On 14 February 2016 at 17:52, Nicolas Pitre <nicolas.pitre@xxxxxxxxxx> wrote:
> On Sun, 14 Feb 2016, Ard Biesheuvel wrote:
>
>> On 13 February 2016 at 22:57, Nicolas Pitre <nicolas.pitre@xxxxxxxxxx> wrote:
>> > 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.
>> >
>>
>> ... and perhaps we should not evaluate cond twice either?
>
> It is not. The value of the argument to __builtin_constant_p() is not
> itself evaluated and therefore does not produce side effects.
>
Interesting, thanks for clarifying.
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