weird optimization

PIEBALDconsult

Try it this way (ANSI C):

int old \[ size \] ;
int i = 0 ;
int j ;

srand ( NULL ) ;

while ( i < size )
{
    old \[ i \] = rand() ;

    for ( j = 0 ; j < i && old \[ j \] != old \[ i \] ; j++ ) ;

    if ( i == j ) i++ ;
}

Luc Pattyn

PIEBALDconsult wrote:

I doubt that belongs in this forum

It might if there were a mistake in the code, but I don't see it. It might help if some good and bad output were also presented... :)

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PIEBALDconsult

PIEBALDconsult

If you use index = rand() % size ; do you get more duplicates?

Robert Surtees

I don't see a srand() to seed rand(). I think the default is srand( 1 ) if none is given but that might not be the case in your implementation.

Mike Dimmick

Congratulations, you've found an optimization bug. Please report this to your compiler vendor. I'm guessing it's erroneously reusing a register, or not saving a value back to a memory location, or something like that. volatile requires the compiler to go back to the memory location every time (because it could have changed in a different code path) rather than use a previous value stored in a register.

Stability. What an interesting concept. -- Chris Maunder

Mladen Jankovic

Posted code was just part of a method, and originally it's not usin built-in random generator. I have made test project, to see what's wrong.

int _tmain(int argc, _TCHAR* argv[])
{
srand( (unsigned int) time(NULL) );

int size = 10;
int\* old = new int\[ size \];

for( int i = 0; i < size; i++ )
{
	int index;
	bool duplicate = false;
	do	
	{		
		index = rand() % size;
		for( int j = 0; j < i; j++ )
		{			
			duplicate = old\[ j \] == index;			
			if( duplicate )				
				break;		
		}	
	} while( duplicate );	

	old\[ i \] = index;
}

for( int i = 0; i < size; i++ )
	cout << old\[ i \] << "\\n";

for( int i = 0; i < size; i++ )
{
	for( int j = i + 1; j < size; j++ )
	{
		if( old\[ i \] == old\[ j \] )
			cout << "old\[" << i << "\]==old\[" << j << "\]==" << old\[ i \] << "\\n";
	}
}

delete old;

return 0;

}

Ouput: 8 3 7 6 0 9 8 3 1 4 old[0]==old[6]==8 old[1]==old[7]==3 Compiler used is MS VC++ 8.0. Building project as Debug produces no duplicates, but as Release produces more the one duplicate.

Mostly, when you see programmers, they aren't doing anything. One of the attractive things about programmers is that you cannot tell whether or not they are working simply by looking at them. Very often they're sitting there seemingly drinking coffee and gossiping, or just staring into space. What the programmer is trying to do is get a handle on all the individual and unrelated ideas that are scampering around in his head. (Charles M Strauss)

Mladen Jankovic

And what if I don't? :D By the way, I'm I the only one who think this message board (last two threads) are messed up?

Mostly, when you see programmers, they aren't doing anything. One of the attractive things about programmers is that you cannot tell whether or not they are working simply by looking at them. Very often they're sitting there seemingly drinking coffee and gossiping, or just staring into space. What the programmer is trying to do is get a handle on all the individual and unrelated ideas that are scampering around in his head. (Charles M Strauss)

PIEBALDconsult

Mladen Jankovic wrote:

Compiler used is MS VC++ 8.0

Oh, well, there's your problem. :-D Could you try my version and let me know whether or not it works?

Mladen Jankovic

PIEBALDconsult wrote:

Could you try my version and let me know whether or not it works?

Yes it works.

Mostly, when you see programmers, they aren't doing anything. One of the attractive things about programmers is that you cannot tell whether or not they are working simply by looking at them. Very often they're sitting there seemingly drinking coffee and gossiping, or just staring into space. What the programmer is trying to do is get a handle on all the individual and unrelated ideas that are scampering around in his head. (Charles M Strauss)

peterchen

Just out of curiosity, which compiler are you using?

We are a big screwed up dysfunctional psychotic happy family - some more screwed up, others more happy, but everybody's psychotic joint venture definition of CP
My first real C# project | Linkify!|FoldWithUs! | sighist

Mladen Jankovic

VC++ 8 SP1, but it looks like that Beta version of Orcas has same bug. I have posted message on MSDN forum, and their moderator submited bug to VS team: https://connect.microsoft.com/VisualStudio/feedback/ViewFeedback.aspx?feedbackID=292132

Mostly, when you see programmers, they aren't doing anything. One of the attractive things about programmers is that you cannot tell whether or not they are working simply by looking at them. Very often they're sitting there seemingly drinking coffee and gossiping, or just staring into space. What the programmer is trying to do is get a handle on all the individual and unrelated ideas that are scampering around in his head. (Charles M Strauss)

peterchen

That's freaking scary!

We are a big screwed up dysfunctional psychotic happy family - some more screwed up, others more happy, but everybody's psychotic joint venture definition of CP
My first real C# project | Linkify!|FoldWithUs! | sighist

Mladen Jankovic

Bug or what? ;)

Mostly, when you see programmers, they aren't doing anything. One of the attractive things about programmers is that you cannot tell whether or not they are working simply by looking at them. Very often they're sitting there seemingly drinking coffee and gossiping, or just staring into space. What the programmer is trying to do is get a handle on all the individual and unrelated ideas that are scampering around in his head. (Charles M Strauss)

David Crow

Your code snippet seems a bit inefficient. Try something like:

__inline void Swap( int &a, int &b )
{
int t = b;
b = a;
a = t;
}

void main( void )
{
int array[10] = {0};
int n = 10;

for (int i = 0; i < n; i++)
    array\[i\] = i;

for (i = 0; i < n; i++)
{ 
    int j = rand() % n;
    Swap(array\[j\], array\[i\]);
}

}

"A good athlete is the result of a good and worthy opponent." - David Crow

"To have a respect for ourselves guides our morals; to have deference for others governs our manners." - Laurence Sterne

ghle

Good programmers don't "break" out of "for" loops.

Member 1229083

Well done, good catch! "Codegen problem with nested do-while / for loops (/O2)" I just gave my vote of Very-Important. Note-to-self: try to avoid flying with Microsoft powered airplanes.