Would you please file a bug report [0] with a reproduction of the missed optimizations that you see? I'm interested in taking a look at it. Today LLVM trunk enabled the vectorizer for -O2 and -Os.
On a related note, I have a dream one day to discover a real example where undefined behavior can be used constructively as a license for the compiler to optimize: the following post alludes to this idea but assembly dumps at the bottom show that the compiler is not taking advantage of the information encoded into the undefined behavior:
http://blog.frama-c.com/index.php?post/2012/07/25/On-the-red...
More seriously, an annotation language for expressing properties that are supposed to be true at various points of the program can be useful to transmit information from the programmer to the compiler and enabling optimizations that would otherwise require difficult full-program analysis. And these annotations can be used to analyze the program too!
http://blog.frama-c.com/index.php?post/2012/07/25/The-restri...
Though, it does seem to always store the old stack pointer in r7, even though it doesn't restore from r7, and even though my inline assembly block specifies r7 on the clobber list. That might be a bug, but it's a single 'add', so who cares.