test-avx2.cpp

/*
Benchmark for Eigen change, see <https://gitlab.com/libeigen/eigen/-/merge_requests/734#note_743674873>

Comment reported here for posterity:

Here's a synthetic "benchmark" which I _believe_ shows the difference: https://gist.github.com/bitonic/2d09df858ba2233b7f472f5f8c0512b4 .

I say that I believe that it exhibits the difference because it shows the runtime differences that I'd expect, with some caveats (see comments on number of instructions below).

However, I have not inspected the assembly manually to check that the code varies in the way I'd expect, which would be a requirement to ensure that things change in the way we expect. That is a bit more labor intensive, and while I might do it, I don't have time to do it right now.

The code inline:

```cpp
#include <Eigen/Core>
#include <iostream>

using ArrType = Eigen::Array<float, 2, 169>;

__attribute__((noinline))
static void print_array(const char* name, const ArrType& arr) {
  std::cout << name << ": " << arr << std::endl;
}

__attribute__((noinline))
static void test_packet(float x) {
  ArrType xs(x);
  ArrType ys(0.0f);

  print_array("xs", xs);
  print_array("ys", ys);

  for (size_t i = 0; i < 100000000; i++) {
    if (i % 2 == 0) {
      ys += xs;
    } else {
      ys -= xs;
    }
  }

  print_array("ys", ys);

  return;
}

int main() {
  test_packet(5.0f);

  return 0;
}
```

I compile it with

```
% clang++ -std=c++20 -I. -Wall -Werror -mavx2 -O3 test-avx2.cpp -o test-avx2
```

In the `eigen` repo.

We just add and subtract from an array which is 169 elements wide. What I realized is that this change only affects arrays of static size -- which was the case in the proprietary code this perf improvement came up in. In fact I am using the same size I were using in that code -- 169. We might want to extend it to `Dynamic` (see the stop condition on the same line on why it does not work with `Dynamic`.

If we compile with the improvement, this is perf stat:

```
 Performance counter stats for './test-avx2-new':

          2,526.48 msec task-clock                #    1.000 CPUs utilized          
                 5      context-switches          #    0.002 K/sec                  
                 2      cpu-migrations            #    0.001 K/sec                  
                89      page-faults               #    0.035 K/sec                  
     9,079,877,582      cycles                    #    3.594 GHz                    
    12,968,042,398      instructions              #    1.43  insn per cycle         
       203,464,968      branches                  #   80.533 M/sec                  
            91,320      branch-misses             #    0.04% of all branches        

       2.527443809 seconds time elapsed

       2.525190000 seconds user
       0.001999000 seconds sys
```

Numbers of note: 2.5 seconds runtime, 12B instructions. With the old code:

```
          3,704.16 msec task-clock                #    0.999 CPUs utilized          
                 8      context-switches          #    0.002 K/sec                  
                 0      cpu-migrations            #    0.000 K/sec                  
                86      page-faults               #    0.023 K/sec                  
    13,027,668,290      cycles                    #    3.517 GHz                    
    38,871,811,483      instructions              #    2.98  insn per cycle         
     2,904,199,848      branches                  #  784.037 M/sec                  
           139,444      branch-misses             #    0.00% of all branches        

       3.706167382 seconds time elapsed

       3.702763000 seconds user
       0.002999000 seconds sys
```

3.7 seconds runtime (1.5x speedup), 40B instructions. I actually do not have a great explanation for the 3x jump in instruction, I was expecting a 2x jump, roughly.

Again, I've learnt to not make definitive statements when it comes to micro benchmarks unless I have checked the assembly, but I think the above already gives some confidence that the code does what I think it does.

*/

#include <Eigen/Core>
#include <iostream>

using ArrType = Eigen::Array<float, 2, 169>;

__attribute__((noinline))
static void print_array(const char* name, const ArrType& arr) {
  std::cout << name << ": " << arr << std::endl;
}

__attribute__((noinline))
static void test_packet(float x) {
  ArrType xs(x);
  ArrType ys(0.0f);

  print_array("xs", xs);
  print_array("ys", ys);

  for (size_t i = 0; i < 100000000; i++) {
    if (i % 2 == 0) {
      ys += xs;
    } else {
      ys -= xs;
    }
  }

  print_array("ys", ys);

  return;
}

int main() {
  test_packet(5.0f);

  return 0;
}