[GHC] #9370: unfolding info as seen when building a module depends on flags in a previously-compiled module

[GHC]

#9370: unfolding info as seen when building a module depends on flags in a
previously-compiled module
-------------------------------------+-------------------------------------
        Reporter:  carter            |                Owner:
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.2.1
       Component:  Compiler          |              Version:  7.8.3
      Resolution:                    |             Keywords:  newcomer,
                                     |  Inlining
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #8635             |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 Actually we really don't want to disable ALL unfoldings, just unfoldings
 for imported things.  Locally-bound things (esp in local lets) should be
 fine.

 I suppose that means that every use of `idUnfolding` has to be guarded
 with a test of `DynFlags`.

 The "is it imported?" test can be `isLocalVar`.

 I took a look at uses of `idUnfolding`.

 * Some, like `idUnfoldingFVs` can be dealt with by testing for
 `isLocalVar`.  Global vars will have no fvs, so no point in looking at the
 unfolding.  This might even improve perf!

 * Some, like the use of `idUnfolding` in `CoreLint` are guaranteed to be
 on a `LocalId`, so that can be an assertion.

 * The`isCompulsoryUnfolding` in `CoreSubst` is OK; we do want to inline
 conpulsory unfoldings regardless.

 Etc.  It's tiresome to look at all these call sites, but I see no
 alternative.  We have an Id that does ''have'' an unfolding, but we don't
 want to look at it....

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9370#comment:44>
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