[GHC] #15113: Do not make CAFs from literal strings

2 May 2018

      #15113: Do not make CAFs from literal strings
-------------------------------------+-------------------------------------
           Reporter:  simonpj        |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.6.1
          Component:  Compiler       |           Version:  8.2.2
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 Currently (as I discovered in #15038), we get the following code for
 `GHC.Exception.Base.patError`:
 {{{
 lvl2_r3y3 :: [Char]
 [GblId]
 lvl2_r3y3 = unpackCString# lvl1_r3y2

 -- RHS size: {terms: 7, types: 6, coercions: 2, joins: 0/0}
 patError :: forall a. Addr# -> a
 [GblId, Arity=1, Str=x, Unf=OtherCon []]
 patError
   = \ (@ a_a2kh) (s_a1Pi :: Addr#) ->
       raise#
         @ SomeException
         @ 'LiftedRep
         @ a_a2kh
         (Control.Exception.Base.$fExceptionPatternMatchFail_$ctoException
            ((untangle s_a1Pi lvl2_r3y3)
             `cast` (Sym (Control.Exception.Base.N:PatternMatchFail[0])
                     :: (String :: *) ~R# (PatternMatchFail :: *))))
 }}}
 That stupid `lvl2_r3y3 :: String` is a CAF, and hence `patError` has CAF-
 refs, and hence so does any function that calls `patError`, and any
 function that calls them.

 That's bad! Lots more CAF entries in SRTs, lots more work traversing those
 SRTs in the garbage collector.  And for what?  To share the work of
 unpacking a C string!  This is nuts.

 What to do?

 * Somehow refrain from floating `unpackCSTring# lit` to top level, even if
 you could otherwise do so. But that seems very ad-hoc, and it make the
 function bigger and less inlinable.

 * Treat a top level definition
 {{{
 x :: [Char]
 x = unpackCString# y
 }}}
   as NOT a CAF, and make it single-entry so that the thunk is not updated.
 Then every use of `x` will unpack the string afresh, which is probably a
 good idea anyhow.

   I like this more.  It would be implemented somewhere in the code
 generator.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15113
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