[dpdk-dev] [PATCH] eal/common: better likely() and unlikely()

Aleksey Baulin Aleksey.Baulin at gmail.com
Sat Jan 13 23:45:42 CET 2018


Please see my comments inline.

On Sun, Jan 14, 2018 at 1:24 AM, Thomas Monjalon <thomas at monjalon.net>
wrote:

> Hi,
>
> I moved your top-post below and did some comments inline.
> More opinions are welcome.
>
> 13/01/2018 23:05, Aleksey Baulin:
> > On Fri, Jan 12, 2018 at 6:35 PM, Thomas Monjalon <thomas at monjalon.net>
> > wrote:
> > > 21/11/2017 08:05, Aleksey Baulin:
> > > > On Mon, Nov 20, 2017 at 4:36 PM, Wiles, Keith <keith.wiles at intel.com
> >
> > > wrote:
> > > > > > On Nov 19, 2017, at 4:16 PM, Aleksey Baulin <
> > > aleksey.baulin at gmail.com>
> > > > > wrote:
> > > > > > -#define unlikely(x)  __builtin_expect((x),0)
> > > > > > +#define unlikely(x)  __builtin_expect(!!(x), 0)
> > > > >
> > > > > I have not looked at the generated code, but does this add some
> extra
> > > > > instruction now to do the !!(x) ?
> > > >
> > > > Sorry for late response. Jim had given the correct answer already.
> > > > You won't get an extra instruction with compiler optimization turned
> on.
> > >
> > > So this patch is adding an instruction in not optimized binary.
> > > I don't understand the benefit.
> > > Is it just to avoid to make pointer comparison explicit?
> > > likely(pointer != NULL) looks better than likely(pointer).
> >
> > This is an interesting question. Perhaps, even a philosophical one. :-)
> >
> > 'likely(pointer)' is a perfectly legal statement in C language, as well
> as
> > a concise one as
> > compared to a more explicit (and redundant/superfluous) 'likely(pointer
> !=
> > NULL)'. If you
> > _require_ this kind of explicitness in cases like this in the code style,
> > then I have no
> > argument here. However, I don't see that anywhere.
>
> It is stated here:
>         http://dpdk.org/doc/guides/contributing/coding_style.
> html#null-pointers


​Oh, thanks for pointing that out! I am sincerely ashamed for missing it.​
I lose that argument as I certainly do submit to the coding style. My only
excuse is that I am actually developing an app and not the DPDK core.


> > There're other cases of explicitness, with the most widespread being a
> > series of logical and
> > compare operations in one statement. For instance, 'if (a > b && a < c)'.
> > Explicitness would
> > require writing it like this: 'if ((a > b) && (a < c))'. I've seen cases
> on
> > this list where that was
> > frowned upon as it's totally unnecessary due to C operator precedence
> > rules, even though
> > those statements, I think, looked better to their authors (actually, they
> > do to me). Granted,
> > it didn't lead to compiler errors, which is the case with the current
> > implementation of 'likely()'.
> >
> > So, my answer to the question is yes, it's to avoid making pointer
> > comparison explicit. I would
> > add though, that it is to avoid making a perfectly legal C statement an
> > illegal one, as with the
> > way the current macro is constructed, compiler emits an error when DPDK
> is
> > built. I write in C
> > for many years with the most time spent in kernels, Linux and not, and I
> > find it unnatural to
> > always add a redundant '!= NULL' just to satisfy the current macro
> > implementation. I would
> > have to accept that though if it's a requirement clearly stated somewhere
> > like a code style.
> >
> > As for an extra instruction, I believe that everything in DPDK
> distribution
> > is compiled with
> > optimization. So the execution speed in not a concern here. Perhaps there
> > are cases where
> > it's compiled without optimization, like debugging, but then I believe
> it's
> > a non-issue.
>
> Yes you're right about optimization.
> But can we be 100% sure that it is always well optimized?
>

​I believe we can. I hope we get other opinions as well.​

> Hope my explanations shed some more light on this patch. :-)
>
> If we can be sure that there is no cost on optimized code,
> I am not against this patch.
> It may be especially useful when not complying to the DPDK
> coding rules, like in applications.
>

​Yes, that's exactly my case. Thanks.​

-- 
Aleksey Baulin


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