[GHC] #14137: Do more inlining into non-recursive join points

[GHC]

#14137: Do more inlining into non-recursive join points
-------------------------------------+-------------------------------------
        Reporter:  simonpj           |                Owner:  nomeata
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.1
      Resolution:                    |             Keywords:  JoinPoints
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by simonpj):

 > Also, safe is recursive, so we don’t inline into it anyways.

 Ah!  Here is yet another place where join points are special. Consider
 {{{
 join j = e in
 joinrec j2 x = ...j...
 }}}
 where there is just one occurrence of `j` in `j2`.  Normally we'd be leery
 about inlining `j` under that `\x` because we might lose sharing.  But
 nullary join points aren't thunks, so the situation is just as if we'd
 said
 {{{
 join j _ = e in
 joinrec j2 x = ...(j ())...
 }}}
 when we'd cerainly inline it (via `preInlineUnconditinoally`).  So let's
 make join points do that.  In `preInlineUnconditionally`:
 {{{
   | otherwise = case idOccInfo bndr of
                   IAmDead                    -> True -- Happens in ((\x.1)
 v)
                   occ at OneOcc { occ_one_br = True }
                                              -> try_once (occ_in_lam occ)
                                                          (occ_int_cxt occ)
                   _                          -> False
 }}}
 In the `occ_one_br = True` alternative, add a guard
 {{{
                        | isJoinId bndr -> True    -- New
                        | otherwise     -> try_Once (occ_in_lam occ) ... as
 before
 }}}
 That should be good for everyone!

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14137#comment:8>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler