The following Cmm code snippet compiles fine

foo ()
{
  STK_CHK_GEN_N (((1)*8)); /* works */
  return (0);
}

The following one, however, only compiles with -O0, but panics with -O1 or more:

foo ()
{
  STK_CHK_GEN_N (8); /* panics */
  return (0);
}

ghc: panic! (the 'impossible' happened)
  (GHC version 7.8.3 for x86_64-unknown-linux):
	bundle
    c1 foo [(c1,
             label: block{v c1}_info
             rep:StackRep [])]
    [(c0, {}), (c3, {})]
    foo() //  []
            { info_tbl: [(c1,
                          label: block{v c1}_info
                          rep:StackRep [])]
              stack_info: arg_space: 8 updfr_space: Just 8
            }
        {offset
          c0: R1 = 0;
              call (P64[Sp])(R1) args: 8, res: 0, upd: 8;
        }
    }

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