bug#7928: mktime test in configure: UB resulting in infinite loop

[Paul Eggert]

On 01/27/11 09:28, Rich Felker wrote:
> On Thu, Jan 27, 2011 at 08:14:56AM -0700, Eric Blake wrote:
>> > # define TYPE_MINIMUM(t) \
>> >   ((t) (! TYPE_SIGNED (t) \
>> >         ? (t) 0 \
>> >         : TYPE_SIGNED_MAGNITUDE (t) \
>> >         ? ~ (t) 0 \
>> >         : ~ (t) 0 << (sizeof (t) * CHAR_BIT - 1)))
>> > # define TYPE_MAXIMUM(t) \
>> >   ((t) (! TYPE_SIGNED (t) \
>> >         ? (t) -1 \
>> >         : ~ (~ (t) 0 << (sizeof (t) * CHAR_BIT - 1))))
> The last line of this macro has UB due to signed integer overflow in
> the << operation.

No it doesn't.  ~ (t) 0 evaluates to -1, and -1 << 31 does not
overflow.