Re:Intel i7 loop performance anomaly (2013)
by nkurz on Friday December 15, 2017 @07:46PM
(#15936594)
I'm sorry, but like the other comment at the bottom, your guesses are so far from reality that they are hard to respond to. IACA is great for what it does, but it's a static analyzer and knows nothing about alignment. L0 doesn't even exist on modern Intel processors. Memory fences would change things, but aren't part of the problem as stated. And your guess that "it probably never even gets executed because the instruction should fit in instruction cache and branch prediction cache and all" just doesn't have any bearing on they way processors work.
Your disclaimer does indicate that you have the self-awareness that you are not an expert, but the fact that you are trying to make an argument would normally indicate that you think you understand what's happening to some extent. Rather than just guessing, I think you'd benefit from trying some things out and seeing what the results are. Play with perf, it's fun!
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.
https://software.intel.com/en-us/articles/intel-architecture...
Tangential video worth watching:
https://www.youtube.com/watch?v=2EWejmkKlxs&feature=youtu.be...
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.