Re: [PATCH] Make -Wint-in-bool-context warn on suspicious shift ops

[Jason Merrill <jason@redhat.com>]

On Tue, Sep 27, 2016 at 10:28 AM, Bernd Edlinger
<bernd.edlinger@hotmail.de> wrote:
> On 09/27/16 16:10, Florian Weimer wrote:
>> * Bernd Edlinger:
>>
>>>> “0 << 0” is used in a similar context, to create a zero constant for a
>>>> multi-bit subfield of an integer.
>>>>
>>>> This example comes from GDB, in bfd/elf64-alpha.c:
>>>>
>>>> |   insn = INSN_ADDQ | (16 << 21) | (0 << 16) | (0 << 0);
>>>>
>>>
>>> Of course that is not a boolean context, and will not get a warning.
>>>
>>> Question is if "if (1 << 0)" is possibly a miss-spelled "if (1 < 0)".
>>>
>>> Maybe 1 and 0 come from macro expansion....
>>
>> But what's the intent of treating 1 << 0 and 0 << 0 differently in the
>> patch, then?
>
> I am not sure if it was a good idea.
>
> I saw, we had code of the form
> bool flag = 1 << 2;
>
> another value LOOKUP_PROTECT is  1 << 0, and
> bool flag = 1 << 0;
>
> would at least not overflow the allowed value range of a boolean.

Assigning a bit mask to a bool variable is still probably not what was
intended, even if it doesn't change the value.

Jason