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?
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