#5535: Performance regression vs. 7.2.1
---------------------------------+------------------------------------------
Reporter: simonmar | Owner: igloo
Type: bug | Status: new
Priority: highest | Milestone: 7.4.1
Component: Compiler | Version: 7.2.1
Keywords: | Os: Unknown/Multiple
Architecture: Unknown/Multiple | Failure: None/Unknown
Difficulty: Unknown | Testcase:
Blockedby: | Blocking:
Related: |
---------------------------------+------------------------------------------
Changes (by igloo):
* difficulty: => Unknown
Comment:
OK, I think the essence of the problem is here:
{{{
{-# LANGUAGE MagicHash, UnboxedTuples #-}
module Q (whenDiverge) where
import GHC.Base
import GHC.Integer
whenDiverge :: Int -> Int -> Double -> Bool
whenDiverge limit radius d
= walkIt (replicate limit d)
where
walkIt [] = True
walkIt (x : _)
| diverge x radius = True
| otherwise = False
diverge :: Double -> Int -> Bool
diverge d radius = exponentDouble d > radius
exponentDouble :: Double -> Int
exponentDouble x = case decodeDouble x of
(m,n) -> if m == (0 :: Integer)
then (0 :: Int)
else n + floatDigits x
decodeDouble :: Double -> (Integer, Int)
decodeDouble (D# x#) = case decodeDoubleInteger x# of
(# i, j #) -> (i, I# j)
}}}
When compiled with 7.2.1 `-O2`, the two top-level functions (`whenDiverge`
and its wrapper) have `Caf=NoCafRefs`. When compiled with HEAD, they
don't. It looks like the problem is the `(0 :: Integer)` literal, which
under the hood uses `GHC.Integer.Type.mkInteger`, which calls
`negateInteger`, which has a CAF for the (minBound :: Int) case.
So I changed it not to use a CAF:
{{{
-negateInteger (S# INT_MINBOUND) = NEG_INT_MINBOUND
+negateInteger i@(S# INT_MINBOUND) = negateInteger (toBig i)
}}}
and now `negateInteger` and `mkInteger` have `HasNoCafRefs` in the
`GHC.Integer.Type` interface file, but `whenDiverge` still doesn't have
it.
Simon, any idea what's going wrong please?
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5535#comment:2>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
_______________________________________________
Glasgow-haskell-bugs mailing list
[email protected]
http://www.haskell.org/mailman/listinfo/glasgow-haskell-bugs
