CppCon 2017: Chandler Carruth “Going Nowhere Faster”

Disclaimer: I am not an expert and have not measured. This is armchair theory. But, I would argue two things. First, the former appears to have at least one unaligned arithmetic: > 400538: mov 0x200b01(%rip),%rdx # 601040 <counter> ...while the latter's equivalent instruction is 4-byte aligned: > 40057d: mov 0x200abc(%rip),%rdx # 601040 <counter> So, I would argue that's the biggest source of _speedup_ in the second case. However, I'm really interested in whether that's true since I don't see a memory fence; so the memory should be in L0 cache for both cases; I have trouble believing that an unaligned access can be so much slower with the data in cache. As for the `callq` to `repz retq`, I would venture a guess that the CPU's able to identify that there are no data dependencies there and the data's never even stored; I'd argue that it probably never even gets executed because the instruction should fit in instruction cache and branch prediction cache and all. Arguably. Like I said, I'm not an expert. I'd say run it through Intel's code analyzer tool. link Tangential video worth watching: this video Edit: actually, thinking about it, it's not unaligned access , it's unaligned math . I don't think that should affect performance at all? Fun.

Chandler Carruth has a good talk with an example of why speculative execution is critical for performance. this video It starts around 36m13s time frame.