On 14/04/12 21:05, ReneSac wrote:
Lang| Comp | Binary size | Time (lower is better)
C++ (g++) - 13kb - 2.42s (100%) -O3 -s
D (DMD) - 230kb - 4.46s (184%) -O -release -inline
D (GDC) - 1322kb - 3.69s (152%) -O3 -frelease -s
Try using extra optimizations for GDC. Actually, GDC has a "dmd-like"
interface, gdmd, and
gdmd -O -release -inline
corresponds to
gdc -O3 -fweb -frelease -finline-functions
... so there may be some optimizations you were missing. (If you call gdmd with
the -vdmd flag, it will tell you exactly what gdc statement it's using.)
The only diference I could see between the C++ and D versions is that C++ has
hints to the compiler about which functions to inline, and I could't find
anything similar in D. So I manually inlined the encode and decode functions:
GDC has all the regular gcc optimization flags available IIRC. The ones on the
GDC man page are just the ones specific to GDC.
Still, the D version is slower. What makes this speed diference? Is there any
way to side-step this?
In my (brief and limited) experience GDC produced executables tend to have a
noticeable but minor gap compared to equivalent g++ compiled C++ code -- nothing
on the order of 150%.
E.g. I have some simulation code which models a reputation system where users
rate objects and are then in turn judged on the consistency of their ratings
with the general consensus. A simulation with 1000 users and 1000 objects takes
~22s to run in C++, ~24s in D compiled with gdmd -O -release -inline.
Scale that up to 2000 users and 1000 objects and it's 47s (C++) vs 53s (D).
2000 users and 2000 objects gives 1min 49s (C++) and 2min 4s (D).
So, it's a gap, but not one to write home about really, especially when you
count that D is safer and (I think) easier/faster to program in.
It's true that DMD is much slower -- the GCC backend is much better at
generating fast code. If I recall right the DMD backend's encoding of floating
point operations is considerably less sophisticated.
Note that this simple C++ version can be made more than 2 times faster with
algoritimical and io optimizations, (ab)using templates, etc. So I'm not asking
for generic speed optimizations, but only things that may make the D code "more
equal" to the C++ code.
I'm sure you can make various improvements to your D code in a similar way, and
there are some things that improve in D when written in idiomatic "D style" as
opposed to a more C++ish way of doing things (e.g. if you want to copy 1 vector
to another, as happens in my code, write x[] = y[] instead of doing any kind of
loop).
Best wishes,
-- Joe
