Quick set a Byte Variable in a 3D struct

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Hi,

I have a 3d structure that contains a Byte variable cCtrlVal.
I would like to set this at runtime to 255 as fast as possible.
There are 5760 instances of cCtrlVal.

Currently I am looping through the matrix with 3 nested for loops.

This there a better quicker way to quick set this var?
Thanks in advance.

:Ron

C++

struct typControlMatrix
	{
	BOOL bEnabled; 
	BOOL bChanged;
	BOOL LastChange;	
	BOOL AdjStripNo;
        BYTE cCtrlVal; 
	enum enControlType eCtrlType;
	DWORD lCtrlNo;
	char sStripDesc[MAX_SCRIBBLE_STRIP_DESC]; 
	CString sCtrlVal;
	BOOL bDescChanged;
	int TrackPage; 
	int CompRatioIndex ;
	DWORD lStripNo;
	int lParamNo;
	} m_tControlMatrix[128][5][9];

Posted 21-Nov-12 16:33pm

Ron Anders

Updated 21-Nov-12 18:45pm

v3

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Comments

Mohibur Rashid 22-Nov-12 0:48am

i don't think you have other choice

Sergey Alexandrovich Kryukov 22-Nov-12 2:13am

Agree; this is easy to make worse, but hard to improve.
By the way, using ++index instead of usual index++ of "for" statement usually makes it faster -- try it...
--SA

2 solutions

Solution 1

You can flatten the loop to a 1D affair by taking the address of the first element of the 3D array and then do 128*5*9 iterations.

Today's optimizing compilers might do the same and in that case this technique would not yield any speed advantage. On the other hand, it's worth a try.

Posted 21-Nov-12 22:51pm

nv3

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CPallini · Accepted Answer · 2012-11-21T23:42:00

With nv3's suggested 'flat approach', using pointers I obtained a somewhat surprising result (about 3x speed improvement):

C++

register int i, j, k;
  LARGE_INTEGER t[4];

  QueryPerformanceCounter(&t[0]);

  //-> 3 loops
  for (i=0; i<128; i++)
  {
    for (j=0; j<5; j++)
    {
      for (k=0; k<9; k++)
        m_tControlMatrix[i][j][k].cCtrlVal = 255;
    }
  }
  //<-
  QueryPerformanceCounter(&t[1]);

  QueryPerformanceCounter(&t[2]);
  //-> flat pointers
  register BYTE * p = &m_tControlMatrix[0][0][0].cCtrlVal;
  register BYTE * q = p + 5760 * sizeof(typControlMatrix);
  while (p < q)
  {
    *p = 255;
    p += sizeof(typControlMatrix);
  }
  //<-
  QueryPerformanceCounter(&t[3]);
  
  CString s;
  s.Format("3 loops: %I64d flat pointers: %I64d speed ratio %g ", (t[1].QuadPart-t[0].QuadPart), (t[3].QuadPart-t[2].QuadPart), ((double)(t[1].QuadPart-t[0].QuadPart))/(t[3].QuadPart-t[2].QuadPart));
  MessageBox(s, "Test");

The output:

3 loops: 195327 flat pointers: 67743 speed ratio 2.88335