I can see there's a mistake in my reply - we were working on a 32-bit architecture for this problem. We didn't pack 8 bytes in a 64-bit word, we packed 4 bytes in a 32-bit word. Sorry.

And in short, our target CPUs don't have SSE. I've only been vaguely aware of SSE, although I used to program a Cray on occasion, so am familiar with vector processing. I must see if the SSE instructions can accelerate some of our other work which isn't yet critical, but may become so.

The demand driven stuff was done by taking 32x32 tiles in the target and reverse transforming each to the source, hence finding out which 32x32 bit tiles in the the source needed to be computed. Flags prevented redundant multiple conversions. The transformation doesn't preserve straight lines, so some care was required on boundaries.