You should be able to reduce the bit-twiddling a great deal IIRC in the
word case.
SW a + SW b
| c <- a + b, c >= min a b = SW c
| otherwise = throw Overflow
There is a similar trick that escapes me at the moment for the signed case.
On Sun, Jun 28, 2015 at 6:15 PM, Nikita Karetnikov
Haskell is often marketed as a safe (or safer) language, but there's an issue that makes it less safe as it could be. I'm talking about arithmetic overflows and division by zero. The safeint package tries to address this, but it only supports the Int type because (as I understand it) there are no useful primitives for other common types defined in Data.Int and Data.Word.
I've tried adding Int64 support to safeint just to see how it would work without primops. Here's a snippet (I haven't tested this code well, so it may be wrong, sorry about that):
shiftRUnsigned :: Word64 -> Int -> Word64 shiftRUnsigned = shiftR
-- http://git.haskell.org/ghc.git/blob/HEAD:/compiler/codeGen/StgCmmPrim.hs#l93... plusSI64 :: SafeInt64 -> SafeInt64 -> SafeInt64 plusSI64 (SI64 a) (SI64 b) = if c == 0 then SI64 r else overflowError where r = a + b c = (fromIntegral $ (complement (a `xor` b)) .&. (a `xor` r)) `shiftRUnsigned` ((finiteBitSize a) - 1)
-- http://git.haskell.org/ghc.git/blob/HEAD:/compiler/codeGen/StgCmmPrim.hs#l96... minusSI64 :: SafeInt64 -> SafeInt64 -> SafeInt64 minusSI64 (SI64 a) (SI64 b) = if c == 0 then SI64 r else overflowError where r = a - b c = (fromIntegral $ (a `xor` b) .&. (a `xor` r)) `shiftRUnsigned` ((finiteBitSize a) - 1)
-- https://stackoverflow.com/a/1815371 timesSI64 :: SafeInt64 -> SafeInt64 -> SafeInt64 timesSI64 (SI64 a) (SI64 b) = let x = a * b in if a /= 0 && x `div` a /= b then overflowError else SI64 x
I may be wrong, but my understanding is that new primops could reduce overhead here. If so, would a patch adding them be accepted? Are there any caveats?
In the safeint package, would it be reasonable to return an Either value instead of throwing an exception? Or would it be too much?
I haven't created a wiki page or ticket because I don't know much, so I want to get some feedback before doing so. That would be my first patch to GHC (if ever), so maybe I'm not the best candidate, but I've been thinking about it for too long to ignore. :\ _______________________________________________ ghc-devs mailing list ghc-devs@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs