diff --git a/hw-prim.cabal b/hw-prim.cabal
--- a/hw-prim.cabal
+++ b/hw-prim.cabal
@@ -3,7 +3,7 @@
 -- see: https://github.com/sol/hpack
 
 name:           hw-prim
-version:        0.6.2.12
+version:        0.6.2.13
 synopsis:       Primitive functions and data types
 description:    Primitive functions and data types.
 category:       Data
diff --git a/src/HaskellWorks/Data/Vector/AsVector64.hs b/src/HaskellWorks/Data/Vector/AsVector64.hs
--- a/src/HaskellWorks/Data/Vector/AsVector64.hs
+++ b/src/HaskellWorks/Data/Vector/AsVector64.hs
@@ -4,11 +4,12 @@
   ( AsVector64(..)
   ) where
 
-import Data.Bits
+import Data.Semigroup ((<>))
 import Data.Word
 
-import qualified Data.ByteString      as BS
-import qualified Data.Vector.Storable as DVS
+import qualified Data.ByteString          as BS
+import qualified Data.ByteString.Internal as BSI
+import qualified Data.Vector.Storable     as DVS
 
 class AsVector64 a where
   asVector64 :: a -> DVS.Vector Word64
@@ -18,40 +19,10 @@
   {-# INLINE asVector64 #-}
 
 instance AsVector64 BS.ByteString where
-  asVector64 bs = DVS.constructN ((BS.length bs + 7) `div` 8) go
-    where go :: DVS.Vector Word64 -> Word64
-          go u = let bsi = DVS.length u * 8 in
-            if bsi + 8 <= BS.length bs
-              then  let w0 = fromIntegral $ BS.index bs (bsi + 0)
-                        w1 = fromIntegral $ BS.index bs (bsi + 1)
-                        w2 = fromIntegral $ BS.index bs (bsi + 2)
-                        w3 = fromIntegral $ BS.index bs (bsi + 3)
-                        w4 = fromIntegral $ BS.index bs (bsi + 4)
-                        w5 = fromIntegral $ BS.index bs (bsi + 5)
-                        w6 = fromIntegral $ BS.index bs (bsi + 6)
-                        w7 = fromIntegral $ BS.index bs (bsi + 7)
-                    in  (w0 `shiftL`  0) .|.
-                        (w1 `shiftL`  8) .|.
-                        (w2 `shiftL` 16) .|.
-                        (w3 `shiftL` 24) .|.
-                        (w4 `shiftL` 32) .|.
-                        (w5 `shiftL` 40) .|.
-                        (w6 `shiftL` 48) .|.
-                        (w7 `shiftL` 56)
-              else  let w0 = let i = bsi + 0 in if i < BS.length bs then fromIntegral $ BS.index bs (bsi + 0) else 0
-                        w1 = let i = bsi + 1 in if i < BS.length bs then fromIntegral $ BS.index bs (bsi + 1) else 0
-                        w2 = let i = bsi + 2 in if i < BS.length bs then fromIntegral $ BS.index bs (bsi + 2) else 0
-                        w3 = let i = bsi + 3 in if i < BS.length bs then fromIntegral $ BS.index bs (bsi + 3) else 0
-                        w4 = let i = bsi + 4 in if i < BS.length bs then fromIntegral $ BS.index bs (bsi + 4) else 0
-                        w5 = let i = bsi + 5 in if i < BS.length bs then fromIntegral $ BS.index bs (bsi + 5) else 0
-                        w6 = let i = bsi + 6 in if i < BS.length bs then fromIntegral $ BS.index bs (bsi + 6) else 0
-                        w7 = let i = bsi + 7 in if i < BS.length bs then fromIntegral $ BS.index bs (bsi + 7) else 0
-                    in  (w0 `shiftL`  0) .|.
-                        (w1 `shiftL`  8) .|.
-                        (w2 `shiftL` 16) .|.
-                        (w3 `shiftL` 24) .|.
-                        (w4 `shiftL` 32) .|.
-                        (w5 `shiftL` 40) .|.
-                        (w6 `shiftL` 48) .|.
-                        (w7 `shiftL` 56)
+  asVector64 bs = if vLen * 8 == BS.length bs
+    then case BSI.toForeignPtr bs of
+      (fptr, start, offset) -> DVS.unsafeCast (DVS.unsafeFromForeignPtr fptr start offset)
+    else case BSI.toForeignPtr (bs <> BS.replicate (vLen * 8 - BS.length bs) 0) of
+      (fptr, start, offset) -> DVS.unsafeCast (DVS.unsafeFromForeignPtr fptr start offset)
+    where vLen = (BS.length bs + 7) `div` 8
   {-# INLINE asVector64 #-}