Re: [GHC] #13027: Core lint errors compiling containers HEAD with GHC HEAD

21 Dec 2016

      #13027: Core lint errors compiling containers HEAD with GHC HEAD
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        Reporter:  erikd             |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.2.1
       Component:  Compiler          |              Version:  8.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Building GHC      |  Unknown/Multiple
  failed                             |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Changes (by RyanGlScott):

 * cc: dfeuer (added)

Comment:

 I'm not entirely sure if this is a GHC bug. For one thing, you don't need
 GHC HEAD to reproduce this - you could just as well use GHC 8.0.1. Here is
 a minimized test case (taken from the source code of `containers` HEAD):

 {{{#!hs
 {-# LANGUAGE BangPatterns #-}
 {-# LANGUAGE MagicHash #-}
 module Containers (insert) where

 import GHC.Exts (isTrue#, reallyUnsafePtrEquality#)

 data Set a = Bin {-# UNPACK #-} !Size !a !(Set a) !(Set a)
            | Tip

 type Size = Int

 insert :: Ord a => a -> Set a -> Set a
 insert = go
   where
     go :: Ord a => a -> Set a -> Set a
     go !x Tip = Bin 1 x Tip Tip
     go !x t@(Bin sz y l r) = case compare x y of
         LT | l' `ptrEq` l -> t
            | otherwise -> undefined -- balanceL y l' r
            where !l' = go x l
         GT | r' `ptrEq` r -> t
            | otherwise -> undefined -- balanceR y l r'
            where !r' = go x r
         EQ | x `ptrEq` y -> t
            | otherwise -> Bin sz x l r
 {-# INLINABLE insert #-}

 ptrEq :: a -> a -> Bool
 ptrEq x y = isTrue# (reallyUnsafePtrEquality# x y)
 {-# INLINE ptrEq #-}
 }}}

 If you compile this with `/opt/ghc/8.0.1/bin/ghc Containers.hs -dcore-lint
 -O2`, you'll get a very similar Core Lint error.

 The culprit seems to be the suspicious use of `reallyUnsafePtrEquality#`
 in `ptrEq`. Any thoughts on this, David?

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13027#comment:1
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