2 ms·

You're using C intrinsics instead of assembly, right? Are you sure the compiler isn't doing the scheduling well enough on its own?

by gok 8y ago

glangdale 8y ago
If you're think about algorithms in terms of how they might embed into a processor, you need to know the throughput and latency numbers even if the compiler is getting things right. For example, there are some nice possibilities with the TBL instruction (or PSHUFB, or VERMB, etc) for doing character class membership tests. Which version you use would have a lot to do with whether you think TBL is going to issue 2/cycle, 1/cycle or 0.5/cycle (just fr'instance) - you might design the algorithm quite differently. It's not just a case of picking the one design choice and then having the compiler schedule that - if you're not thinking about this during design, you're not going to be remotely near peak performance.
- gok 8y ago
  Sure but the compiler can compile your intrinsics to different-but-equivalent instructions, and modern compilers actually do that pretty aggressively. Are you sure the code output was actually using the instructions you expected?
  - burntsushi 8y ago
    Often times you wind up needing to change the entire algorithm to use different instructions with different latency/throughput numbers. The classic case off the top of my head (for Intel at least) is PCMPESTRI versus completely different approaches for substring search.
  - glangdale 8y ago
    Yes. I look at the asm all the time, having not fallen off the turnip truck yesterday.
Rebelgecko 8y ago
I'm not an expert assembly programmer and I'm definitely not an expert at using SIMD instructions, but the last time I created a NEON implementation of an algorithm I was able to beat GCC's output (from compiling C intrinsics) after an afternoon of tweaking things by hand. From looking at GCC's output it didn't seem like it was doing a great job interleaving ARM and NEON ops to take advantage of that pipeline, although maybe it's gotten better since GCC 5.

You're using C intrinsics instead of assembly, right? Are you sure the compiler isn't doing the scheduling well enough on its own?

by gok 8y ago

You're using C intrinsics instead of assembly, right? Are you sure the compiler isn't doing the scheduling well enough on its own?

glangdale 8y ago

If you're think about algorithms in terms of how they might embed into a processor, you need to know the throughput and latency numbers even if the compiler is getting things right. For example, there are some nice possibilities with the TBL instruction (or PSHUFB, or VERMB, etc) for doing character class membership tests. Which version you use would have a lot to do with whether you think TBL is going to issue 2/cycle, 1/cycle or 0.5/cycle (just fr'instance) - you might design the algorithm quite differently. It's not just a case of picking the one design choice and then having the compiler schedule that - if you're not thinking about this during design, you're not going to be remotely near peak performance.

gok 8y ago
Sure but the compiler can compile your intrinsics to different-but-equivalent instructions, and modern compilers actually do that pretty aggressively. Are you sure the code output was actually using the instructions you expected?
- burntsushi 8y ago
  Often times you wind up needing to change the entire algorithm to use different instructions with different latency/throughput numbers. The classic case off the top of my head (for Intel at least) is PCMPESTRI versus completely different approaches for substring search.
- glangdale 8y ago
  Yes. I look at the asm all the time, having not fallen off the turnip truck yesterday.

Rebelgecko 8y ago

I'm not an expert assembly programmer and I'm definitely not an expert at using SIMD instructions, but the last time I created a NEON implementation of an algorithm I was able to beat GCC's output (from compiling C intrinsics) after an afternoon of tweaking things by hand. From looking at GCC's output it didn't seem like it was doing a great job interleaving ARM and NEON ops to take advantage of that pipeline, although maybe it's gotten better since GCC 5.