On Sat, 13 Feb 2016, Ard Biesheuvel wrote:
> On 12 February 2016 at 22:01, Arnd Bergmann <arnd@xxxxxxxx> wrote:
> > However, I did stumble over an older patch I did now, which I could
> > not remember what it was good for. It does fix the problem, and
> > it seems to be a better solution.
> >
> > Arnd
> >
> > diff --git a/include/linux/compiler.h b/include/linux/compiler.h
> > index b5acbb404854..b5ff9881bef8 100644
> > --- a/include/linux/compiler.h
> > +++ b/include/linux/compiler.h
> > @@ -148,7 +148,7 @@ void ftrace_likely_update(struct ftrace_branch_data *f, int val, int expect);
> > */
> > #define if(cond, ...) __trace_if( (cond , ## __VA_ARGS__) )
> > #define __trace_if(cond) \
> > - if (__builtin_constant_p((cond)) ? !!(cond) : \
> > + if (__builtin_constant_p(!!(cond)) ? !!(cond) : \
> > ({ \
> > int ______r; \
> > static struct ftrace_branch_data \
> >
>
> I remember seeing this patch, but I don't remember the exact context.
> But when you think about it, !!cond can be a build time constant even
> if cond is not, as long as you can prove statically that cond != 0. So
You're right. I just tested it and to my surprise gcc is smart enough
to figure that case out.
> I think this change is obviously correct, and an improvement since it
> will remove the profiling overhead of branches that are not true
> branches in the first place.
Indeed.
Nicolas
--
To unsubscribe from this list: send the line "unsubscribe linux-media" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
