inline functions

[Caligo]

On Sat, Mar 26, 2011 at 3:47 AM, Jonathan M Davis <jmdavisProg at gmx.com> wrote:
> On 2011-03-26 01:06, Caligo wrote:
>> On Fri, Mar 25, 2011 at 11:56 PM, Jonathan M Davis <jmdavisProg at gmx.com>
> wrote:
>> > On 2011-03-25 21:21, Caligo wrote:
>> >> On Fri, Mar 25, 2011 at 10:49 PM, Jonathan M Davis <jmdavisProg at gmx.com>
>> >
>> > wrote:
>> >> > On 2011-03-25 19:04, Caligo wrote:
>> >> >> T[3] data;
>> >> >>
>> >> >> T dot(const ref Vector o){
>> >> >>     return data[0] * o.data[0] + data[1] * o.data[1] + data[2] *
>> >> >> o.data[2]; }
>> >> >>
>> >> >> T LengthSquared_Fast(){ return data[0] * data[0] + data[1] * data[1]
>> >> >> + data[2] * data[2]; }
>> >> >> T LengthSquared_Slow(){ return dot(this); }
>> >> >>
>> >> >>
>> >> >> The faster LengthSquared() is twice as fast, and I've test with GDC
>> >> >> and DMD.  Is it because the compilers don't inline-expand the dot()
>> >> >> function call?  I need the performance, but the faster version is too
>> >> >> verbose.
>> >> >
>> >> > It sure sounds like it didn't inline it. Did you compile with -inline?
>> >> > If you didn't then it definitely won't inline it.
>> >> >
>> >> > - Jonathan M Davis
>> >>
>> >> I didn't know I had to supply GDC with -inline, so I did, and it did
>> >> not help.  In fact, with the -inline option the performance gets worse
>> >> (for DMD and GDC), even for code that doesn't contain any function
>> >> calls.  In any case, code compiled with DMD is always behind GDC when
>> >> it comes to performance.
>> >
>> > I don't know what gdc does, but you have to use -inline with dmd if you
>> > want it to inline anything. It also really doesn't make any sense at all
>> > that inlining would harm performance. If that's the case, something
>> > weird is going on. I don't see how inlining could _ever_ harm
>> > performance unless it just makes the program's binary so big that _that_
>> > harms performance. That isn't very likely though. So, if using -inline
>> > is harming performance, then something weird is definitely going on.
>> >
>> > - Jonathan M Davis
>>
>> The only time that -inline has no effect is when I turn on -O3.  This
>> is also when the code performs the best.  I've never used -O3 in my
>> C++ code, but I guess things are different in D even with the same
>> back-end.
>
> I really don't know what gdc does. With dmd, inlining is not turned on unless
> -inline is used. Also, -inline with dmd does not force inlining, it merely
> turns on the optimization. The compiler still chooses where and when it's best
> to inline.
>
> With gcc, I believe that inlining is normally turned on at a pretty low
> optimization level (probably -O), and like dmd, it chooses where and when it's
> best to inline, but unlike dmd, it uses the inline keyword in C++ as a hint as
> to what it should do. However, -O3 forces inlining on all functions marked
> with inline. How gdc deals with that given that D doesn't have an inline
> keyword, I don't know.
>
> Regardless, given what inlining does, I have a _very_ hard time believing that
> it would ever degrade performance unless it's buggy.
>
> - Jonathan M Davis
>


I was going to post my code, but I take back what I said.  What is
happening is that there is a lot of fluctuation in performance.  The
low performance always occurred when I had -inline enabled, which made
me think -inline degrades performance.  The performance should be
consistent, but for some reason it's not.

The important thing is that -inline doesn't make any difference with
GDC.  The -O3 does make a big difference.