[Q] Inlining done: evtRead

[Simon Peyton Jones]

Oh well, your INLINE pragma is saying "please inline evtRead at every call site". And so GHC does exactly that.

That seems like obeying the pragma doesn't it?

Simon

| -----Original Message-----
| From: Gabor Greif <ggreif at gmail.com>
| Sent: 08 January 2019 00:06
| To: Simon Peyton Jones <simonpj at microsoft.com>
| Cc: ghc-devs <ghc-devs at haskell.org>
| Subject: Re: [Q] Inlining done: evtRead
| 
| I think you have to follow this:
| 
| -- | Data is available to be read.
| evtRead :: Event
| evtRead = Event 1
| {-# INLINE evtRead #-}
| 
| 
| On 1/8/19, Simon Peyton Jones <simonpj at microsoft.com> wrote:
| > 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
| >