[GHC] #11155: Trivial error thunk gives "undefined reference to stg_ap_0_upd_info"

2 Dec 2015

      #11155: Trivial error thunk gives "undefined reference to stg_ap_0_upd_info"
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           Reporter:  simonpj        |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  7.10.2
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 This error popped up when Alan was coding the solution for #11028.  The
 error is utterly unrelated to what Alan was working on.  Here's a
 reproducer
 {{{
 {-# OPTIONS_GHC -O -fno-full-laziness #-}
 module Main where

 foo :: Bool
 {-# NOINLINE foo #-}
 foo = error "rk"

 bar x = let t :: Char
             t = case foo of { True -> 'v'; False -> 'y' }
         in [t]

 main = print (bar ())
 }}}
 Just compile that and you get
 {{{
 Foo.o: In function `c1Sm_info':
 (.text+0x29a): undefined reference to `stg_ap_0_upd_info'
 }}}
 Why do we get that unresolved symbol?   The STG code for `bar` looks like
 {{{
 Main.bar :: forall t_aup. t_aup -> [GHC.Types.Char]
 [GblId, Arity=1, Str=DmdType m2, Unf=OtherCon []] =
     \r srt:SRT:[rf :-> Main.foo] [x_s1RR]
         let {
           sat_s1RT [Occ=Once] :: GHC.Types.Char
           [LclId, Str=DmdType] =
               \u srt:SRT:[rf :-> Main.foo] [] Main.foo;
         } in  : [sat_s1RT GHC.Types.[]];
 }}}
 Look at that: an updatable thunk saying `sat_s1RT = Main.foo`!
 The error message is terrible, but the problem is a thunk whose only
 payload is a single variable.

 Why does that happen?  The Core is
 {{{
 bar =
   \ (@ t_aup) _  -> let t::Char = case foo of wild_00 { }
                     in : @ Char t ([] @ Char)
 }}}
 The `case` is needed to change `foo`'s type from `Bool` to `Char`.
 The Core-to-STG pass drops the empty case alternatives as useless
 (rightly),
 but leaves a bare variable as the RHS, which confuses the code generator.

 We should clearly substitute `Main.foo` for `t`, either in Core-to-STG, or
 during code generation.

 Why hasn't this happened before now?  It is quite hard to provoke, because
 floating the thunk for `t` to top level stops it happening.  So it only
 happens if you switch off full laziness (as my test case here does), or
 if some very delicate inlining happens after the last float-out. The
 latter
 is very rare, but it's what happened to Alan.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11155
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