linux-kernel - Re: [PATCH v2] pinctrl: ingenic: Improve unreachable code generation

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Date:   Fri, 21 Feb 2020 16:31:05 +0100
From:   Linus Walleij <linus.walleij@...aro.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Paul Cercueil <paul@...pouillou.net>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Randy Dunlap <rdunlap@...radead.org>
Subject: Re: [PATCH v2] pinctrl: ingenic: Improve unreachable code generation

On Thu, Feb 20, 2020 at 4:35 PM Josh Poimboeuf <jpoimboe@...hat.com> wrote:

> In the second loop of ingenic_pinconf_set(), it annotates the switch
> default case as unreachable().  The annotation is technically correct,
> because that same case would have resulted in an early function return
> in the previous loop.
>
> However, the compiled code is suboptimal.  GCC seems to work extra hard
> to ensure that the unreachable code path triggers undefined behavior.
> The function would fall through to start executing whatever function
> happens to be next in the compilation unit.
>
> This is problematic because:
>
>   a) it adds unnecessary 'ensure undefined behavior' logic, and
>      corresponding i-cache footprint; and
>
>   b) it's less robust -- if a bug were to be introduced, falling through
>      to the next function would be catastrophic.
>
> Yet another issue is that, while objtool normally understands
> unreachable() annotations, there's one special case where it doesn't:
> when the annotation occurs immediately after a 'ret' instruction.  That
> happens to be the case here because unreachable() is immediately before
> the return.
>
> Remove the unreachable() annotation and replace it with a comment.  This
> simplifies the code generation and changes the unreachable error path to
> just silently return instead of corrupting execution.
>
> This fixes the following objtool warning:
>
>   drivers/pinctrl/pinctrl-ingenic.o: warning: objtool: ingenic_pinconf_set() falls through to next function ingenic_pinconf_group_set()
>
> Reported-by: Randy Dunlap <rdunlap@...radead.org>
> Signed-off-by: Josh Poimboeuf <jpoimboe@...hat.com>

Patch applied.

Yours,
Linus Walleij