[GHC] #11179: Allow plugins to access "dead code"

[GHC]

#11179: Allow plugins to access "dead code"
-------------------------------------+-------------------------------------
        Reporter:  lerkok            |                Owner:  (none)
            Type:  feature request   |               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:  #10823            |  Differential Rev(s):  Phab:D3073
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by abakst):

 Here is one benefit of moving `simpleOptPgm` to a separate pass: I am
 currently writing a static analysis as a CoreToCore plugin pass. Consider
 the following:

 {{{
 {-#
   OPTIONS_GHC -fplugin      Analysis.Plugin
               -fplugin-opt  Analysis.Plugin:dataNodeProcess
 #-}
 module DataNode where

 import Control.Distributed.Process.ManagedProcess

 dataNodeProcess :: ProcessDefinition Int
 dataNodeProcess = defaultProcess {
   apiHandlers = [handleCall $ \i () -> reply () i]
   }
 }}}

 The associated core *without* optimizations (`ghc -O0 DataNode.hs -ddump-
 ds`) is
 {{{
 dataNodeProcess
 dataNodeProcess =
   case defaultProcess
   of _
   { ProcessDefinition _ ds_d4tk ds_d4tl ds_d4tm ds_d4tn ds_d4to ->
   ProcessDefinition
     (: ($ (handleCall $dSerializable_a462 $dSerializable_a462)
           (\ i_a30r ds_d4tf ->
              case ds_d4tf of _ { () -> reply $dSerializable_a462 () i_a30r
 }))
        [])
     ds_d4tk
     ds_d4tl
     ds_d4tm
     ds_d4tn
     ds_d4to
   }
 }}}

 The detail isn't important, but since `defaultProcess` is exported
 by`ManagedProcess`, the analysis knows what to do. On the other hand, if I
 run `ghc -O2 DataNode.hs -ddump-ds`, this is (part of) the resulting core:

 {{{
 dataNodeProcess
 dataNodeProcess =
   ProcessDefinition
     (build
        (\ @ a_d4y4 c_d4y5 n_d4y6 ->
           c_d4y5
             ($ (handleCall $dSerializable_a4an $dSerializable_a4an)
                (\ i_a33Z ds_d4y0 ->
                   case ds_d4y0 of _ { () -> reply $dSerializable_a4an ()
 i_a33Z }))
             n_d4y6))
     []
     []
     (defaultProcess2 `cast` ...)
     (defaultProcess1 `cast` ...)
     Terminate
 }}}

 As `defaultProcess1` and `defaultProcess2` are not exported by
 `ManagedProcess`, the analysis has no idea what do (which can be quite
 annoying if it's in a place where we want precision).

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11179#comment:19>
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