Re: [PATCH] pinctrl: ingenic: Make unreachable path more robust

[Paul Cercueil]

Hi Josh,

Le lun., févr. 17, 2020 at 09:18, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> 
a écrit :
> On Fri, Feb 14, 2020 at 11:37:04PM -0300, Paul Cercueil wrote:
> >  > >  I don't like the idea that you change this driver's code just 
> > to
> >  > > work around
> >  > >  a bug in objtool, and I don't like the idea of working around a
> >  > > future bug
> >  > >  that shouldn't be introduced in the first place.
> >  >
> >  > It's not an objtool bug.  It's a byproduct of the fact that GCC's
> >  > undefined behavior is inscrutable, and there's no way to 
> > determine that
> >  > it actually *wants* to jump to a random function.
> >  >
> >  > And anyway, regardless of objtool, the patch is meant to make the 
> > code
> >  > more robust.
> >  >
> >  > Do you not agree that BUG (defined behavior) is more robust than
> >  > unreachable (undefined behavior)?
> >
> >  It's a dead code path. That would be an undefined behaviour, if it 
> > was
> >  taken, but it's not.
>
> Given your confidence that humans don't introduce bugs, would you
> recommend that we
>
>   s/BUG()/unreachable()/
>
> tree-wide?

Of course not.

> Another option would be to remove the unreachable() statement, which
> would actually improve the generated code by making it more compact 
> (16
> bytes of i-cache savings), on top of removing the "fallthrough to next
> function" nastiness.

I'd prefer that, yes.

-Paul

> diff --git a/drivers/pinctrl/pinctrl-ingenic.c 
> b/drivers/pinctrl/pinctrl-ingenic.c
> index 96f04d121ebd..13c7d3351ed5 100644
> --- a/drivers/pinctrl/pinctrl-ingenic.c
> +++ b/drivers/pinctrl/pinctrl-ingenic.c
> @@ -2158,7 +2158,8 @@ static int ingenic_pinconf_set(struct 
> pinctrl_dev *pctldev, unsigned int pin,
>  			break;
>
>  		default:
> -			unreachable();
> +			/* unreachable */
> +			break;
>  		}
>  	}