Calc/Performance/Cell size

Basic problem: the most basic cell consumes about 50bytes all told, more complex cells consume far more memory, there are a number of simple & obvious things to re-factor here.

It's trivial to calculate the average cost - simply create a sheet with several thousand cells in it, and measure the heap allocation change on load - with memprof or some other tool, then divide by the number of cells. Similarly, wins are easy to measure this way.

ScBaseCell

sc/inc/cell.hxx; code in sc/source/core/data/cell.cxx & cell2.cxx

class ScBaseCell
{
protected:
	ScPostIt*		pNote;
	SvtBroadcaster*	pBroadcaster;
	USHORT			nTextWidth;
	BYTE			eCellType;		// enum CellType - BYTE spart Speicher
	BYTE			nScriptType;

Every cell carries this overhead; note that a chunk of it is not necessary for many cells:

• ScPostIt pointer - very, very infrequently used - we have almost no post-it note per cell.
• SvtBroadcaster - used by cells that are referenced (by a single cell (ie. non-range) reference) from another cell - again, a sub-set of all cells.

Solutions: a little re-factoring required, but stealing a bit-field from eCellType to denote a 'special' cell:

class ScBaseCell
{
protected:
	USHORT			nTextWidth;
	BYTE			eCellType : 7;  // enum CellType - BYTE spart Speicher
        bool                    bSpecial : 1;   // other information to be looked up elsewhere
	BYTE			nScriptType;

The 'bSpecial' flag could be used to denote that there is a 'note' for this cell (in a separate hash), or that this cell has a single-cell dependant. So - we can save 2/3rds of the base size with fairly little effort.

ScFormulaCell

There are a number of problems here:

• ScFormulaCell inherits from svt/inc/listener.h - which has a virtual destructor, hence we have a vtable pointer per instance too (most likely unnecessary), as well as the listener list.

• Document pointer - as in the ScEditCell structure we lug around a document pointer we should 'know' as implicit context.

• Shared formulae - Excel will 'share' formulae - ie. very little state is duplicated if you fill a column 'D' with =((A1+B1)/C1)* SQRT(A1) or whatever. Calc by contrast will duplicate this formulae innumerable times. We need to extract immutable, position independant formula objects, reference count & share these; plus of course, elide duplicates on import. This would give a massive memory saving for large sheets - it's very common to share formulae.