[Q] Inlining done: evtRead

[Simon Peyton Jones]

Are you sure?   GHC.Event isn't used on Windows, so I did this:

=================
module Bar where

newtype Evt = Evt Int

evtRead :: Evt
evtRead = Evt 33

instance Show Evt where
  show = showEvt

showEvt :: Evt -> String
{-# NOINLINE showEvt #-}
showEvt (Evt x) = show x
============

module Foo where

import Bar

main = print evtRead
===============

And indeed when I compile these with -O I get

Foo.main1
  = showEvt (Bar.evtRead1 `cast` (Sym (Bar.N:Evt[0]) :: Int ~R# Evt))

where Bar.evtRead1 is the static (I# 33) box. 

No duplication.

Can you give me a repro case that isn't OS-specific?  (I suppose I can try on Linux tomorrow, but I'm sure that the OS is only accidentally involved here.)

Simon

| -----Original Message-----
| From: Gabor Greif <ggreif at gmail.com>
| Sent: 07 January 2019 23:28
| To: Simon Peyton Jones <simonpj at microsoft.com>
| Cc: ghc-devs <ghc-devs at haskell.org>
| Subject: [Q] Inlining done: evtRead
| 
| Hi Simon,
| 
| a simplifier question...
| 
| Roughly a year ago I started learning about imported Ids, their unfoldings
| etc.
| 
| I have very small example program that compiles on Linux.
| 
| ```haskell
| import GHC.Event
| 
| main = print evtRead
| ```
| 
| `evtRead` is a newtype-wrapped Int. When you compile above program
| with HEAD GHC without optimisation, you'll see that `evtRead` gets
| passed directly to `show`.
| 
| But with -O1 it's unfolding will be inlined, floated to toplevel, and
| dumped as static global data into the using Main module. This was not
| the case in GHC 8.4. Not sure about 8.6 (will check). Anyway here is
| the inlining notice that the simplifier gave me (-ddump-inlinings
| -dverbose-core2core)
| 
| > Inlining done: GHC.Event.Internal.evtRead
| >     Inlined fn:  (GHC.Types.I# 1#)
| >                  `cast` (Sym (GHC.Event.Internal.N:Event[0])
| >                          :: GHC.Types.Coercible GHC.Types.Int
| > GHC.Event.Internal.Event)
| >     Cont:   Stop[BoringCtxt] GHC.Event.Internal.Event
| >
| 
| I believe this is a regression, as copies of global data can pop up in
| potentially many different modules.
| 
| What do you think? Which change could have caused this?
| 
| Cheers,
| 
|     Gabor