From: Kees Cook > Sent: 01 March 2024 04:45 > To: Rasmus Villemoes <linux@xxxxxxxxxxxxxxxxxx> > > The __is_constexpr() macro is dark magic. Shed some light on it with > a comment to explain how and why it works. All the 8s don't help... I don't think you need that much explanation. Perhaps just saying that the type of ?: depends on the types of the values and is independent of the condition. The type of (0 ? (void *)p : (foo *)q) is normally 'void *' (so that both values can be assigned to it). But if 'p' is 'an integer constant expression with value 0' then (void *)p is NULL and the type is 'foo *'. The type can then be checked to find out it 'p' is constant 0. A non-zero constant 'p' can be multiples by 0. I need to replace the definition with (the more portable): #define __if_constexpr(cond, if_const, if_not_const) \ _Generic(0 ? (void *)((long)(cond) * 0) : (char *)0, \ char *: (if_const), \ void *: (if_not_const)) which is arguably less cryptic. #define __is_constexpr(cond) __if_constexpr(cond, 1, 0) So that I can write: #define is_non_neg_const(x) (__if_constexpr(x, x , -1) >= 0) and avoid the compiler bleating about some comparisons in unreachable code. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
