Re: C vs C++

Jon,

what stands beyond the abbreviation RFE?

Jevgeni.


On Mon, Feb 16, 2015 at 9:45 PM, Jon Paris <[email protected]> wrote:
> I had missed the packed bit - good catch Mark.
>
> As far as “unclear why” - I believe that while the ILE C compiler was IBM i specific, the C++ compiler is basically just the AIX version running in PASE. I’m disappointed that they apparently made no changes in an area where there was obviously such a huge potential for performance issues as packed decimal operations.
>
> Perhaps an RFE is appropriate.
>
>
> Jon Paris
>
> www.partner400.com
> www.SystemiDeveloper.com
>
> On Feb 16, 2015, at 2:31 PM, Jevgeni Astanovski <[email protected]> wrote:
>
>> Mark, frankly speaking packed decimal was my suspect since I saw the
>> difference. I saw the difference - it was intuitively clear that the
>> implementations of packed decimal is fundamentally different in C and C++
>> (however unclear why). What I'll do tomorrow, I'll write my own trivial
>> benchmark using decimal arithmetics.
>> Hopefully I shall see the difference.
>> The program uses decimal (15,0), decimal (15,9) and decimal (11,7) - It
>> seems that they all fit into 8 byte int.
>> I'll then see which is better - staying on C and decimals or switch to C++
>> and integers...
>>
>> Thanks,
>> Jevgeni.
>> On Feb 16, 2015 8:29 PM, "Mark S Waterbury" <[email protected]>
>> wrote:
>>
>>> Jevjeni:
>>>
>>> I just noticed near the bottom of your post, where you said:
>>>
>>>        Program makes a lot of calculations with packed decimal.
>>>
>>> There is a big difference in the way ILE C implements support for "packed
>>> decimal" as a "native" data type -- versus the way it is "supported" in ILE
>>> C++ -- in ILE C, the compiler directly genrerates in-line code (in WCode /
>>> NMI)  to work on packed decimal data.  In the ILE C++ compiler, support for
>>> "packed decimal" is through a C++ library called "BCD" -- there are a lot
>>> of macros and such defined in the QSYSINC/H include member named BCD ... if
>>> you take a peek at that member, I think you will see that this uses
>>> "operator overloading" in C++ to trick the compiler into calling the
>>> desired library functions when operations are performed on "packed decimal"
>>> data.
>>>
>>> This difference could easily account for the big differences you are
>>> seeing.
>>>
>>> And then, you said:
>>>
>>>        ... is there anything that can be done about it?
>>>
>>> In C++, you could look into converting the packed decimal data into 64-bit
>>> integers, and then performing any calculations using those long integers.
>>> This would be much faster than using packed decimal arithmetic, then
>>> convert back to packed decimal format, if needed.  Or, just stick with ILE
>>> C/400 for those programs that require doing a lot of packed decimal
>>> arithmetic operations.
>>>
>>> Hope that helps,
>>>
>>> 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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