[GHC] #13429: Optimizer produces Core with an infinite <<loop>>

16 Mar 2017

      #13429: Optimizer produces Core with an infinite <<loop>>
-------------------------------------+-------------------------------------
           Reporter:  lehins         |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.2
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Incorrect result
  Unknown/Multiple                   |  at runtime
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 While using vector package to implement convolution and supplying `-O1` or
 `-02` to ghc compilation results in core with an infinite loop. In order
 to trigger this behavior at least two modules is required. Attached is the
 minimal setup that I could come up with, that demonstrates the issue. Here
 is the stack trace:

 {{{
 $ stack --install-ghc --resolver lts-8.3 exec -- ghc -O1 -prof -fprof-auto
 main.hs && ./main +RTS -xc
 [1 of 2] Compiling Loop             ( Loop.hs, Loop.o )
 [2 of 2] Compiling Main             ( main.hs, main.o )
 Linking main ...
 *** Exception (reporting due to +RTS -xc): (THUNK_STATIC), stack trace:
   Main.CAF
   --> evaluated by: Main.main,
   called from Main.CAF
   --> evaluated by: Loop.toKernel.\,
   called from Data.Vector.Fusion.Util.>>=,
   called from Loop.toKernel,
   called from Main.main,
   called from Main.CAF
   --> evaluated by: Data.Vector.Fusion.Util.>>=,
   called from Loop.toKernel,
   called from Main.main,
   called from Main.CAF
 *** Exception (reporting due to +RTS -xc): (THUNK_STATIC), stack trace:
   Main.CAF
 main: <<loop>>
 }}}

 At first I though that it might be a bug in a vector package, which is
 still a possibility, but since I was not able to observe that issue with
 ghc-7.8.4, I decided to open an ticket here. I tested attached code with
 all of subsequent released ghc versions (7.10.1 - 8.0.2), which resulted
 in the infinite loop.

 Worth noting that sometimes, when recompilation of only the `main.hs` file
 is enforced, program starts to work as expected. Here is an example:

 {{{
 $ stack --resolver ghc-7.10.3 exec --package vector-0.11.0.0 --package
 primitive-0.6.1.0 -- ghc -O1 main.hs && ./main
 [1 of 2] Compiling Loop             ( Loop.hs, Loop.o )
 [2 of 2] Compiling Main             ( main.hs, main.o )
 Linking main ...
 main: <<loop>>
 $ touch main.hs
 $ stack --resolver ghc-7.10.3 exec --package vector-0.11.0.0 --package
 primitive-0.6.1.0 -- ghc -O1 main.hs && ./main
 [2 of 2] Compiling Main             ( main.hs, main.o )
 Linking main ...

 }}}

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13429
GHC http://www.haskell.org/ghc/
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