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

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Message-ID: <20170622094333.GB25967@leverpostej>
Date:   Thu, 22 Jun 2017 10:43:33 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     David Rientjes <rientjes@...gle.com>
Cc:     Sodagudi Prasad <psodagud@...eaurora.org>, 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 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))