[GHC] #11157: (Optimized prime generation) ghc: panic! (the 'impossible' happened)

[GHC]

#11157: (Optimized prime generation) ghc: panic! (the 'impossible' happened)
-------------------------------------+-------------------------------------
           Reporter:  jachymb        |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  7.10.2
           Keywords:                 |  Operating System:  Linux
       Architecture:  x86_64         |   Type of failure:  Compile-time
  (amd64)                            |  crash
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 I try to compile the example from
 https://wiki.haskell.org/Prime_numbers#Using_ST_Array
 {{{#!hs
 {-# OPTIONS_GHC -O2 #-}

 import Control.Monad
 import Control.Monad.ST
 import Data.Array.ST
 import Data.Array.Unboxed

 sieveUA :: Int -> UArray Int Bool
 sieveUA top = runSTUArray $ do
     let m = (top-1) `div` 2
         r = floor . sqrt $ fromIntegral top + 1
     sieve <- newArray (1,m) True      -- :: ST s (STUArray s Int Bool)
     forM_ [1..r `div` 2] $ \i -> do
       isPrime <- readArray sieve i
       when isPrime $ do               -- ((2*i+1)^2-1)`div`2 == 2*i*(i+1)
         forM_ [2*i*(i+1), 2*i*(i+2)+1..m] $ \j -> do
           writeArray sieve j False
     return sieve
 }}}

 I get the following error:
 {{{
 [1 of 1] Compiling Main             ( p111.hs, interpreted )
 ghc: panic! (the 'impossible' happened)
   (GHC version 7.10.2 for x86_64-unknown-linux):
         floatExpr tick break<29>()

 Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

 }}}

 The error does not happen if I remove the OPTIONS_GHC -O2 directive.
 I use ArchLinux and the GHC compiled package from it's standard
 repositories.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11157>
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