#16294: Code generation corrupts writes to Addr# -------------------------------------+------------------------------------- Reporter: andrewthad | Owner: (none) Type: bug | Status: new Priority: highest | Milestone: Component: Compiler | Version: 8.6.3 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- This issue affects at least GHC 8.4.4 and GHC 8.6.3. Here is a somewhat minimal example: {{{#!hs {-# language BangPatterns #-} {-# language MagicHash #-} {-# language UnboxedTuples #-} {-# options_ghc -Wall -Werror -O2 #-} import Data.Primitive import Data.Void import Data.Word import Data.Monoid import GHC.IO (IO(..)) import Foreign.Storable import Numeric (showHex) import qualified GHC.Exts as E import qualified Data.Primitive as PM main :: IO () main = do arr <- compute 0xABCD 0x79 putStrLn (showString "raw packet: " . appEndo (foldMap (Endo . showHex) (E.toList arr)) $ "") compute :: Word16 -> Word8 -> IO ByteArray compute totlen prot = do buf <- PM.newPinnedByteArray 28 PM.setByteArray buf 0 28 (0 :: Word8) let !(Addr addr) = PM.mutableByteArrayContents buf !ptr = E.Ptr addr :: E.Ptr Void pokeByteOff ptr 0 (0x45 :: Word8) pokeByteOff ptr 1 (0 :: Word8) pokeByteOff ptr 2 (totlen :: Word16) pokeByteOff ptr 4 (0 :: Word16) pokeByteOff ptr 6 (0 :: Word16) pokeByteOff ptr 8 (0x40 :: Word8) pokeByteOff ptr 9 (prot :: Word8) touchMutableByteArray buf PM.unsafeFreezeByteArray buf touchMutableByteArray :: MutableByteArray E.RealWorld -> IO () touchMutableByteArray (MutableByteArray x) = touchMutableByteArray# x touchMutableByteArray# :: E.MutableByteArray# E.RealWorld -> IO () touchMutableByteArray# x = IO $ \s -> case E.touch# x s of s' -> (# s', () #) }}} For those curious about the particular interleaving of 8-bit and 16-bit writes, this was adapted from code that fills out an `iphdr` for use with raw sockets. The output will be dependent on your platform's endianness. On my little-endian architecture, I get: {{{ raw packet: 450cdab00004079000000000000000000 }}} As we expect, the `abcd` from the source gets flipped to `cdab` because of the little endian architecture this ran on. However, it starts in an unusual place. It's not even byte-aligned. Something, possible a cmm optimization or a codegen optimization, makes the `writeWord16OffAddr#` end up straddling three bytes. Someone will probably want to write a more minimal example that eliminates the use of the `primitive` library. Sorry to be the bearer of bad news :( -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16294 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler