3 ms·

Yes and no. “yes” because what clang did is a very good job, pretty sure it substantially faster than scalar code. Especially on newish CPUs like Zen2. “no“ b

Yes and no. “yes” because what clang did is a very good job, pretty sure it substantially faster than scalar code. Especially on newish CPUs like Zen2. “no“ because from what they have currently for that particular test case, they only need a few improvements to make their code as fast as a manually vectorized version. Apparently, the compiler is already aware of the cost of RAM loads. They just need to account for these bypass delays between integer and FP execution units in their code generator.

nkurz 6y ago
Frequently, when one is using SIMD one is more concerned about throughput (how many operations you can do per second) and less about latency (how many nanoseconds until you get there first result back). I'm not sure if you are equating the two in your comments, or have an actual preference for lower latency, but realize that one can often gain in throughput by giving up some latency. I mention this because I don't share your certainty that avoiding duplicate loads is necessarily "faster". I've occasionally been able beat compilers at getting higher throughput by making the duplicate load, despite the fact the latency increases. This is because vector shuffles (and I think aligns and inserts) are limited to a single execution port (at least on recent Intel processors), while loads can be executed twice per cycle. Depending on the mix of the rest of instructions, this means that sometimes the redundant load ends up being the better choice. Which is to say, rather than being sure, measure!
- Const-me 6y ago
  I agree in general, and yes, I always use a profiler before writing these intrinsics. But still, that particular clang output is suboptimal. They have already optimized loads with these shuffles. I think they did 80% of the work already. They are just using the wrong shuffles.

nkurz 6y ago

Frequently, when one is using SIMD one is more concerned about throughput (how many operations you can do per second) and less about latency (how many nanoseconds until you get there first result back). I'm not sure if you are equating the two in your comments, or have an actual preference for lower latency, but realize that one can often gain in throughput by giving up some latency. I mention this because I don't share your certainty that avoiding duplicate loads is necessarily "faster". I've occasionally been able beat compilers at getting higher throughput by making the duplicate load, despite the fact the latency increases. This is because vector shuffles (and I think aligns and inserts) are limited to a single execution port (at least on recent Intel processors), while loads can be executed twice per cycle. Depending on the mix of the rest of instructions, this means that sometimes the redundant load ends up being the better choice. Which is to say, rather than being sure, measure!

Const-me 6y ago
I agree in general, and yes, I always use a profiler before writing these intrinsics. But still, that particular clang output is suboptimal. They have already optimized loads with these shuffles. I think they did 80% of the work already. They are just using the wrong shuffles.