Re: [PATCH] scripts: coccinelle: check for !(un)?likely usage
From: Denis Efremov
Date: Wed Aug 28 2019 - 08:33:52 EST
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 <efremov@xxxxxxxxx> 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
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 (!__builtin_expect(!!(cond), 1))
if (!((!!(cond)) == 1))
if ((!!(cond)) != 1) and since !! could result in 0 or 1
if ((!!(cond)) == 0)
if (__builtin_expect(!!(cond), 0))
if ((!!(cond)) == 0))