On 8/28/19 2:33 PM, Rasmus Villemoes wrote: > On 25/08/2019 21.19, Julia Lawall wrote: >> >> >>> On 26 Aug 2019, at 02:59, Denis Efremov <[email protected]> wrote: >>> >>> >>> >>>> On 25.08.2019 19:37, Joe Perches wrote: >>>>> On Sun, 2019-08-25 at 16:05 +0300, Denis Efremov wrote: >>>>> This patch adds coccinelle script for detecting !likely and !unlikely >>>>> usage. It's better to use unlikely instead of !likely and vice versa. >>>> >>>> Please explain _why_ is it better in the changelog. >>>> >>> >>> In my naive understanding the negation (!) before the likely/unlikely >>> could confuse the compiler >> >> As a human I am confused. Is !likely(x) equivalent to x or !x? > > #undef likely > #undef unlikely > #define likely(x) (x) > #define unlikely(x) (x) > > should be a semantic no-op. So changing !likely(x) to unlikely(x) is > completely wrong. If anything, !likely(x) can be transformed to > unlikely(!x).
As far as I could understand it: # define likely(x) __builtin_expect(!!(x), 1) # define unlikely(x) __builtin_expect(!!(x), 0) >From GCC doc: __builtin_expect compares the values. The semantics of the built-in are that it is expected that exp == c. if (!likely(cond)) if (!__builtin_expect(!!(cond), 1)) if (!((!!(cond)) == 1)) if ((!!(cond)) != 1) and since !! could result in 0 or 1 if ((!!(cond)) == 0) if (unlikely(cond)) if (__builtin_expect(!!(cond), 0)) if ((!!(cond)) == 0)) Thanks, Denis
