CppCon 2017: Chandler Carruth “Going Nowhere Faster”

The best presentation I've seen about CPU performance related to pipelining, branch prediction, and speculative execution was Chandler Carruth's "Going Nowhere Faster" presentation at CppCon 2017 [0]. I do recommend watching the whole presentation, but if you watch nothing else then just watch the 5 minutes or so from the linked timestamp. [0]: [VIDEO]

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: [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. [VIDEO] It starts around 36m13s time frame.

>The YouTube inteface says "auto-generated" Ok, I never noticed that. I just read the captions and assumed obvious misspellings were auto-generated. For example in this video[1], the caption text is "L1D cache misses" but he's actually saying "L1-dcache misses" . (The Linux terminal screen he's showing does display "L1-dcache" .) Even though that video is not labeled as "auto-generated", I assumed it was because of the bad caption. Based on your info, I guess CppCon uses humans like Mechanical Turk or other non-domain typists to manually add the captions. [1] [VIDEO]

Sure, but wouldn’t it make sense to extend the instruction set to allow the compiler to use these registers instead of reserving them for speculative / out-of-order execution? It was just a thought i had after watching a talk by a compiler guy: [VIDEO]