The off hand answer is so you can run off the end safely with an sse load or store, but as you say size+align-1 would be sufficient. So I don't know, may be you can do some archaeology and find an answer. Here is a link to
version 1.1 of Avisyntn.cpp, Thu Jul 4 13:16:41 2002 UTC (11 years, 2 months ago) by richardberg as originally supplied by BenRG. Around version 1.9 Sh0dan started doing something with improving alignment.