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

[GHC]

#11179: Allow plugins to access "dead code"
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        Reporter:  lerkok            |                Owner:
            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:                    |
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Comment (by gridaphobe):

 After a closer I don't think it's actually worth opening a separate
 ticket, as the optimizations the desugarer performs are truly simple. It's
 just

 1. the dead code removal this ticket already aims to address, and
 2. inlining single-use functions, which is also problematic for analysis
 tools (we may have annotated the function with some internal invariant!).

 As an example,

 {{{
 foo :: Int -> Int
 foo x = g x
   where
   -- g is only used once so it is immediately inlined by
   -- CoreSubst.simple_opt_expr
   g y = y + 1
 }}}

 Before `simpleOptPgm`:
 {{{
 foo :: Int -> Int
 foo =
   \ (x :: Int) ->
     letrec {
       g :: Int -> Int
       g =
         letrec {
           g :: Int -> Int
           g = \ (y :: Int) -> + y 1; } in
         g; } in
     g x
 }}}

 After:
 {{{
 foo :: Int -> Int
 foo = \ (x :: Int) -> (\ (y :: Int) -> + y 1) x
 }}}

 Granted, the nested definition of `g` pre-optimization is weird, but
 LiquidHaskell (at least) already handles this pattern without any special
 effort on our end.

 I think a cleaner solution would be my suggestion to move `simpleOptPgm`
 from the desugarer to an initial Core2Core pass (perhaps guarded by a flag
 so it doesn't bug regular users).

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11179#comment:15>
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