linux-kernel - Re: [PATCH 00/23] Make use of unlikely() more consistently.

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Message-ID: <46369585-4d67-5235-d57d-a5162cc85e56@gmail.com>
Date:   Fri, 31 Aug 2018 23:35:08 +0300
From:   Igor Stoppa <igor.stoppa@...il.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
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Subject: Re: [PATCH 00/23] Make use of unlikely() more consistently.

On 31/08/18 17:09, Arnd Bergmann wrote:

[...]

>> #define assert(condition)
>> ...
>>          if (unlikely(!(condition)))
>>                  error_action()
>> ...
> 
> There is a potential that this introduces false-postive -Wmaybe-uninitialized
> warnings when CONFIG_PROFILE_ANNOTATED_BRANCHES is
> set, since that turns unlikely() into a complex operation that in turn
> confuses the compiler so it no longer keeps track of which variables
> are initialized or not.
> 
> It's possible that none of your patches do that, but one needs to be aware
> of the problem, and possibly revert some of your patches if it does cause
> warning regressions in drivers that don't actually benefit from the
> micro-optimization.

I see.
But if such case might arise, it might be possible, instead of reverting 
these patches, to locally - and conditionally - turn unlikely() into an 
identity macro.

For example:

#ifdef CONFIG_PROFILE_ANNOTATOED_BRANCHES
#define unlikely(x) (x)
#endif

Without giving up the hint for the case of normal compilation.

--
igor