linux-kernel - Re: [PATCH 1/8] Compiler attributes: GCC function alignment workarounds

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Date:   Mon, 9 Jan 2023 15:43:16 +0100
From:   Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
To:     Mark Rutland <mark.rutland@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, catalin.marinas@....com,
        lenb@...nel.org, linux-acpi@...r.kernel.org,
        linux-kernel@...r.kernel.org, mhiramat@...nel.org,
        ndesaulniers@...gle.com, ojeda@...nel.org, peterz@...radead.org,
        rafael.j.wysocki@...el.com, revest@...omium.org,
        robert.moore@...el.com, rostedt@...dmis.org, will@...nel.org
Subject: Re: [PATCH 1/8] Compiler attributes: GCC function alignment workarounds

On Mon, Jan 9, 2023 at 2:58 PM Mark Rutland <mark.rutland@....com> wrote:
>
> As far as I can tell, GCC doesn't respect '-falign-functions=N':
>
> * When the __weak__ attribute is used
>
>   GCC seems to forget the alignment specified by '-falign-functions=N',
>   but will respect the '__aligned__(N)' function attribute. Thus, we can
>   work around this by explciitly setting the alignment for weak
>   functions.
>
> * When the __cold__ attribute is used
>
>   GCC seems to forget the alignment specified by '-falign-functions=N',
>   and also doesn't seem to respect the '__aligned__(N)' function
>   attribute. The only way to work around this is to not use the __cold__
>   attibute.

If you happen to have a reduced case, then it would be nice to link it
in the commit. A bug report to GCC would also be nice.

I gave it a very quick try in Compiler Explorer, but I couldn't
reproduce it, so I guess it depends on flags, non-trivial functions or
something else.

> + * '-falign-functions=N', and require alignment to be specificed via a function

Nit: specificed -> specified

> +#if CONFIG_FUNCTION_ALIGNMENT > 0
> +#define __function_aligned             __aligned(CONFIG_FUNCTION_ALIGNMENT)
> +#else
> +#define __function_aligned
> +#endif

Currently, the file is intended for attributes that do not depend on
`CONFIG_*` options.

What I usually mention is that we could change that policy, but
otherwise these would go into e.g. `compiler_types.h`.

> +#if !defined(CONFIG_CC_IS_GCC) || (CONFIG_FUNCTION_ALIGNMENT == 0)
>  #define __cold                          __attribute__((__cold__))
> +#else
> +#define __cold
> +#endif

Similarly, in this case this could go into `compiler-gcc.h` /
`compiler-clang.h` etc., since the definition will be different for
each.

Cheers,
Miguel