[GHC] #11688: Bytestring break failing rewrite to breakByte and failing to eliminate boxing/unboxing

[GHC]

#11688: Bytestring break failing rewrite to breakByte and failing to eliminate
boxing/unboxing
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        Reporter:  alexbiehl         |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.10.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by bgamari):

 I briefly looked at this; I left a comment on the `bytestring` ticket
 which I'll reproduce here,

 > I suspect the issue here is that `(==)` is being inlined too early,
 hiding the rule fire opportunity. By contrast, if you give `(==)` another
 name with a later inline pragma the `break -> breakByte` rule fires as
 expected.
 >
 > {{{#!hs
 > main = do
 >   chunk <- DB.hGetSome stdin 32768
 >   let newline = c2w '\n'
 >   let (prefix,suffix) = DB.break (eq newline) chunk
 >   DB.hPut stdout prefix
 >   DB.hPut stdout suffix
 >
 > eq :: Eq a => a -> a -> Bool
 > eq = (==)
 > {-# INLINE [1] eq #-}
 >
 > {-# RULES
 > "ByteString specialise break (eq x)" forall x.
 >     DB.break (eq x) = DB.breakByte x
 >   #-}
 > }}}
 >
 > Actually, even if one doesn't specify an explicit phase on `eq` the rule
 still fires! This is likely accidental, since GHC just happens to look at
 the `breakByte` rule before the `eq` inlining.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11688#comment:3>
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