plusWord2# is buggy

Looks like its behavior differs between optimization levels:

{-# LANGUAGE MagicHash, UnboxedTuples #-}

import GHC.Prim
import GHC.Word

big :: Word
big = maxBound

good = let x = case big of W# w -> w in case plusWord2# x x of (# a, b #) -> (W# a, W# b)

main = do
  putStrLn $ case good of
    (0, z) -> "Such optimal, much sad"
    (1, z) -> "No optimization here"

I thought I was going crazy! Try the above code with and without -O2 and you'll get different output.

Trac metadata

Trac field	Value
Version	7.8.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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