[EXTERNAL] Unexpected duplicate join points in "Core" output?

[Simon Peyton Jones]

| For two join points to be duplicates they need to not only be alpha
| equivalent but to also have the same continuation.  

Yes exactly. And it would not be hard to adapt the existing CSE pass to support this.  Just needs doing.

A ticket and a repo case would be really helpful.

Simon

PS: I am leaving Microsoft at the end of November 2021, at which point simonpj at microsoft.com will cease to work.  Use simon.peytonjones at gmail.com instead.  (For now, it just forwards to simonpj at microsoft.com.)

| -----Original Message-----
| From: ghc-devs <ghc-devs-bounces at haskell.org> On Behalf Of Viktor
| Dukhovni
| Sent: 24 November 2021 21:27
| To: ghc-devs at haskell.org
| Subject: Re: [EXTERNAL] Unexpected duplicate join points in "Core"
| output?
| 
| On Sun, Nov 21, 2021 at 06:53:53AM -0500, Carter Schonwald wrote:
| 
| > On Sat, Nov 20, 2021 at 4:17 PM Simon Peyton Jones via ghc-devs <
| > ghc-devs at haskell.org> wrote:
| >
| > > There is absolutely no reason not to common-up those to join
| points.
| > > But we can't common up some join points when we could if they were
| let's.
| > > Consider
| > >
| > > join j1 x = x+1
| > > in case v of
| > >       A -> f (join j2 x = x+1 in ...j2...)
| > >       B -> ....j1...
| > >       C -> ....j1...
| > >
| > > Even though j2 is identical to j1's, we can't eliminate j2 in
| favour
| > > of j1 because then j1 wouldn't be a join point any more.
| >
| > In this example: why would it stop being a join point ?
| >
| > Admittedly, my intuition might be skewed by my own ideas about how
| > join points are sortah a semantic special case of other constructs.
| 
| I think the point is that join points are tail calls that don't return
| to the caller.  But here even though `j1` and `j2` have the same body
| j1's continuation is not the same as j2's continuation.
| 
| Rather the result of `j2` is the input to `f`, but the result of j1 is
| a possible output of the whole `case` block in the B and C branches.
| For two join points to be duplicates they need to not only be alpha
| equivalent but to also have the same continuation.  Something like
| 
|     join j1 x = x + 1 in
|     join j2 y = y + 1 in
|         ... j1 ...
|         ... j2 ...
| 
| where eliminating j2 in favour of j1 should be correct.
| 
| --
|     VIktor.
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