Constant folding on 'mod/Word' - incorrect result
Compiling this with GHC 7.10.3 on both MacOS and Linux with '-O' producess results '5 0'.
module Main where testfn :: Word -> IO () testfn wseq = do print $ wseq `mod` 1 main = do testfn 5 print $ (5 :: Word) `mod` 1
Changing type to Int produces correct result. It has probably something to do with compiler/prelude/PrelRules.hs - the rules for Int and Word differ. It seems to me that it should be optimized the same way, but the culprit seems to be the 'rightIdentityDynFlags onew' - that seems to be a clear bug (if it does what I think it does).
primOpRules nm IntRemOp = mkPrimOpRule nm 2 [ nonZeroLit 1 >> binaryLit (intOp2 rem) , leftZero zeroi , do l <- getLiteral 1 dflags <- getDynFlags guard (l == onei dflags) retLit zeroi , equalArgs >> retLit zeroi , equalArgs >> retLit zeroi ] primOpRules nm WordRemOp = mkPrimOpRule nm 2 [ nonZeroLit 1 >> binaryLit (wordOp2 rem) , rightIdentityDynFlags onew ]
I found it in different code where lots of inlining reduced some branch of code into this and produced wrong result.