#7286: GHC doesn't optimise away primitive identity conversions
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Reporter: rl | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler
Version: 7.7 | Keywords:
Os: Unknown/Multiple | Architecture: Unknown/Multiple
Failure: Compile-time performance bug | Testcase:
Blockedby: | Blocking:
Related: |
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Here is a small example:
{{{
import Data.Char (ord)
import GHC.Base (unsafeChr)
import GHC.Word
foo :: Word8 -> Word8
foo = fromIntegral . ord . unsafeChr . fromIntegral
bar :: Word8 -> Word8
bar = baz . baz
where
baz = fromIntegral . (fromIntegral :: Word8 -> Word)
}}}
GHC produces the following code:
{{{
foo =
\ (x_arR :: Word8) ->
case x_arR of _ { W8# x#_avQ ->
W8# (narrow8Word# (int2Word# (ord# (chr# (word2Int# x#_avQ)))))
}
bar =
\ (x_arR :: Word8) ->
case x_arR of _ { W8# x#_avQ ->
W8# (narrow8Word# (narrow8Word# x#_avQ))
}
}}}
This could be optimised if GHC knew that:
* `ord# (chr# x) = x`
* `int2Word# (word2Int# x) = x`
* `narrow8Word# (narrow8Word# x) = narrow8Word# x`
There are a lot of similar rules so I'm not sure if enumerating them all
in base is the way to go.
I assume the backend(s) will optimise all of this away but this is quite a
lot of unnecessary intermediate code. This is taken from vector-bytestring
so it actually happens in the wild.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7286>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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