Re: A few cases of left shifting negative integers

Andres Freund <andres@anarazel.de> writes:
> On 2015-08-21 13:03:42 -0400, Tom Lane wrote:
>> The behavior is well-defined, at least as long as we don't shift far
>> enough to have integer overflow

> Unfortunately not:
> 5.8.2: The value of E1 << E2 is E1 left-shifted E2 bit positions;
> vacated bits are zero-filled. 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. Otherwise, if E1 has a signed type and
> non-negative value, and E1 � 2 E2 is representable in the result type,
> then that is the resulting value; >>otherwise, the behavior is undefined<<.

[ rolls eyes... ]  There isn't any machine in the world where the behavior
isn't well-defined short of overflow.  Why do the C spec authors persist
in pretending otherwise?
        regards, tom lane