Take a look at awfy.com (http://arewefastyet.com/?runs=1000&view=regress&machine=11), there are two obvious performance regressions.
The root cause is related to r125637, and it's further exposed in r126387. It's a typo in the baseline JIT compiler for op_get_by_val which causes the baseline JIT incorrectly falls back to the slow case. The value profiler knows the fact that the operation goes to the slow path, the DFG compiler then thinks it might clobber the world and the high level optimizations especially the local CSE are affected greatly.
The fix should recover both regressions. My local test indicates 5% on SunSpider, 2% on v8 and 23% on Kraken.
Created attachment 162453 [details]
Created attachment 162454 [details]
Performance result (32bit)
Comment on attachment 162453 [details]
Clearing flags on attachment: 162453
Committed r127778: <http://trac.webkit.org/changeset/127778>
All reviewed patches have been landed. Closing bug.