Re: [PATCH 1/2] if_enabled.h: Add IF_ENABLED_OR_ELSE() and IF_ENABLED() macros

[Paul Cercueil]

Hi Ard,

Le mer. 9 déc. 2020 à 10:38, Ard Biesheuvel <ardb@xxxxxxxxxx> a 
écrit :
> On Tue, 8 Dec 2020 at 17:49, Paul Cercueil <paul@xxxxxxxxxxxxxxx> 
> wrote:
> >  Introduce a new header <linux/if_enabled.h>, that brings two new 
> > macros:
> >  IF_ENABLED_OR_ELSE() and IF_ENABLED().
> >
> >  IF_ENABLED_OR_ELSE(CONFIG_FOO, a, b) evaluates to (a) if CONFIG_FOO 
> > is set
> >  to 'y' or 'm', (b) otherwise. It is used internally to define the
> >  IF_ENABLED() macro. The (a) and (b) arguments must be of the same 
> > type.
> >
> >  IF_ENABLED(CONFIG_FOO, ptr) evaluates to (ptr) if CONFIG_FOO is set 
> > to 'y'
> >  or 'm', NULL otherwise. The (ptr) argument must be a pointer.
> >
> >  The IF_ENABLED() macro can be very useful to help GCC drop dead 
> > code.
> >
> >  For instance, consider the following:
> >
> >      #ifdef CONFIG_FOO_SUSPEND
> >      static int foo_suspend(struct device *dev)
> >      {
> >         ...
> >      }
> >      #endif
> >
> >      static struct pm_ops foo_ops = {
> >          #ifdef CONFIG_FOO_SUSPEND
> >          .suspend = foo_suspend,
> >          #endif
> >      };
> >
> >  While this works, the foo_suspend() macro is compiled conditionally,
> >  only when CONFIG_FOO_SUSPEND is set. This is problematic, as there 
> > could
> >  be a build bug in this function, we wouldn't have a way to know 
> > unless
> >  the config option is set.
> >
> >  An alternative is to declare foo_suspend() always, but mark it as 
> > maybe
> >  unused:
> >
> >      static int __maybe_unused foo_suspend(struct device *dev)
> >      {
> >         ...
> >      }
> >
> >      static struct pm_ops foo_ops = {
> >          #ifdef CONFIG_FOO_SUSPEND
> >          .suspend = foo_suspend,
> >          #endif
> >      };
> >
> >  Again, this works, but the __maybe_unused attribute is required to
> >  instruct the compiler that the function may not be referenced 
> > anywhere,
> >  and is safe to remove without making a fuss about it. This makes the
> >  programmer responsible for tagging the functions that can be
> >  garbage-collected.
> >
> >  With this patch, it is now possible to write the following:
> >
> >      static int foo_suspend(struct device *dev)
> >      {
> >         ...
> >      }
> >
> >      static struct pm_ops foo_ops = {
> >          .suspend = IF_ENABLED(CONFIG_FOO_SUSPEND, foo_suspend),
> >      };
> >
> >  The foo_suspend() function will now be automatically dropped by the
> >  compiler, and it does not require any specific attribute.
> >
> >  Signed-off-by: Paul Cercueil <paul@xxxxxxxxxxxxxxx>
>
> Hi Paul,
>
> I understand the issue you are trying to address here, but please note
> that there are many cases where the struct member in question will not
> even be declared if the associated CONFIG option is not set, in which
> case this will cause a compile error.

Of course. This is for the case where the field is always present. For 
instance, (struct device_driver *)->pm is always present, independently 
of CONFIG_PM.

Cheers,
-Paul

> >  ---
> >   include/linux/if_enabled.h | 22 ++++++++++++++++++++++
> >   1 file changed, 22 insertions(+)
> >   create mode 100644 include/linux/if_enabled.h
> >
> >  diff --git a/include/linux/if_enabled.h b/include/linux/if_enabled.h
> >  new file mode 100644
> >  index 000000000000..8189d1402340
> >  --- /dev/null
> >  +++ b/include/linux/if_enabled.h
> >  @@ -0,0 +1,22 @@
> >  +/* SPDX-License-Identifier: GPL-2.0 */
> >  +#ifndef __LINUX_IF_ENABLED_H
> >  +#define __LINUX_IF_ENABLED_H
> >  +
> >  +#include <linux/build_bug.h>
> >  +#include <linux/compiler_types.h>
> >  +#include <linux/kconfig.h>
> >  +
> >  +/*
> >  + * IF_ENABLED_OR_ELSE(CONFIG_FOO, a, b) evaluates to (a) if 
> > CONFIG_FOO is set
> >  + * to 'y' or 'm', (b) otherwise.
> >  + */
> >  +#define IF_ENABLED_OR_ELSE(option, a, b) \
> >  +       (BUILD_BUG_ON_ZERO(__same_type((a), (b))) || 
> > IS_ENABLED(option) ? (a) : (b))
> >  +
> >  +/*
> >  + * IF_ENABLED(CONFIG_FOO, ptr) evaluates to (ptr) if CONFIG_FOO is 
> > set to 'y'
> >  + * or 'm', NULL otherwise.
> >  + */
> >  +#define IF_ENABLED(option, ptr) IF_ENABLED_OR_ELSE(option, ptr, 
> > NULL)
> >  +
> >  +#endif /* __LINUX_IF_ENABLED_H */
> >  --
> >  2.29.2
> >
> >
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