[GHC] #13645: whoCreated produces an uninformative stack trace when an exception is raised in a CAF

4 May 2017

      #13645: whoCreated produces an uninformative stack trace when an exception is
raised in a CAF
-------------------------------------+-------------------------------------
           Reporter:  refold         |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           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:
-------------------------------------+-------------------------------------
 Consider the following program:

 {{{#!hs
 {-# LANGUAGE ScopedTypeVariables #-}
 module Main where

 import Control.Exception
 import GHC.Stack

 {-# NOINLINE caf #-}
 caf :: [Int]
 caf = [1..500]

 {-# NOINLINE caf_exc #-}
 caf_exc :: Int
 caf_exc = caf !! 1000

 {-# NOINLINE foo #-}
 foo :: Int -> Int
 foo 0 = caf_exc
 foo n = foo $ n - 1

 {-# NOINLINE bar #-}
 bar :: Int -> Int
 bar n = bar' n
   where
     bar' 0 = foo n
     bar' m = bar' $ m - 1

 main :: IO ()
 main = print (bar 10) `catch`
        \(e :: SomeException) -> do stacktrace <- whoCreated e
                                    print stacktrace
 }}}

 By default, when built with profiling, `whoCreated` in the example above
 produces a quite uninformative stack trace:

 {{{#!shell
 $ ./caf-nostack
 ["GHC.List.CAF (<entire-module>)"]
 }}}

 However, if you run the program with `+RTS -xc`, you'll see that it prints
 a stack trace with much more context:

 {{{#!shell
 $ ./caf-nostack +RTS -xc
 *** Exception (reporting due to +RTS -xc): (THUNK_2_0), stack trace:
   GHC.List.CAF
   --> evaluated by: Main.caf_exc,
   called from Main.CAF
   --> evaluated by: Main.foo,
   called from Main.bar.bar',
   called from Main.bar,
   called from Main.main,
   called from Main.CAF
   --> evaluated by: Main.main
 ["GHC.List.CAF (<entire-module>)"]
 }}}

 It'd be nice if `whoCreated` produced something closer to the `+RTS -xc`
 output in this case.

 Cabalised test project: https://github.com/23Skidoo/caf-nostack

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13645
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