[GHC] #13513: Incorrect behavior on arm64 with optimisations

2 Apr 2017

      #13513: Incorrect behavior on arm64 with optimisations
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           Reporter:  achirkin       |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.2
           Keywords:                 |  Operating System:  Linux
       Architecture:  aarch64        |   Type of failure:  Incorrect result
                                     |  at runtime
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 I compiled ghc manually on a chromebook running ubuntu on aarch64 (arm64).
 I used `perf-llvm` build during compilation. This might be related to
 [https://ghc.haskell.org/trac/ghc/ticket/11578], but I am not sure how.

 While working with stack and cabal I experienced unusually long
 compilation times or seemingly random build failures from time to time.
 Luckily, at some point I got very interesting error in package `text`:
 {{{#!hs
 Prelude> import qualified Data.Text as T
 Prelude T> T.splitOn (T.pack " ") (T.pack "Hello world!")
 ["Hello","world!\54497\44724?\NUL\54521\44724?\NUL\NUL\NUL\NUL\NUL\34560...BIG
 BLOB OF BYTES HERE...\NUL"]
 }}}
 By default this package compiles with `-O2`, so I tried various options to
 determine when the result is correct and when is not.
 To test this I used bare GHC installation and `cabal-install-1.24.0.2`,
 (`cabal sandbox init && cabal install text --ghc-options="..."`).

 Here is what I tried so far:

   * BAD: `-O2`
   * BAD: `-O2 -fllvm -optlc-O3`
   * OK: `-O1 -fllvm -optlc-O3`
   * OK: `-O1 -fllvm -fliberate-case -fregs-graph -fspec-constr -optlc-O3`

 That is: when `text` package is compiled with `-O2` the result of
 `Text.splitOn` function is garbaged with random data; when it is compiled
 with `-O1`, the result is correct.
 Since `text` is represented using `ByteArray#` internally, I would suggest
 something is wrong with `ByteArray#`'s length attribute, but I am not
 sure.

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13513
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