Implement utf8LengthByLeader via bit magic

src/Data/Text/Internal/Encoding/Utf8.hs

@@ -39,7 +39,7 @@ module Data.Text.Internal.Encoding.Utf8
 import Control.Exception (assert)
 import GHC.Stack (HasCallStack)
 #endif
-import Data.Bits (Bits(..))
+import Data.Bits (Bits(..), FiniteBits(..))
 import Data.Char (ord)
 import GHC.Exts
 import GHC.Word (Word8(..))
@@ -68,12 +68,21 @@ utf8Length :: Char -> Int
 utf8Length (C# c) = I# ((1# +# geChar# c (chr# 0x80#)) +# (geChar# c (chr# 0x800#) +# geChar# c (chr# 0x10000#)))
 {-# INLINE utf8Length #-}
 
+-- This is a branchless version of
+-- utf8LengthByLeader w
+--   | w < 0x80  = 1
+--   | w < 0xE0  = 2
+--   | w < 0xF0  = 3
+--   | otherwise = 4
+--
+-- c `xor` I# (c# <=# 0#) is a branchless equivalent of c `max` 1.
+-- It is crucial to write c# <=# 0# and not c# ==# 0#, otherwise
+-- GHC is tempted to "optimize" by introduction of branches.
 utf8LengthByLeader :: Word8 -> Int
-utf8LengthByLeader w
-  | w < 0x80  = 1
-  | w < 0xE0  = 2
-  | w < 0xF0  = 3
-  | otherwise = 4
+utf8LengthByLeader w = c `xor` I# (c# <=# 0#)
+  where
+    !c@(I# c#) = countLeadingZeros (complement w)
+{-# INLINE utf8LengthByLeader #-}
 
 ord2 ::
 #if defined(ASSERTS)