Subject: Re: bug#15924: [PATCH] dfa: avoid undefined behavior of "1 << 31"

From: Jim Meyering <[email-masked]>
Date: Mon, 18 Nov 2013 21:25:25 -0800

On Mon, Nov 18, 2013 at 6:16 PM, Paul Eggert <[email-masked]> wrote:
> Jim Meyering wrote:
>>  static int
>>  tstbit (unsigned int b, charclass const c)
>>  {
>> -  return c[b / INTBITS] & 1 << b % INTBITS;
>> +  return c[b / INTBITS] & 1U << b % INTBITS;
>>  }
>
> On a machine with 32-bit int and where b % INTBITS is 31,
> the expression c[b / INTBITS] & 1U << b % INTBITS
> is of type 'unsigned' and can have the value 2**31, and
> this will overflow when tstbit converts that value as an int,
> leading to implementation-defined behavior, which can include
> raising a signal.
>
> Better would be something like this:
>
> static bool
> tstbit (unsigned int b, charclass const c)
> {
>   return c[b / INTBITS] >> b % INTBITS & 1;
> }
>
> and it'd probably be better to encourage this style in
> other places where the problem occurs, e.g., quotearg.

Good point.  "bool" is a better return type, too.
I will adjust.

Thanks, Paul.