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

[Thomas Monjalon <thomas@monjalon.net>]

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@monjalon.net>
> wrote:
> > 21/11/2017 08:05, Aleksey Baulin:
> > > On Mon, Nov 20, 2017 at 4:36 PM, Wiles, Keith <keith.wiles@intel.com>
> > wrote:
> > > > > On Nov 19, 2017, at 4:16 PM, Aleksey Baulin <
> > aleksey.baulin@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

> 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?

> 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.