* Jason Merrill:
> On Sun, Sep 25, 2016 at 3:46 AM, Bernd Edlinger
> <[email protected]> wrote:
>> This patch makes -Wint-in-bool-context warn on suspicious integer left
>> shifts, when the integer is signed, which is most likely some kind of
>> programming error, for instance using "<<" instead of "<".
>>
>> The warning is motivated by the fact, that an overflow on integer shift
>> left is undefined behavior, even if gcc won't optimize the shift based
>> on the undefined behavior.
>>
>> So in absence of undefined behavior the boolean result does not depend
>> on the shift value, thus the whole shifting is pointless.
>
> It's pointless for unsigned integers, too; why not warn for them as
> well? And why not warn for 0 << 0 and 1 << 0, which are just as
> pointless?
“1 << 0“ is often used in a sequence of flag mask definitions. This
example is from <bits/termios.h>:
| /* Terminal control structure. */
| struct termios
| {
| /* Input modes. */
| tcflag_t c_iflag;
| #define IGNBRK (1 << 0) /* Ignore break condition. */
| #define BRKINT (1 << 1) /* Signal interrupt on break. */
| #define IGNPAR (1 << 2) /* Ignore characters with parity errors. */
| #define PARMRK (1 << 3) /* Mark parity and framing errors. */
“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);
