> > diff --git a/include/linux/compiler.h b/include/linux/compiler.h > > index a2a56a50dd85227a4fdc62236a2710ca37c5ba52..30ce06df4153cfdc0fad9bc7bffab9097f8b0450 100644 > > --- a/include/linux/compiler.h > > +++ b/include/linux/compiler.h > > @@ -316,6 +316,47 @@ static inline void *offset_to_ptr(const int *off) > > #define statically_true(x) (__builtin_constant_p(x) && (x)) > > #define statically_false(x) (__builtin_constant_p(x) && (x) == 0) > > > > +/* > > + * Whether x is the integer constant expression 0 or something else. > > + * > > + * Details: > > + * - The C11 standard defines in §6.3.2.3.3 > > + * (void *)<integer constant expression with the value 0> > > + * as a null pointer constant (c.f. the NULL macro). > > + * - If x evaluates to the integer constant expression 0, > > + * (void *)(x) > > + * is a null pointer constant. Else, it is a void * expression. > > + * - In a ternary expression: > > + * condition ? operand1 : operand2 > > + * if one of the two operands is of type void * and the other one > > + * some other pointer type, the C11 standard defines in §6.5.15.6 > > + * the resulting type as below: > > + * if one operand is a null pointer constant, the result has the > > + * type of the other operand; otherwise [...] the result type is > > + * a pointer to an appropriately qualified version of void. > > + * - As such, in > > + * 0 ? (void *)(x) : (char *)0 > > + * if x is the integer constant expression 0, operand1 is a null > > + * pointer constant and the resulting type is that of operand2: > > + * char *. If x is anything else, the type is void *. > > + * - The (long) cast silences a compiler warning for when x is not 0. > > + * - Finally, the _Generic() dispatches the resulting type into a > > + * Boolean. > > The comment is absolutely excessive. I like this comment. Particularly I like the references to the standard followed by a step-by-step explanation of how the macro is built. > I'm sure I managed about 2 lines in one of the patches I did. Sorry, don't understand this. Thanks, Yury > > + * > > + * Glory to Martin Uecker <Martin.Uecker@xxxxxxxxxxxxxxxxxxxxx> > > IIRC Martin has agreed in the past that the accreditation can > be removed - especially since it refers to the 'sizeof (void)' trick. > > > + */ > > +#define __is_const_zero(x) \ > > + _Generic(0 ? (void *)(long)(x) : (char *)0, char *: 1, void *: 0) > > + > > +/* > > + * Returns a constant expression while determining if its argument is a > > + * constant expression, most importantly without evaluating the argument. > > You need to differentiate between a 'constant integer expression' > and a 'compile time constant'. > > > + * > > + * If getting a constant expression is not relevant to you, use the more > > + * powerful __builtin_constant_p() instead. > > __builtin_constant_p() is not 'more powerful' it is testing for > something different. > > David > > > + */ > > +#define is_const(x) __is_const_zero(0 * (x)) > > + > > /* > > * This is needed in functions which generate the stack canary, see > > * arch/x86/kernel/smpboot.c::start_secondary() for an example. > > > > -- > > 2.45.2 > > > > - > Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK > Registration No: 1397386 (Wales)
