Re: C vs C++

What I saw in my tests is that whatever approach you use in an attempt
to convert packed decimal in C++ to something "faster" (tried
__dectoi, QXXPTOI, cpynv) the result is exactly the same as if I make
a plain assignment of packed decimal variable value to integer (or
long long) value. So these functions provide no work around.
And at the same time I saw that ANY operation with packed decimal (or
as they call it BCD) is extremely expensive in terms of performance.
At the same time in C plain assignment is absolutely fastest of all
the ways to convert packed to int.

In this situation there are 2 things that surprise me:
1. Why IBM decided not to use the same engine for packed decimal in C
as they use in C++? What was the trade off? I mean did they gain
anything by loosing so much in performance?
2. How did it happen that noone have mentioned that earlier (at least
I googled and found no evidence that anyone new about it)?

But these questions can potentially be answered only by "insiders", I
suppose....



On Tue, Feb 17, 2015 at 4:55 PM, Mark S Waterbury
<[email protected]> wrote:
> Jevgeni:
>
> Yes, the overhead of converting the data to and from packed decimal needs to
> be considered, when working with the data in other formats.
>
> Support for financial data is why IBM developed "packed decimal" support in
> the first place, on the IBM mainframes (S/360, S/370, S/390, z/Series). It
> looks like the IBM z/OS XL C compiler also supports packed decimal directly,
> in the same way that ILE C/400 does, and the IBM C++ compiler on the
> mainframe uses the same "BCD" library, the same way ILE C++ does.
>
> IBM has made some improvements in each new generation -- Power4, Power5,
> Power6, Power7 and Power8, with regard to better support for "packed
> decimal" arithmetic and what is called "decimal floating point,"  so I am
> not surprised that your results are different from mine.  (That is why I
> posted the source code, so that anyone can try it on their systems.)
>
> I think this is the right approach -- to test various coding alternatives,
> and measure the results, before deciding which way to go, for your "live"
> production applications.
>
> All the best,
>
> Mark S. Waterbury
>
>> On 2/17/2015 8:56 AM, Jevgeni Astanovski wrote:
>>
>> Mark,
>>
>> Spent some time and rewrote the API to use long long.
>> However I have nothing to do about the data that I'm processing.
>> I work in the finance area so effectively my API works not with
>> numbers - rather with money.
>> And all the money in the tables is kept as packed decimals.
>> So what I did - I read packed decimals and immediately convert them to
>> long long before I make some arithmetics with them.
>> Result is still the same - 2 times slower than the C version...
>>
>> I'll now go and experiment with the benchmarking program of yours to
>> see what consumes time. If conversion of packed to int (or long) is
>> most ti,e consuming, then probably I'm digging in the wrong
>> direction.....
>>
>>
>
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