Don't perform unaligned writes

Changelog.md

@@ -1,6 +1,7 @@
 0.10.10.0 TBD
 
  * Add conversions between ShortByteString and CString (#126)
+ * Don't perform unaligned writes when it isn't known to be safe (#133)
 
 0.10.8.2 Duncan Coutts <[email protected]> Feb 2017
 

Data/ByteString/Builder/Prim/Binary.hs

@@ -272,9 +272,6 @@ int64LE :: FixedPrim Int64
 int64LE = fromIntegral >$< word64LE
 
 
--- TODO: Ensure that they are safe on architectures where an unaligned write is
--- an error.
-
 -- | Encode a single native machine 'Int'. The 'Int's is encoded in host order,
 -- host endian form, for the machine you are on. On a 64 bit machine the 'Int'
 -- is an 8 byte value, on a 32 bit machine, 4 bytes. Values encoded this way

Data/ByteString/Builder/Prim/Internal.hs

@@ -198,7 +198,19 @@ liftFixedToBounded = toB
 
 {-# INLINE CONLIKE storableToF #-}
 storableToF :: forall a. Storable a => FixedPrim a
+-- Not all architectures are forgiving of unaligned accesses; whitelist ones
+-- which are known not to trap (either to the kernel for emulation, or crash).
+#if defined(i386_HOST_ARCH) || defined(x86_64_HOST_ARCH) \
+    || ((defined(arm_HOST_ARCH) || defined(aarch64_HOST_ARCH)) \
+        && defined(__ARM_FEATURE_UNALIGNED)) \
+    || defined(powerpc_HOST_ARCH) || defined(powerpc64_HOST_ARCH) \
+    || defined(powerpc64le_HOST_ARCH)
 storableToF = FP (sizeOf (undefined :: a)) (\x op -> poke (castPtr op) x)
+#else
+storableToF = FP (sizeOf (undefined :: a)) $ \x op ->
+    if (ptrToWordPtr op) `mod` (fromIntegral (alignment (undefined :: a))) == 0 then poke (castPtr op) x
+    else with x $ \tp -> copyBytes op (castPtr tp) (sizeOf (undefined :: a))
+#endif
 
 {-
 {-# INLINE CONLIKE liftIOF #-}