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.
Trac metadata
Trac field | Value |
---|---|
Version | 7.10.3 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Prelude |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | |
Operating system | |
Architecture |