#5152: GHC generates poor code for large 64-bit literals
---------------------------------+------------------------------------------
Reporter: bos | Owner: igloo
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 7.0.3
Keywords: | Testcase:
Blockedby: | Difficulty:
Os: Unknown/Multiple | Blocking:
Architecture: Unknown/Multiple | Failure: Runtime performance bug
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Changes (by simonpj):
* owner: => igloo
Comment:
I see. The reason for this is that, in general
* An integer literal `k` means `fromInteger (k::Integer)`
* If the `Integer` `k` is big, then then it is represented using a chain
of multiplies and adds; see `MkCore.mkIntegerExpr`.
So the your 64-bit word constant is getting represented as
{{{
integerToWord64 (plusInteger (S# blah1) (S# blah2))
}}}
where `blah, blah2 :: Int#` are (small) int literals in Core.
GHC already has a mechanism to short-cut this stuff when we know all the
types. The short cut is embodied in `TcHsSyn.shortCutLit`:
{{{
shortCutLit :: OverLitVal -> TcType -> Maybe (HsExpr TcId)
shortCutLit (HsIntegral i) ty
| isIntTy ty && inIntRange i = Just (HsLit (HsInt i))
| isWordTy ty && inWordRange i = Just (mkLit wordDataCon (HsWordPrim i))
...etc...
}}}
All we need to do is to add cases for `Word64` ... and for all the other
fixed-width types.
Ian would you like to do that?
Simon
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5152#comment:1>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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