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Message-ID: <CANiq72kgmFYEO_EB_NxAF=S7VOf45KM7W3uwxxvftVErwfWzjg@mail.gmail.com>
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
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