#5152: GHC generates poor code for large 64-bit literals
---------------------------------+------------------------------------------
Reporter: bos | Owner: igloo
Type: bug | Status: new
Priority: normal | Milestone: 7.4.1
Component: Compiler | Version: 7.0.3
Keywords: | Testcase:
Blockedby: | Difficulty:
Os: Unknown/Multiple | Blocking:
Architecture: Unknown/Multiple | Failure: Runtime performance bug
---------------------------------+------------------------------------------
Comment(by simonpj):
The whole approach in this ticket is a bit unsatisfactory (I know, I
recommended it!) because it only works if the typechecker knows enough at
the moment it encounters the literal. The vaguaries of inference might
mean it doesn't know enough, and it ''certainly'' won't know enough if you
have constant in code that is subsequently specialised:
{{{
f :: Num a => a -> a
f a = a + 0x9ddfea08eb382d69
{-# SPECIALISE f :: Word64 -> Word64 #-}
}}}
The Right Thing is really for the optimiser to do the right constant-
folding thing. I can see two reasonable alternative approaches.
The first is to treat big integer literals using a list of coefficients
thus:
{{{
makeInteger :: [Int] -> Integer
}}}
(This would replace the chain of plus/multiply stuff at the moment.) With
the idea that
{{{
makeInteger [a,b,c] = a + 10000*(b + 10000*c)
}}}
(Actually 2**30 would probably be a better constant than 10000.) Now you
could sensibly have a rule
{{{
RULE "fromInteger/Float"
fromInteger (makeInteger (a:as)) = a + 10000*fromInteger (makeInteger as)
:: Word64
}}}
or something like that. (This would allow out of range Word constants,
which would just overflow. I'd prefer a static error...)
A second approach would be to make Core have a built-in, boxed, literal
type `Integer`, not (visibly) represented with a data type. It would
directly support constant-folding etc just as `Int#` does. Then in the
back end it'd be expanded to a call to `makeInteger` or something like
that.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5152#comment:7>
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
