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

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Date:   Thu, 22 Jun 2017 23:45:26 -0700
From:   Sodagudi Prasad <psodagud@...eaurora.org>
To:     Mark Rutland <mark.rutland@....com>
Cc:     David Rientjes <rientjes@...gle.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 2017-06-22 02:43, Mark Rutland wrote:
> On Tue, Jun 20, 2017 at 04:12:32PM -0700, David Rientjes wrote:
>> On Tue, 20 Jun 2017, Mark Rutland wrote:
>> 
>> > As with my reply to David, my preference would be that we:
>> >
>> > 1) Align compiler-clang.h with the compiler-gcc.h inlining behaviour, so
>> >    that things work by default.
>> >
>> > 2) Fix up the arm64 core code (and drivers for architected / common
>> >    peripherals) to use __always_inline where we always require inlining.
>> >
>> > 3) Have arm64 select CONFIG_ARCH_SUPPORTS_OPTIMIZED_INLINING, and have
>> >    people test-build configurations with CONFIG_OPTIMIZE_INLINING, with
>> >    both GCC and clang.
>> >
>> > 4) Fix up drivers, etc, as appropriate.
>> >
>> > 5) Once that's largely stable, and if there's a benefit, have arm64
>> >    select CONFIG_OPTIMIZE_INLINING by default.
>> >
>> > That should avoid undue breakage, while enabling this ASAP.
>> >
>> 
>> Sounds good, but I think we should simply deal with the
>> __attribute__((unused)) needed for clang as part of compiler-gcc.h by
>> simply adding it to the inline override there to avoid duplicated 
>> code.
> 
> Agreed. That looks much better.
> 
> Thanks,
> Mark.
> 
>> 
>> diff --git a/include/linux/compiler-clang.h 
>> b/include/linux/compiler-clang.h
>> --- a/include/linux/compiler-clang.h
>> +++ b/include/linux/compiler-clang.h
>> @@ -15,11 +15,3 @@
>>   * with any version that can compile the kernel
>>   */
>>  #define __UNIQUE_ID(prefix) __PASTE(__PASTE(__UNIQUE_ID_, prefix), 
>> __COUNTER__)
>> -
>> -/*
>> - * GCC does not warn about unused static inline functions for
>> - * -Wunused-function.  This turns out to avoid the need for complex 
>> #ifdef
>> - * directives.  Suppress the warning in clang as well.
>> - */
>> -#undef inline
>> -#define inline inline __attribute__((unused)) notrace
>> diff --git a/include/linux/compiler-gcc.h 
>> b/include/linux/compiler-gcc.h
>> index 0efef9cf014f..71fe0994cf1a 100644
>> --- a/include/linux/compiler-gcc.h
>> +++ b/include/linux/compiler-gcc.h
>> @@ -66,18 +66,22 @@
>> 
>>  /*
>>   * Force always-inline if the user requests it so via the .config,
>> - * or if gcc is too old:
>> + * or if gcc is too old.
>> + * GCC does not warn about unused static inline functions for
>> + * -Wunused-function.  This turns out to avoid the need for complex 
>> #ifdef
>> + * directives.  Suppress the warning in clang as well by using 
>> "unused"
>> + * function attribute, which is redundant but not harmful for gcc.
>>   */
>>  #if !defined(CONFIG_ARCH_SUPPORTS_OPTIMIZED_INLINING) ||		\
>>      !defined(CONFIG_OPTIMIZE_INLINING) || (__GNUC__ < 4)
>> -#define inline		inline		__attribute__((always_inline)) notrace
>> -#define __inline__	__inline__	__attribute__((always_inline)) notrace
>> -#define __inline	__inline	__attribute__((always_inline)) notrace
>> +#define inline inline		__attribute__((always_inline,unused)) notrace
>> +#define __inline__ __inline__	__attribute__((always_inline,unused)) 
>> notrace
>> +#define __inline __inline	__attribute__((always_inline,unused)) 
>> notrace
>>  #else
>>  /* A lot of inline functions can cause havoc with function tracing */
>> -#define inline		inline		notrace
>> -#define __inline__	__inline__	notrace
>> -#define __inline	__inline	notrace
>> +#define inline inline		__attribute__((unused)) notrace
>> +#define __inline__ __inline__	__attribute__((unused)) notrace
>> +#define __inline __inline	__attribute__((unused)) notrace
>>  #endif
>> 
>>  #define __always_inline	inline __attribute__((always_inline))
Hi David,

I just want to cross check with you.
Do you want me to update this change in next patch set ? Or are you 
going to add this ?

-Thanks, Prasad
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