GHC doesn't optimise away primitive identity conversions
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.
Trac metadata
Trac field | Value |
---|---|
Version | 7.7 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Compiler |
Test case | |
Differential revisions | |
BlockedBy | |
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Blocking | |
CC | |
Operating system | |
Architecture |