Re: [Bulk] C vs C++

Jevgeni:

I just ran a small "CPU speed benchmark" program written in C, compiled 
it with CRTBNDC and ran it, then compiled the same source, using 
CRTBNDCPP, and ran that version.  Both versions reported exactly the 
elapsed time. Here is this "benchmark" program:

     http://code.midrange.com/f4200b6200.html

so you can try this program on your system, using both ILE C and ILE C++ 
to see if you get the same results.

Questions / suggestions:

 1. what version of OS/400 or IBM i is this running on?
 2. what Target Release (TGTRLS) was specified on the compile and bind
    commands?
 3. show commands used to compile and bind, e.g. CRTCMOD or CRTCPPMOD
    and CRTPGM  with any parameters used
 4. explain how you ran the "benchmark" -- how did you measure the
    elapsed times that you reported
 5. create a "cut down" version of a program that demonstrates the
    problem, and post it at http://code.midrange.com -- then reply to
    the list with the generated link --   (you will also need this for
    reporting a problem to IBM.)

HTH,

Mark S. Waterbury

 > On 2/16/2015 9:56 AM, Jevgeni Astanovski wrote:
> Hi all.
> Does anyone have any idea why C++ program is significantly slower than
> C program?
>
> Explain the situation.
> I have a number of API-s that are called via RPC, access some tables,
> make some calculations and return some structures to the caller.
> They've all been written on ILE/C.
> Recently I started to experiment with porting them to ILE/C++. There
> was a number of reasons why it would be nice.
> After I rewrote the first one, I started to measure performance. This
> is an issue for me as some of them are called hundreds of thousands
> times per day.
> Found out that it is approximately 2 times slower than C version.
> Shared a code to colleges - no one found anything suspicious. However
> one guy came out with a "brilliant" idea - he advised me to try to
> recompile my C program with CPP compiler and see the performance.
> Guess what? C program compiled with CPP compiler was the same 2 times
> slower than C program compiled with C compiler....
>
> And the only modifications that I did for compiling C with CPP compiler was:
> 1. Substituted "#pragma mapinc" table structue definitions with those
> generated by GENCSRC;
> 2. Changed char and unsigned char in a couple of places as C++ is more strict;
> 3. Made a substitution for "NNNND" constants (like 100D for example)
> as C++ does not support that.
>
> What I saw is that time increase looks like it has a proportional nature.
> I tested API in 3 cases (number of tests is over 100): simple request,
> medium request; complicated request and the timing is:
> Simple C: 1ms; Simple C++ : 2ms
> Medium C: 3ms; Simple C++ : 7ms
> Long C: 25ms; Long C++: 60ms.
>
> The difference is in number of records read from table and a number of
> calculations.
> That is a simple one retrieves less that 10 records; medium - more
> than 10 and long - more than 100. Program makes a lot of calculations
> with packed decimal.
>
> Any idea why it is like that and is there anything that can be done about it?
>
> Thanks in advance,
>
> Jevgeni.

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