[GHC] #765: x86_64 NCG goes via unsigned for FFI Int return types.

#765: x86_64 NCG goes via unsigned for FFI Int return types.
--------------------------------------------+-------------------------------
    Reporter:  duncan.coutts@xxxxxxxxxxxxx  |        Owner:         
        Type:  bug                          |       Status:  new    
    Priority:  high                         |    Milestone:         
   Component:  Compiler (NCG)               |      Version:  6.4.2  
    Severity:  normal                       |     Keywords:         
          Os:  Linux                        |   Difficulty:  Unknown
Architecture:  x86_64 (amd64)               |  
--------------------------------------------+-------------------------------
With ghc-6.4.2 on x86_64 and using the -fasm NCG we get the wrong answer
 for this FFI test case:

 cbit.c:
 {{{
 int c_id (int n) {
         return n;
 }
 }}}

 hsbit.hs:
 {{{
 module Foo where
 foreign import ccall unsafe "c_id" c_id :: Int -> IO Int
 }}}

 {{{
 ghc -c cbit.c
 ghc -fffi --make test.hs cbit.o
 ./a.out
 4294967295
 }}}

 Going via C we get the correct answer:

 {{{
 ghc -fvia-C -fffi --make test.hs cbit.o
 /tmp/ghc32363.hc:135: warning: implicit declaration of function `c_id'
 ./a.out
 -1
 }}}

 Obviously what's going on is that we're converting to an unsigned int
 value at some point so negative values are wrapping round.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/765>
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