Fix "build/elem" RULE.

An redundant constraint prevented the rule from matching.

Fixing this allows a call to elem on a known list to be translated
into a series of equality checks, and eventually a simple case
expression.

Surprisingly this seems to regress elem for strings. To avoid
this we now also allow foldrCString to inline and add an UTF8
variant. This results in elem being compiled to a tight
non-allocating loop over the primitive string literal which
performs a linear search.

In the process this commit adds UTF8 variants for some of the
functions in GHC.CString. This is required to make this work for
both ASCII and UTF8 strings.

There are also small tweaks to the CString related rules.
We now allow ourselfes the luxury to compare the folding function
via eqExpr, which helps to ensure the rule fires before we inline
foldrCString*. Together with a few changes to allow matching on both
the UTF8 and ASCII variants of the CString functions.

2 files changed, 209 insertions, 40 deletions

diff --git a/libraries/ghc-prim/GHC/CString.hs b/libraries/ghc-prim/GHC/CString.hs
index 514fb0e9f9..ad89a5d3e3 100644
--- a/libraries/ghc-prim/GHC/CString.hs
+++ b/libraries/ghc-prim/GHC/CString.hs
@@ -16,13 +16,78 @@
 -----------------------------------------------------------------------------
 
 module GHC.CString (
+        -- * Ascii variants
         unpackCString#, unpackAppendCString#, unpackFoldrCString#,
-        unpackCStringUtf8#, unpackNBytes#, cstringLength#
+        cstringLength#,
+
+        -- * Utf variants
+        unpackCStringUtf8#, unpackAppendCStringUtf8#, unpackFoldrCStringUtf8#,
+
+        -- * Other
+        unpackNBytes#,
     ) where
 
 import GHC.Types
 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
 -----------------------------------------------------------------------------
@@ -70,8 +135,27 @@ Moreover, we want to make it CONLIKE, so that:
 All of this goes for unpackCStringUtf8# too.
 -}
 
-{- Note [unpackCString# iterating over addr]
-   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+{- 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:
@@ -88,6 +172,10 @@ This works since these two expressions will read from the same address.
 
 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]
@@ -110,28 +198,19 @@ unpackAppendCString# addr rest
         -- See Note [unpackCString# iterating over addr]
         !ch = indexCharOffAddr# addr 0#
 
-unpackFoldrCString# :: Addr# -> (Char  -> a -> a) -> a -> a
-
--- Usually the unpack-list rule turns unpackFoldrCString# into unpackCString#
-
--- It also has a BuiltInRule in GHC.Core.Opt.ConstantFold:
---      unpackFoldrCString# "foo" c (unpackFoldrCString# "baz" c n)
---        =  unpackFoldrCString# "foobaz" c n
-
-{-# NOINLINE unpackFoldrCString# #-}
--- At one stage I had 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.  BUT there may be a lot of
--- literal strings, and making a separate 'unpack' loop for
--- each is highly gratuitous.  See nofib/real/anna/PrettyPrint.
-
-unpackFoldrCString# addr f z
-  | isTrue# (ch `eqChar#` '\0'#) = z
-  | True                         = C# ch `f` unpackFoldrCString# (addr `plusAddr#` 1#) f z
+-- 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
-    -- See Note [unpackCString# iterating over addr]
-    !ch = indexCharOffAddr# addr 0#
+    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.
@@ -139,22 +218,43 @@ unpackCStringUtf8# :: Addr# -> [Char]
 {-# NOINLINE CONLIKE unpackCStringUtf8# #-}
 unpackCStringUtf8# addr
     | isTrue# (ch `eqChar#` '\0'#  ) = []
-    | isTrue# (ch `leChar#` '\x7F'#) = C# ch : unpackCStringUtf8# (addr `plusAddr#` 1#)
-    | isTrue# (ch `leChar#` '\xDF'#) =
-        let !c = C# (chr# (((ord# ch                                  -# 0xC0#) `uncheckedIShiftL#`  6#) +#
-                            (ord# (indexCharOffAddr# (addr `plusAddr#` 1#) 0#) -# 0x80#)))
-        in c : unpackCStringUtf8# (addr `plusAddr#` 2#)
-    | isTrue# (ch `leChar#` '\xEF'#) =
-        let !c = C# (chr# (((ord# ch                                             -# 0xE0#) `uncheckedIShiftL#` 12#) +#
-                           ((ord# (indexCharOffAddr# (addr `plusAddr#` 1#) 0#) -# 0x80#) `uncheckedIShiftL#`  6#) +#
-                            (ord# (indexCharOffAddr# (addr `plusAddr#` 2#) 0#) -# 0x80#)))
-        in c : unpackCStringUtf8# (addr `plusAddr#` 3#)
-    | True                           =
-        let !c = C# (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#)))
-        in c : unpackCStringUtf8# (addr `plusAddr#` 4#)
+    | 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#
@@ -187,3 +287,61 @@ foreign import ccall unsafe "strlen" c_strlen :: Addr# -> Int#
 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#)))
diff --git a/libraries/ghc-prim/changelog.md b/libraries/ghc-prim/changelog.md
index 9cfbe99dbe..ac7a138580 100644
--- a/libraries/ghc-prim/changelog.md
+++ b/libraries/ghc-prim/changelog.md
@@ -5,6 +5,17 @@
 - Add known-key `cstringLength#` to `GHC.CString`. This is just the
   C function `strlen`, but a built-in rewrite rule allows GHC to
   compute the result at compile time when the argument is known.
+  
+- In order to support unicode better the following functions in `GHC.CString`
+  gained UTF8 counterparts:
+
+        unpackAppendCStringUtf8# :: Addr# -> [Char] -> [Char]
+        unpackFoldrCStringUtf8# :: Addr# -> (Char -> a -> a) -> a -> a
+
+- unpackFoldrCString* variants can now inline in phase [0].
+
+  If the folding function is known this allows for unboxing of the
+  Char argument resulting in much faster code.
 
 ## 0.6.1 (edit as necessary)