[Processing/x86] Add an SSE4.1 implementation of GeneralBilinearAccurateDownsample

Keep track of relative pixel offsets and utilize pshufb to efficiently extract relevant pixels for horizontal scaling ratios <= 4. Fall back to a generic approach for ratios > 4. The use of blendps makes this require SSE4.1. The pshufb path can be backported to SSSE3 and the generic path to SSE2 for a minor reduction in performance by replacing blendps and preceding instructions with an equivalent sequence. The implementation assumes that data beyond the end of each line, before the next line begins, can be dirtied; which AFAICT is safe with the current usage of these routines. Speedup is ~5.32x/~4.25x (32-bit/64-bit) for horizontal ratios <= 2, ~5.06x/~3.97x for ratios within (2, 4], and ~3.93x/~3.13x for ratios > 4 when not memory-bound on Haswell as compared with the current SSE2 implementation.

SASindre Aamås committed 10 years ago