Re: [Boost-build] B2 C++ engine development.

Subject: Re: [Boost-build] B2 C++ engine development.
From: Rene Rivera (grafikrobot_at_[hidden])
Date: 2018-10-26 02:46:46

On Thu, Oct 25, 2018 at 12:55 PM Rene Rivera <grafikrobot_at_[hidden]> wrote:

> On Thu, Oct 25, 2018 at 12:41 PM Steven Watanabe via Boost-build <
> boost-build_at_[hidden]> wrote:
>
>> AMDG
>>
>> On 10/24/2018 09:58 PM, Rene Rivera via Boost-build wrote:
>> > One of my goals in the future of B2 is to move the engine to something
>> > easier to maintain and improve. As such it's my goal to incrementally
>> > rewrite the engine component of B2 in C++. Yesterday I completed an
>> initial
>> > step in that by having B2 compile as C++ instead of ANSI C. The code
>> > changes are in the "feature/cxx" branch and from outward appearances
>> it's
>> > exactly the same as before except that the build scripts now invoke the
>> C++
>> > compiler (only lightly tested for the compilers I have available). First
>> > con of this is that the executable is slightly fatter (about 20K on
>> OSX).
>> > But also the first pro, it's about 4% faster.
>> >
>>
>> Any explanation for this?  Are the build options
>> exactly the same other than -x c++?  The code
>> changes don't look significant, so is it just
>> that the stricter rules of C++ allow the optimizer
>> more freedom?  It looks like you're not linking
>> the C++ runtime (which was my first guess about
>> the binary size increase).
>>
>
> I think the speedup is from the stricter type rules of C++, probably
> constness and aliasing. But it would take way more investigation and
> looking at assembly to truly figure that out. And I'm not going to bother
> doing that :-) I don't prevent linking of the C++ runtime. But since it's
> not used yet I suspect it's just linked away anyway. My suspicion for size
> increase would be of the slightly different optimization behavior. In that
> it might prefer slightly larger code that performs better in many small
> instances.
>

While adding the "--debug" option for building I added LTO to the release
build. And that gives an overall 11% perf improvement (with xcode 10 clang).

-- 
-- Rene Rivera
-- Grafik - Don't Assume Anything
-- Robot Dreams - http://robot-dreams.net