[GHC] #10052: Panic (something to do with floatExpr?)

[Peter Wortmann]

Simon Peyton Jones wrote:
> Why do you say "we are clearly trying to float past a breakpoint"?  Why is it so clear?

It's the only kind of Tickish that is scoped, counting and not splittable.

This means that we can't

a) Simply float code out of it, because the payload must still be 
covered (scoped)

b) Copy the tick, because it would change entry counts (here: duplicate 
breakpoints)

> Why is it wrong to float a lazy thunk out of a breakpoint?

Good questions - I haven't given breakpoint semantics a lot of thought, 
to be honest. My assumption was that most optimisation passes would 
never see them. And where they did, they should just leave them in peace 
as much as possible.

For whatever it's worth, the source cautions against making breakpoints 
unscoped:

    -- Breakpoints are scoped: eventually we're going to do call
    -- stacks, but also this helps prevent the simplifier from moving
    -- breakpoints around and changing their result type (see #1531).

Hm. We might try to make them pseudo-splittable, with non-counting 
breakpoints being NOPs for now? This might still allow us to implement 
breakpoint-based stack traces if we really want them...

Greetings,
   Peter

> Simon
>
> | -----Original Message-----
> | From: ghc-devs [mailto:ghc-devs-bounces at haskell.org] On Behalf Of Peter
> | Wortmann
> | Sent: 04 February 2015 17:24
> | To: ghc-devs at haskell.org
> | Subject: Re: [GHC] #10052: Panic (something to do with floatExpr?)
> |
> |
> |
> | We are clearly trying to float past a breakpoint here, which is simply
> | impossible. Pretty sure this would have been a panic before my changes
> | too (it would have tried to "mkNoCount" the breakpoint). Guess I was
> | wrong reading a "breakpoints don't appear here" invariant out of that...
> |
> | The quick fix would be to drop all floats in-place:
> |
> |    -- scoped, counting and unsplittable, can't be floated through
> |    | otherwise
> |    = floatBody tOP_LEVEL expr
> |
> | This fixes the panic, but is a bit awkward. Probably better to change
> | SetLevels? Not a piece of code I'm very familiar with...
> |
> | Greetings,
> |    Peter
> |
> | On 04/02/2015 13:31, Simon Peyton Jones wrote:
> | > Peter:
> | >
> | > Here's a bad crash, due to you.   (Doing this by email because I'm
> | offline.)
> | >
> | > The (Tick t e) case of FloatOut.floatExpr is incomplete.  It simply
> | panics in some cases.
> | >
> | > Could you fix this please?  Either that case shouldn't happen, in which
> | case Core Lint should check for it, and whoever is generating it should
> | be fixed.  Or it should happen, in which case floatExpr should do the
> | right thing.
> | >
> | > Could you leave a Note to explain what is happening in the floatExpr
> | (Tick ...) cases?
> | >
> | > Thanks
> | >
> | > Simon
> | >
> | > | -----Original Message-----
> | > | From: ghc-tickets [mailto:ghc-tickets-bounces at haskell.org] On Behalf
> | Of
> | > | GHC
> | > | Sent: 31 January 2015 17:38
> | > | Cc: ghc-tickets at haskell.org
> | > | Subject: [GHC] #10052: Panic (something to do with floatExpr?)
> | > |
> | > | #10052: Panic (something to do with floatExpr?)
> | > | -------------------------------------+-------------------------------
> | ----
> | > | --
> | > |               Reporter:  edsko       |             Owner:
> | > |                   Type:  bug         |            Status:  new
> | > |               Priority:  normal      |         Milestone:
> | > |              Component:  Compiler    |           Version:  7.10.1-rc2
> | > |               Keywords:              |  Operating System:
> | > | Unknown/Multiple
> | > |           Architecture:              |   Type of failure:
> | None/Unknown
> | > |   Unknown/Multiple                   |        Blocked By:
> | > |              Test Case:              |   Related Tickets:
> | > |               Blocking:              |
> | > | Differential Revisions:              |
> | > | -------------------------------------+-------------------------------
> | ----
> | > | --
> | > |  Loading
> | > |
> | > |  {{{
> | > |  main = let (x :: String) = "hello" in putStrLn x
> | > |  }}}
> | > |
> | > |  using a very simple driver for the GHC API (see T145.hs) causes a
> | ghc
> | > |  panic:
> | > |
> | > |  {{{
> | > |  [1 of 1] Compiling Main             ( T145-input.hs, interpreted )
> | > |  T145: T145: panic! (the 'impossible' happened)
> | > |    (GHC version 7.10.0.20150128 for x86_64-apple-darwin):
> | > |          floatExpr tick
> | > |  <<details unavailable>>
> | > |
> | > |  Please report this as a GHC bug:
> | http://www.haskell.org/ghc/reportabug
> | > |  }}}
> | > |
> | > |  This panic is arising in our test case for #8333, so it may be
> | related
> | > | to
> | > |  that bug.
> | > |
> | > | --
> | > | Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10052>
> | > | GHC <http://www.haskell.org/ghc/>
> | > | The Glasgow Haskell Compiler
> | > | _______________________________________________
> | > | ghc-tickets mailing list
> | > | ghc-tickets at haskell.org
> | > | http://www.haskell.org/mailman/listinfo/ghc-tickets
> | >
> |
> | _______________________________________________
> | ghc-devs mailing list
> | ghc-devs at haskell.org
> | http://www.haskell.org/mailman/listinfo/ghc-devs