Re: [PATCH 0/2] Fix invalid left shift of negative value.

["Ulrich Weigand" <uweigand at de dot ibm dot com>]

Kevin Büttner wrote:

> Looking at one of your changes from part 1/2...
> 
> -    (LONGEST) -1 << (TYPE_LENGTH (base_type) * TARGET_CHAR_BIT - 1);
> +    -((LONGEST) 1 << (TYPE_LENGTH (base_type) * TARGET_CHAR_BIT - 1));
> 
> What aspect of the original expression is not defined by the C standard?

The C standard (either C99 or C11) says:

  The result of E1 << E2 is E1 left-shifted E2 bit positions; vacated bits
  are filled with zeros. If E1 has an unsigned type, the value of the result
  is E1 * 2^E2, reduced modulo one more than the maximum value representable
  in the result type. If E1 has a signed type and nonnegative value, and
  E1 * 2^E2 is representable in the result type, then that is the resulting
  value; otherwise, the behavior is undefined.

Note the "otherwise" case includes any E1 of signed type and negative value.

(For >>, the behavior in the latter case is at least implementation-
defined, and not undefined.)

Bye,
Ulrich

-- 
  Dr. Ulrich Weigand
  GNU/Linux compilers and toolchain
  Ulrich.Weigand@de.ibm.com