linux-kernel - Re: [PATCH] compiler, clang: Add always

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Date:   Mon, 19 Jun 2017 14:14:26 -0700
From:   Sodagudi Prasad <psodagud@...eaurora.org>
To:     David Rientjes <rientjes@...gle.com>
Cc:     mark.rutland@....com, will.deacon@....com, catalin.marinas@....com,
        mingo@...nel.org, peterz@...radead.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] compiler, clang: Add always_inline attribute to inline

On 2017-06-19 13:25, David Rientjes wrote:
> On Mon, 19 Jun 2017, Prasad Sodagudi wrote:
> 
>> Commit abb2ea7dfd82 ("compiler, clang: suppress warning for unused
>> static inline functions") re-defining the 'inline' macro but
>> __attribute__((always_inline)) is missing. Some compilers may
>> not honor inline hint if always_iniline attribute not there.
>> So add always_inline attribute to inline as done by
>> compiler-gcc.h file.
>> 
> 
> IIUC, __attribute__((always_inline)) was only needed for gcc versions < 
> 4
> and that the inlining decision making is improved in >= 4.  To make a
> change like this, I would think that we would need to show that clang 
> is
> making suboptimal decisions.  I don't think there's a downside to 
> making
> CONFIG_OPTIMIZE_INLINING specific only to gcc.
> 
> If it is shown that __attribute__((always_inline)) is needed for clang 
> as
> well, this should be done as part of compiler-gcc.h to avoid duplicated
> code.

Hi David,

Here is the discussion about this change -  
https://lkml.org/lkml/2017/6/15/396
Please check mark and will's comments.

-Thanks, Prasad

> 
>> Signed-off-by: Prasad Sodagudi <psodagud@...eaurora.org>
>> ---
>>  include/linux/compiler-clang.h | 5 +++++
>>  1 file changed, 5 insertions(+)
>> 
>> diff --git a/include/linux/compiler-clang.h 
>> b/include/linux/compiler-clang.h
>> index d614c5e..deb65b3 100644
>> --- a/include/linux/compiler-clang.h
>> +++ b/include/linux/compiler-clang.h
>> @@ -22,4 +22,9 @@
>>   * directives.  Suppress the warning in clang as well.
>>   */
>>  #undef inline
>> +#if !defined(CONFIG_ARCH_SUPPORTS_OPTIMIZED_INLINING) ||		\
>> +    !defined(CONFIG_OPTIMIZE_INLINING)
>> +#define inline inline __attribute__((always_inline)) 
>> __attribute__((unused)) notrace
>> +#else
>>  #define inline inline __attribute__((unused)) notrace
>> +#endif

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