Re: [Freepooma-devel] mixing domain and stride information

[ron hylton]

> From: Richard Guenther <address@hidden>
> Maybe you can help me see the actual problems.

Richard,

I often solve problems with coefficients that are homogeneous in one or more 
dimensions.  In Array terms, this means that one or more subscripts really 
does nothing and can be ignored.  So I have an engine type I call a 
FlattenedBrick, descended from BrickBase, which is really a 
lower-dimensional brick embedded in the apparent brick.  This saves memory 
and the unused subscripts are visible to the compiler so there is potential 
for optimization, and I have some functions for manipulating FlattenedBricks 
that I can use where I think manual optimization is worthwhile.

The previous BrickView worked fine because it just asked BrickBase what the 
strides & offset were and got the correct values from FlattenedBrick.  Now 
BrickView assumes one very specific memory layout rather than asking 
BrickBase so FlattenedBrick doesn't work, nor would any other BrickBase 
descendant that wanted to use its own memory layout.

Here's a snippet from the original BrickViewBase sliceInit:

   {
     if (!dom.ignorable(dt))
     {
        PAssert(d < Dim);
        domain_m[d]  = Interval<1>(domain[dt].length());
        strides_m[d] = baseStrides[dt] * domain[dt].stride();
        ++d;
     }

     baseOffset_m += domain[dt].first() * baseStrides[dt];

The new stride & offset is computed from the baseStrides supplied by 
BrickBase.

Here's the current version:

 int baseStride = 1;
 int d = 0;
 for (int dt = 0; dt < BaseDim; ++dt)
 {
   if (!dom.ignorable(dt))
   {
     PBoundAssert(d < Dim);
     domain_m[d]  = Interval<1>(domain[dt].length());
     strides_m[d] = baseStride * domain[dt].stride();
     ++d;
   }

   baseOffset_m += domain[dt].first() * baseStride;
   baseStride *= baseDom[dt].length();

BrickView is now using the domain length from BrickBase to compute the 
strides and offset and assuming that memory is laid out densely with a 
particular ordering of subscripts; control over actual memory layout has 
been taken away from BrickBase.

I doubt that there can be any measurable effect on optimization, the changes 
are all in BrickView initialization code.

Ron