C / C++ / MFC

30-Apr-02 12:09

"delete [] ptr;" when you allocate an array (CString *ptr = new CString[100]). otherwise use "delete ptr;".

-c

<font size=-2>//The Mandelbrot set
o(int O){putchar(O);}main(){float _[8],O,I=.05;char l;for(_[6]=15;_[6]<':';o
(10),_[5]=-'$'*I+_[6]++*I)for(_[7]=-5;_[7]<'@';_[4]=-'('*I+_[7]++*I,o(l?'?':':'))
for(*_=O=0,l=1;++l&&((_[2]=*_**_)+(_[3]=O*O)<4);O=*_*O+_[5]+O**_,*_=_[2]-_[3]+_[4]);}</font>

Fighting the optimizer

C^hris L^osinger30-Apr-02 10:58

30-Apr-02 10:58

I'm trying to optimize a function. I have an implementation in a static library that i'm using as a reference and another built into my app that i'm working on. I'm not recompiling the static lib, just linking to it.

The test code looks like this:

dwStart = GetTickCount();
for (z=0;z<reps;z++)
{
  Fn(params);
}
dwStop = GetTickCount();

Trial 1:
compiled in release mode with "Max Speed" optimizations
run 1000 reps of each over the same data set

New implementation : 2.45 sec
Reference implementation : 6.41 sec

So, I'm doing good. I've halved the speed of the reference.

Trial 2:
compiled in release mode with "Default" optimizations
run 1000 reps of each over the same data set

New implementation : 3.45 sec
Reference implementation : 5.70 sec

So, as expected, the new function is slower without the max-speed optimizations. but, the reference implementation, the one in the static libs that i'm not recompiling has gotten faster when i changed optimization settings. Errrr... WTF??

any ideas?

-c

<font size=-2>//The Mandelbrot set
o(int O){putchar(O);}main(){float l[8],O,I=.05;char _;for(l[6]=15;l[6]<':';o
(10),l[5]=-'$'*I+l[6]++*I)for(l[7]=-5;l[7]<'@';l[4]=-'('*I+l[7]++*I,o(_?'?':':'))for
(*l=O=0,_=1;++_&&((l[2]=*l**l)+(l[3]=O*O)<4);O=*l*O+l[5]+O**l,*l=l[2]-l[3]+l[4]);}</font>

Michael Dunn30-Apr-02 11:08

Michael Dunn

30-Apr-02 11:08

Chris Losinger wrote:
the one in the static libs that i'm not recompiling has gotten faster

Did you reboot in between tests? I always do that, to make sure all caches are emptied. For example, the 6.41s run might have read data from disk, which was then cached when subsequent tests ran, resulting in the 5.7s time later.

--Mike--
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C^hris L^osinger30-Apr-02 11:22

30-Apr-02 11:22

nope. if i switch back to the max speed optimizations, it goes back to the first set of numbers. it's consistent.

-c

<font size=-2>//The Mandelbrot set
o(int O){putchar(O);}main(){float l[8],O,I=.05;char _;for(l[6]=15;l[6]<':';o
(10),l[5]=-'$'*I+l[6]++*I)for(l[7]=-5;l[7]<'@';l[4]=-'('*I+l[7]++*I,o(_?'?':':'))for
(*l=O=0,_=1;++_&&((l[2]=*l**l)+(l[3]=O*O)<4);O=*l*O+l[5]+O**l,*l=l[2]-l[3]+l[4]);}</font>

Joaquín M López Muñoz30-Apr-02 11:31

Joaquín M López Muñoz

30-Apr-02 11:31

It's hard to know without more info about that Fn function and the parms it is accepting. Change the static lib implementation to a do-nothing function, so that you will be basically measuring the cost of doing the function call --maybe this sheds some light on the problem.

Joaquín M López Muñoz
Telefónica, Investigación y Desarrollo

C^hris L^osinger30-Apr-02 11:46

30-Apr-02 11:46

FYI, here are the parameters:

<br />
void FnNew(const BYTE *pIn, const int inW, const int inH, BYTE *pOut, const int outW, const int outH, const int bpp)<br />

the static lib function has essentially the same params, except that they aren't "const" - they are treated as const, but not labelled as such. i tested this angle and got the same results, const or not.

-c

<font size=-2>//The Mandelbrot set
o(int O){putchar(O);}main(){float _[8],O,I=.05;char l;for(_[6]=15;_[6]<':';o
(10),_[5]=-'$'*I+_[6]++*I)for(_[7]=-5;_[7]<'@';_[4]=-'('*I+_[7]++*I,o(l?'?':':'))
for(*_=O=0,l=1;++l&&((_[2]=*_**_)+(_[3]=O*O)<4);O=*_*O+_[5]+O**_,*_=_[2]-_[3]+_[4]);}</font>

Joaquín M López Muñoz30-Apr-02 11:51

Joaquín M López Muñoz

30-Apr-02 11:51

What happens if you replace the static lib "i.e. the reference" implementation with a do-nothing function? Still having worse results in "Max speed"?

Joaquín M López Muñoz
Telefónica, Investigación y Desarrollo

C^hris L^osinger30-Apr-02 12:07

30-Apr-02 12:07

actually, no.

i had to make a new static lib to test this (don't have the source to the other lib with me). but, the do-nothing function with 100000000 reps takes the same amount of time regardless of how the calling app is optimized.

hmm.

-c

<font size=-2>//The Mandelbrot set
o(int O){putchar(O);}main(){float _[8],O,I=.05;char l;for(_[6]=15;_[6]<':';o
(10),_[5]=-'$'*I+_[6]++*I)for(_[7]=-5;_[7]<'@';_[4]=-'('*I+_[7]++*I,o(l?'?':':'))
for(*_=O=0,l=1;++l&&((_[2]=*_**_)+(_[3]=O*O)<4);O=*_*O+_[5]+O**_,*_=_[2]-_[3]+_[4]);}</font>

Joaquín M López Muñoz30-Apr-02 12:13

Joaquín M López Muñoz

30-Apr-02 12:13

Maybe I didn't made myself clear enough... I'm asumming the do-nothing function still accepts the whole lot of parameters (even though it does nothing with them), right?

Joaquín M López Muñoz
Telefónica, Investigación y Desarrollo

C^hris L^osinger30-Apr-02 13:32

30-Apr-02 13:32

yes. it's the same function signature - just nothing in the function.

-c

<font size=-2>//The Mandelbrot set
o(int O){putchar(O);}main(){float _[8],O,I=.05;char l;for(_[6]=15;_[6]<':';o
(10),_[5]=-'$'*I+_[6]++*I)for(_[7]=-5;_[7]<'@';_[4]=-'('*I+_[7]++*I,o(l?'?':':'))
for(*_=O=0,l=1;++l&&((_[2]=*_**_)+(_[3]=O*O)<4);O=*_*O+_[5]+O**_,*_=_[2]-_[3]+_[4]);}</font>

Mike Nordell1-May-02 8:38

Mike Nordell

1-May-02 8:38

Chris Losinger wrote:
FYI, here are the parameters:

void FnNew(const BYTE *pIn, const int inW, const int inH, BYTE *pOut, const int outW, const int outH, const int bpp)

That explains it. Seven stack parameters using the IA32 architecture can be nothing but disaster. You wouldn't gain much by using "register" calling conventions either, since at four (or rather three in C++ functions) GPR's the optimizer can't actually help.

Try creating a struct that wraps your paramaters and do some re-testing. I'm sure you'll get the speed up again.

Note: Using MSVC you'd have to create auto-variables at the callee's site if you're using them more than once, else the MSVC optimizer often don't "cache" the values in registers, it frequently reloads them from the pointed-to object. I.e. do
void FnNew(const* pSmth)
{
const int inW = pSmth->inW;
... use inW multiple times ...
}

Giles1-May-02 9:20

Giles

1-May-02 9:20

It could well be the kernel process scheduler. Well not so much that but say another program starts having a few page faults, in 5 seconds the kernel has switched so many threads and processes that this kind of thing could be affecting your timings. The best way is to do this over a longer period, say 30-60 seconds. As well as that make sure you have things like background file indexing etc turned off. It all makes a difference, and it could be that your thread is not getting its share of the time it wants. Ultimately the scheduler is based on heuristics that allow it to tune itself, and while its accurate most of the time, there’s not reason while it could not mess up for a second or two. Have your tried repeating the same thing – could be asking a stupid question. I’ll shut up. Big Grin | :-D

C^hris L^osinger1-May-02 9:45

1-May-02 9:45

sadly, the times are totally consistent and reproducable.

-c

<font size=-2>//The Mandelbrot set
main(){float _,l,O,I,o,x;char i;for(x=-1;x<1;x+=.05,putchar(10))
for(o=-2;o<.9;o+=.05,putchar(i?'?':':'),_=l=O=I=--(i=1))for
(;((O=_*_)+(I=l*l)<4)&++i;l=_*l*2+x,_=O-I+o);}
</font>

Is there a RAR compression library around?

Li-kai Liu (Angus)30-Apr-02 10:44

Li-kai Liu (Angus)

30-Apr-02 10:44

Hi,

As title.
I know that there are a few ZIP compression libraries either
freely or commercially available.

But I couldn't find RAR compression ones, simple source
codes, or even books about this compression algorithm
as I have the needs to handle RAR files in my program.

So, just wondered if someone knows it. Thanks Smile | :)

Re: Is there a RAR compression library around?

Mike Nordell1-May-02 8:41

Mike Nordell

1-May-02 8:41

ykliu wrote:
But I couldn't find RAR compression ones, simple source
codes, or even books about this compression algorithm
as I have the needs to handle RAR files in my program.

<irony>
How odd, a commercial compression library with a proprietary compression algorithm isn't freely available.

Serisouly, since its proprietary you won't find any free implementations of it.

For free implementations I'd say that for fast compression I recommend you have a look at LZO. For reasonable time/space you should probably use zlib. For better compression you should probably have a look at bz2.

Equivalent of VB's "Preserve" keyword

Anton A. Loukine30-Apr-02 9:52

Anton A. Loukine

30-Apr-02 9:52

Hi:

I am sort of new to C++.

Suppose I created a CString array of n elements.
CString *someArray;
someArray = newCString[n];

Then further in the code I changed my mind and decided that n was too much and half of that will be enough.

Can I resize the array during run time WITHOUT losing the data in the first half of the array. In VB it's easily achieved using PRESERVE keyword. Is there an alternative in C++?

Thank you.

Re: Equivalent of VB's "Preserve" keyword

Paul M Watt30-Apr-02 9:58

Paul M Watt

30-Apr-02 9:58

No there is not.

The best you can do is to allocate a new buffer, and copy all of the data from the first buffer into the second buffer. Delete the memory from the first buffer, then assign the pointer from the second buffer back to the first.

By the way, the VB method is just more convenient, not more effifient, underneath all of this work is done by VB as well, you just do not have to deal with it.

Good Luck!

Build a man a fire, and he will be warm for a day
Light a man on fire, and he will be warm for the rest of his life!

Re: Equivalent of VB's "Preserve" keyword

C^hris L^osinger30-Apr-02 10:04

30-Apr-02 10:04

short answer to this particular question: use CStringArray. you can resize the array all you want.

not as short answer to this particular question: use std::vector<CString>. vectors are dynamic arrays, so you can resize them at will.

but, short answer to the general question "can i resize an array allocated with 'new'?": no.

-c

<font size=-2>//The Mandelbrot set
o(int O){putchar(O);}main(){float l[8],O,I=.05;char _;for(l[6]=15;l[6]<':';o
(10),l[5]=-'$'*I+l[6]++*I)for(l[7]=-5;l[7]<'@';l[4]=-'('*I+l[7]++*I,o(_?'?':':'))for
(*l=O=0,_=1;++_&&((l[2]=*l**l)+(l[3]=O*O)<4);O=*l*O+l[5]+O**l,*l=l[2]-l[3]+l[4]);}</font>