[GHC] #14052: Significant GHCi speed regression with :module and `let` in GHC 8.2.1

[GHC]

#14052: Significant GHCi speed regression with :module and `let` in GHC 8.2.1
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.4.1
       Component:  GHCi              |              Version:  8.2.1-rc2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Runtime           |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by phadej):

 {{{
 --- a/compiler/main/InteractiveEval.hs
 +++ b/compiler/main/InteractiveEval.hs
 @@ -655,7 +655,7 @@ setContext imports
                 liftIO $ throwGhcExceptionIO (formatError dflags mod err)
             Right all_env -> do {
         ; let old_ic         = hsc_IC hsc_env
 -             !final_rdr_env = all_env `icExtendGblRdrEnv` ic_tythings
 old_ic
 +             !final_rdr_env = {-# SCC "final_rdr_env" #-} all_env
 `icExtendGblRdrEnv` ic_tythings old_ic
         ; setSession
           hsc_env{ hsc_IC = old_ic { ic_imports    = imports
                                    , ic_rn_gbl_env = final_rdr_env }}}}
 }}}

 makes

 {{{
 COST CENTRE         MODULE          SRC
 %time %alloc

 final_rdr_env       InteractiveEval
 compiler/main/InteractiveEval.hs:660:59-104   48.0   91.3
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14052#comment:9>
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