Re: [PATCH v3 7/7] dynamic_debug: add jump label support

[Jason Baron]

On 07/08/2016 05:41 PM, Andrew Morton wrote:
> On Wed,  6 Jul 2016 17:42:36 -0400 Jason Baron <jbaron@xxxxxxxxxx> wrote:
>
> > Although dynamic debug is often only used for debug builds, sometimes its
> > enabled for production builds as well. Minimize its impact by using jump
> > labels. This reduces the text section by 7000+ bytes in the kernel image
> > below. It does increase data, but this should only be referenced when
> > changing the direction of the branches, and hence usually not in cache.
> >
> > ...
> >
> > +#ifdef HAVE_JUMP_LABEL
> > +
> > +#define dd_key_init(key, init) key = (init)
> >
> > ...
> >
> > +#else
> > +
> > +#define dd_key_init(key, init)
> > +
> umm, lazy.  One is an lval and returns a value and the other does
> neither.  Lack of parenthesization in the first version doubtless
> exposes various horrors.
>
> Care to do something more robust and conventional here?  Presumably use
> symmetrical do{}while(0) things, neither of which is an lval, both of
> which don't return anything.

Hi,

The 'dd_key_init()' macro is being used here to help initialize
the 'key' field in the 'struct _ddebug', and its not being used as a
statement.

In the 'HAVE_JUMP_LABEL' case, we are initializing the 'key' field, while in
the not-'HAVE_JUMP_LABEL' case, the 'key' field is simply not present
in the structure (to conserve space).

Thanks,

-Jason