On 09/27/16 14:49, Florian Weimer wrote: > * Jason Merrill: > >> On Sun, Sep 25, 2016 at 3:46 AM, Bernd Edlinger >> <bernd.edlin...@hotmail.de> 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); >
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.... Bernd.