From: Linus Torvalds > Sent: 17 November 2024 20:12 > > On Sun, 17 Nov 2024 at 11:23, David Laight <David.Laight@xxxxxxxxxx> wrote: > > > > Since 99% will be 1,0 maybe saving the extra expansion is best anyway. > > So have is_const_zero(x) and add if_const_zero(x, if_z, if_nz) later. > > Ok. So something like this seems to give us the relevant cases: > > #define __is_const_zero(x) \ > _Generic(0?(void *)(long)(x):(char *)0, char *:1, void *:0) > > #define is_const_zero(x) __is_const_zero(!!(x)) > #define is_const_true(x) __is_const_zero(!(x)) > #define is_const(x) __is_const_zero(0*!(x)) > > and should work with all scalar expressions that I can think of (ok, > technically 'void' is a scalar type and it obviously won't work with > that). And should work in all contexts. Seems a reasonable set. Maybe they need a set that are paired with __BUILD_BUG_ON_ZERO_MSG() to generate an error message on failure. Although I would add a few more ' ' characters for readability. > It does want a comment (in addition to the comment about how NULL is > special for the ternary op that makes it work): the '(long)' cast is > so that there are no warnings for casting to 'void *' when it's *not* > a constant zero expression, and the '!' pattern is to turn pointers > and huge constants into 'int' without loss of information and without > warnings. The comments would need to be terse one-liners. I wonder if it reads better (and without extra comments) if the (long) cast is removed and the 'callers' are required to generate 'long' args. So you have: #define __is_const_zero(x) \ _Generic(0 ? (void *)(x) : (char *)0, char *: 1, void *: 0) #define is_const_zero(x) __is_const_zero((x) ? 1L : 0L) #define is_const_true(x) __is_const_zero((x) ? 0L : 1L) #define is_const(x) __is_const_zero((x) ? 0L : 0L) I've done a quick test of the last one in godbolt. David > > Compound types obviously will generate a warning. As they should. > > The above looks reasonable to me, but I didn't actually test any of it > in the actual kernel build. > > Linus - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
