[commit: ghc] master: hopefully fix #7620 (b91f3d2)

[Simon Marlow]

Repository : ssh://darcs.haskell.org//srv/darcs/ghc

On branch  : master

http://hackage.haskell.org/trac/ghc/changeset/b91f3d260f3ccb57e8fcd61f943c6fadf26391c8

>---------------------------------------------------------------

commit b91f3d260f3ccb57e8fcd61f943c6fadf26391c8
Author: Simon Marlow <marlowsd at gmail.com>
Date:   Mon Jan 28 10:04:34 2013 +0000

    hopefully fix #7620

>---------------------------------------------------------------

 compiler/cmm/PprC.hs |    7 +++++--
 1 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/compiler/cmm/PprC.hs b/compiler/cmm/PprC.hs
index bcfb5dc..45f46b8 100644
--- a/compiler/cmm/PprC.hs
+++ b/compiler/cmm/PprC.hs
@@ -358,10 +358,13 @@ pprExpr e = case e of
     CmmRegOff reg 0 -> pprCastReg reg
 
     CmmRegOff reg i
-        | i >  0    -> pprRegOff (char '+') i
-        | otherwise -> pprRegOff (char '-') (-i)
+        | i < 0 && negate_ok -> pprRegOff (char '-') (-i)
+        | otherwise          -> pprRegOff (char '+') i
       where
         pprRegOff op i' = pprCastReg reg <> op <> int i'
+        negate_ok = negate (fromIntegral i :: Integer) <
+                    fromIntegral (maxBound::Int)
+                     -- overflow is undefined; see #7620
 
     CmmMachOp mop args -> pprMachOpApp mop args