[GHC] #15445: SPECIALIZE one of two identical functions does not fire well

[GHC]

#15445: SPECIALIZE one of two identical functions does not fire well
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        Reporter:  nobrakal      |                Owner:  (none)
            Type:  bug           |               Status:  new
        Priority:  normal        |            Milestone:  8.6.1
       Component:  Compiler      |              Version:  8.4.3
      Resolution:                |             Keywords:
Operating System:  Linux         |         Architecture:  x86_64 (amd64)
 Type of failure:  None/Unknown  |            Test Case:
      Blocked By:                |             Blocking:
 Related Tickets:                |  Differential Rev(s):
       Wiki Page:                |
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Comment (by simonpj):

 Aha.  The problem is that we get
 {{{
 RULES: "SPEC plusTwoRec" forall ($dNum :: Num Int).
                          plusTwoRec @ Int $dNum = plusTwoRec_$splusTwoRec
 }}}
 but, as you say, Common Subexpression Elimination has decided to replace
 `plusTwoRec`'s RHS with just `plusTwoRec'`.  This is basically a good
 thing to do (saves code duplication), but if `plusTwoRec` is inlined
 before the rule has a chance to fire, we'll miss the specialisation.

 I thought of disabling CSE for functions with RULES; but that seems wrong.
 Generally, we add a NOINLINE pragma to a function with RULES to ensure
 that the function does not inline before the rule has a chance to fire.  I
 think we should do the same thing with these auto-generated RULES from
 specialisations.

 Patch coming.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15445#comment:2>
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