Funky Word32 arithmetic

module Main where

import Word

main = putStrLn (show (q * 2) ++ "\n" ++ show (q * 2 + 1))

q :: Word32
q = 2147483647

The above program, compiled thusly on a linux/386 box:

ghc -fglasgow-exts t.hs

prints:

4294967294
4294967295

But if we turn on -O, we get:

4294967294
1

This happens in both 4.09 and 4.08.
Amusingly enough, 4.08 comments:
Warning: Integer overflow in: *# 2147483647 2
but that isn't the operation that gives the problem!

Snooping in the Word library, I first suspected the rather suspicious definition of Num for Word32, which uses (signed) Int arithmetic.  But even signed Int arithmetic would give the right answer here.

One fix is to define arithmetic on Word32 using real unsigned primitives - I tried it, it works and the problem goes away.  I can check those changes in if you like.  But I think there's another bug lurking here somewhere, since the signed arithmethic should also give the right answer.

Trac metadata

Trac field	Value
Version	None
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	ResolvedFixed
Component	Compiler
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