[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
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        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):

 when module is loaded into context with `:m`
 we `InteractiveEval.setContext` which builds `ic_rn_gbl_env` with

 {{{
 !final_rdr_env = all_env `icExtendGblRdrEnv` ic_tythings old_ic
 }}}

 Because there aren't shadowing cleanup anymore, `ic_tythings` grows,
 `icExtendGblRdrEnv` is linear in ic_tythings size, which causes the
 quadratic behaviour when we repeadetly `:m` modules.

 So `it` isn't related to this bug. E.g. the `GenExample.hs` version below
 has similar slowdown:

 {{{
 module Main where

 import Control.Monad

 import System.Environment
 import System.Exit
 import System.IO

 main :: IO ()
 main = do
   args <- getArgs
   case args of
     n:_ -> genExamples (read n)
     _ -> do hPutStrLn stderr "usage: runghc GenExamples.hs <num-examples>"
             exitWith $ ExitFailure 1

 genExamples :: Int -> IO ()
 genExamples nExamples = do
   putStrLn ":set +s"
   putStrLn ":l Foo"
   putStrLn "let bar = ()" -- first one
   sequence_ [genExample i | i <- [1..nExamples] ]

 genExample :: Int -> IO ()
 genExample i = putStr $ unlines
   [ ""
   , ":m *Foo"
   , "let foo = bar" -- ask old
   , "let bar = ()"  -- reset, if `let bar = foo` is even slower
   ]
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14052#comment:8>
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