Does anyone have any idea why C++ program is significantly slower than
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
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,
This mailing list archive is Copyright 1997-2019 by midrange.com and David Gibbs as a compilation work. Use of the archive is restricted to research of a business or technical nature. Any other uses are prohibited. Full details are available here. If you have questions about this, please contact