I have a rectangle, with an x and y coordinate which represent the top-left corner of the rectangle. I also have the width and the height.

I need a piece of code that can iterate through all the four lines of the rectangle fast.

My crap code works but have 4 for-loops o_0

Just emit the four corners by hand: (x, y), (x+w, y), (x+w, y+h) and (x, y+h). You already know the shape, the origin and the size, and the number of points to calculate is so trivial that hand calculation is easy enough.

yeah, but that would require four for-loops, which is inefficient. Performance is critical.

are you trying to do something like this ?

#define MAX_POLYVERTS    4
#define MASK_POLYVERTS    (MAX_POLYVERTS - 1)

int rect_pts[MAX_POLYVERTS][2] = { {x, y}, {x+w, y}, {x+w, y+h}, {x, y+h}};
int a, b;

for(a = 0; a <= MAX_POLYVERTS; a++)
{
    point1_x = rect_pts[a][0];
    point1_y = rect_pts[a][1];

    // get coords of next point, use bitmask to 'wrap' value back to zero
    point2_x = rect_pts[(a+1)&MASK_POLYVERTS][0];
    point2_y = rect_pts[(a+1)&MASK_POLYVERTS][1];

    // ... do something here with points 1 and 2
}

yeah, but that would require four for-loops, which is inefficient. Performance is critical.

I am checking if the rectangle is intersecting with solid tile.

Here is my previous code(it was two for loops instead of one:

		int x = entity.x;
		int y = entity.y;
		int width =  entity.width;
		int height = entity.height;
		
		//Check the top and bottom line
		for(int i = 0; i < width; i++)
			if(foo(i, y) || foo(i, y + height))
				...
			
                //Check left and right line
		for(int i = 0; i < height; i++)
			if(foo(x, i) || foo(x, i + width))
				...

I'm sure that whatever language you are using (your code snippet can be several) there is a Rectangle implementation in the core API which has an intersects( anotherRectangle ) method?

I am checking if the rectangle is intersecting with solid tile.

Here is my previous code(it was two for loops instead of one:

		int x = entity.x;
		int y = entity.y;
		int width =  entity.width;
		int height = entity.height;
		
		//Check the top and bottom line
		for(int i = 0; i < width; i++)
			if(foo(i, y) || foo(i, y + height))
				...
			
                //Check left and right line
		for(int i = 0; i < height; i++)
			if(foo(x, i) || foo(x, i + width))
				...

AABB vs AABB :

http://www.miguelcasillas.com/?p=30