[GHC] #10844: CallStack should not be inlined

[GHC]

#10844: CallStack should not be inlined
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        Reporter:  nomeata           |                Owner:  gridaphobe
            Type:  task              |               Status:  patch
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.10.2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D1259
       Wiki Page:                    |
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Comment (by gridaphobe):

 Unfortunately the rules don't seem to fire in the right order. The key
 step appears to be when we get to the following state.

 {{{
 boo = build (\ @ b_aog -> unpackFoldrCString# "foo"#)

 lvl_spB = \ @ b_aot c_aou n_aov -> foldr c_aou n_aov boo

 foo = \ x_anI -> augment lvl_spB x_anI
 }}}

 At this point, I would want `fold/build` to fire on `lvl_spB`, giving us

 {{{
 lvl_spB = \ @ b_aot c_aou n_aov -> unpackFoldrCString# "foo"# c_aou n_aov
 }}}

 followed by `unpack-append` on `foo`, which would finally give us

 {{{
 foo = \ x_anI -> unpackAppendCString# "foo"# x_anI
 }}}

 But instead `foldr/app` fires, collapsing `foo` back into a use of `++`.

 If I add a special rule for `++` on strings

 {{{
 {-# RULES

 "++-string"  forall xs ys. unpackCString# xs ++ ys = unpackAppendCString#
 xs ys

 #-}
 }}}

 I can coax GHC into generating the same Core as on master, but this is not
 very satisfying. It seems like we already have all of the necessary rules
 available.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10844#comment:15>
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