[GHC] #14085: powModInteger sometimes ignores sign of argument

[GHC]

#14085: powModInteger sometimes ignores sign of argument
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        Reporter:  ocheron           |                Owner:  ocheron
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Core Libraries    |              Version:  8.2.1
      Resolution:                    |             Keywords:  integer-gmp
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Incorrect result  |  Unknown/Multiple
  at runtime                         |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D3826
       Wiki Page:                    |
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Comment (by ocheron):

 Working on a patch I found another issue with `gcdExtInteger` when first
 argument is negative.

 Program:
 {{{#!haskell
 {-# LANGUAGE MagicHash #-}
 {-# LANGUAGE UnboxedTuples #-}

 import GHC.Integer.GMP.Internals

 f a b = let (# g, s #) = gcdExtInteger a b in (g, s)

 main = do
     print $ f 100000000000000000000000000000 7
     print $ f (-100000000000000000000000000000) 7
 }}}
 crashes with assertion when executed:
 {{{
 $ ghc -V && ghc --make a
 The Glorious Glasgow Haskell Compilation System, version 8.2.1
 [1 of 1] Compiling Main             ( a.hs, a.o )
 Linking a ...
 $ ./a
 (1,3)
 Assertion failed: (sn <= xn), function integer_gmp_gcdext, file libraries
 /integer-gmp/cbits/wrappers.c, line 315.
 Abort trap: 6
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14085#comment:2>
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