From: Andrew Morton > Sent: 30 March 2023 20:51 > > On Thu, 30 Mar 2023 13:42:39 +0300 Jani Nikula <jani.nikula@xxxxxxxxx> wrote: > > > is_power_of_2() only works for types <= sizeof(unsigned long) and it's > > also not a constant expression. There are a number of places in kernel > > where is_power_of_2() is called on u64, which fails on 32-bit > > builds. Try to remedy that. While at it, make it a constant expression > > when possible. > > Yes, the current `is_power_of_2(unsigned long n)' isn't very general. > > But wouldn't all these issues be addressed by simply doing > > #define is_power_of_2(n) (n != 0 && ((n & (n - 1)) == 0)) > > ? > > (With suitable tweaks to avoid evaluating `n' more than once) I think you need to use the 'horrid tricks' from min() to get a constant expression from constant inputs. For non-constants this looks ok (see https://godbolt.org/z/G73MTr9jn) David static inline int lisp2(unsigned long n) { return n && !(n & (n - 1)); } static inline int llisp2(unsigned long long lln) { #if 0 // I think this looks worse, esp. for gcc on x86 return lln && !(lln & (lln - 1)); #else unsigned long n = lln; if (lln >= 1ull << 32) { if (n) return 0; n = lln >> 32; } return lisp2(n); #endif } #define isp2(n) (sizeof ((n)+0) == sizeof (long) ? lisp2(n) : llisp2(n)) int is12(unsigned long long i) { return isp2(i); } - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
