_ch = unpackUtf8Char# byte_count ch addr
                !addr' = addr `plusBytes` byte_count
            in  C# utf_ch : unpackCStringUtf8# addr'
          where
            -- See Note [unpackCString# iterating over addr]
            !ch = indexCharOffAddr# addr 0#

    unpackAppendCStringUtf8# :: Addr# -> [Char] -> [Char]
    {-# NOINLINE unpackAppendCStringUtf8# #-}
         -- See the NOINLINE note on unpackCString#
    unpackAppendCStringUtf8# addr rest
        | isTrue# (ch `eqChar#` '\0'#) = rest
        | True =
            let !byte_count = getByteCount ch
                !utf_ch = unpackUtf8Char# byte_count ch addr
                !addr' = (addr `plusBytes` byte_count)
            in  C# utf_ch : unpackAppendCStringUtf8# addr' rest
          where
            -- See Note [unpackCString# iterating over addr]
            !ch = indexCharOffAddr# addr 0#

    -- See Note [Inlining of unpackFoldrCString]
    {-# NOINLINE[0] unpackFoldrCStringUtf8# #-}
    unpackFoldrCStringUtf8# :: Addr# -> (Char -> a -> a) -> a -> a
    unpackFoldrCStringUtf8# addr_init f z_init
      = go addr_init z_init
      where
        go addr z
          | isTrue# (ch `eqChar#` '\0'#) = z
          | True =
              let !byte_count = getByteCount ch
                  !utf_ch = unpackUtf8Char# byte_count ch addr
                  !addr' = (addr `plusBytes` byte_count)
              in C# utf_ch `f` go addr' z
          where
            -- See Note [unpackCString# iterating over addr]
            !ch = indexCharOffAddr# addr 0#

    -- There's really no point in inlining this for the same reasons as
    -- unpackCString. See Note [Inlining unpackCString#] above for details.
    unpackNBytes# :: Addr# -> Int# -> [Char]
    {-# NOINLINE unpackNBytes# #-}
    unpackNBytes# _addr 0#   = []
    unpackNBytes#  addr len# = unpack [] (len# -# 1#)
        where
         unpack :: [Char] -> Int# -> [Char]
         unpack acc i#
          | isTrue# (i# <# 0#)  = acc
          | True                =
             case indexCharOffAddr# addr i# of
                ch -> unpack (C# ch : acc) (i# -# 1#)

    -- The return type is not correct here. We really want CSize,
    -- but that type is defined in base. However, CSize should always
    -- match the size of a machine word (I hope), so this is probably
    -- alright on all platforms that GHC supports.
    foreign import ccall unsafe "strlen" c_strlen :: Addr# -> Int#

    -- | Compute the length of a NUL-terminated string. This address
    -- must refer to immutable memory. GHC includes a built-in rule for
    -- constant folding when the argument is a statically-known literal.
    -- That is, a core-to-core pass reduces the expression
    -- @cstringLength# "hello"#@ to the constant @5#@.
    cstringLength# :: Addr# -> Int#
    {-# INLINE[0] cstringLength# #-}
    cstringLength# = c_strlen

    ------------------------------
    --- UTF8 decoding utilities
    ------------------------------
    --
    -- These functions make explicit the logic that was originally
    -- part of unpackCStringUtf8. Since we want the same support for ascii
    -- and non-ascii a variety of functions needs the same logic. Instead
    -- of C&P'in the decoding logic all over we have it here once, and then
    -- force GHC to inline it.
    --
    -- All the overhead of the Bytes argument and calls goes away once all is
    -- said and done. And what remains is readable code in Haskell land and
    -- performant code in the resulting binary.

    data Bytes = One | Two | Three | Four

    {-# INLINE getByteCount #-}
    getByteCount :: Char# -> Bytes
    getByteCount ch
        | isTrue# (ch `leChar#` '\x7F'#) = One
        | isTrue# (ch `leChar#` '\xDF'#) = Two
        | isTrue# (ch `leChar#` '\xEF'#) = Three
        | True                           = Four

    {-# INLINE plusBytes #-}
    plusBytes :: Addr# -> Bytes -> Addr#
    plusBytes addr bytes =
      case bytes of
        One   -> addr `plusAddr#` 1#
        Two   -> addr `plusAddr#` 2#
        Three -> addr `plusAddr#` 3#
        Four  -> addr `plusAddr#` 4#

    -- | Take the current address, read unicode char of the given size.
    -- We obviously want the number of bytes, but we have to read one
    -- byte to determine the number of bytes for the current codepoint
    -- so we might as well reuse it and avoid a read.
    --
    -- Side Note: We don't dare to decode all 4 possibilities at once.
    -- Reading past the end of the addr might trigger an exception.
    -- For this reason we really have to check the width first and only
    -- decode after.
    {-# INLINE unpackUtf8Char# #-}
    unpackUtf8Char# :: Bytes -> Char# -> Addr# -> Char#
    unpackUtf8Char# bytes ch addr =
      case bytes of
        One -> ch
        Two ->   (chr# (((ord# ch                                           -# 0xC0#) `uncheckedIShiftL#`  6#) +#
                         (ord# (indexCharOffAddr# (addr `plusAddr#` 1#) 0#) -# 0x80#)))
        Three -> (chr# (((ord# ch                                           -# 0xE0#) `uncheckedIShiftL#` 12#) +#
                        ((ord# (indexCharOffAddr# (addr `plusAddr#` 1#) 0#) -# 0x80#) `uncheckedIShiftL#`  6#) +#
                         (ord# (indexCharOffAddr# (addr `plusAddr#` 2#) 0#) -# 0x80#)))
        Four ->  (chr# (((ord# ch                                           -# 0xF0#) `uncheckedIShiftL#` 18#) +#
                        ((ord# (indexCharOffAddr# (addr `plusAddr#` 1#) 0#) -# 0x80#) `uncheckedIShiftL#` 12#) +#
                        ((ord# (indexCharOffAddr# (addr `plusAddr#` 2#) 0#) -# 0x80#) `uncheckedIShiftL#`  6#) +#
                         (ord# (indexCharOffAddr# (addr `plusAddr#` 3#) 0#) -# 0x80#)))