[Bug tree-optimization/105062] Suboptimal vectorization for reduction with several elements

[glisse at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105062

--- Comment #2 from Marc Glisse <glisse at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #1)
> But since not all of the std::max are recognized as
> MAX_EXPR but some only after loop if-conversion

Ah, I hadn't noticed. I tried replacing std::max with a simpler by-value
version so we get MAX_EXPR already in early inline, but that didn't help.
Actually, it made things worse: 

#include <vector>
#include <tuple>
#include <chrono>
#include <algorithm>
#include <random>
#include <iostream>

int my_max(int a, int b){ return (a<b)?b:a; }

int main(){
  const long n = 100000000;
  std::vector<std::tuple<int,int,double>> vec;
  vec.reserve(n);
  std::random_device rd;
  std::default_random_engine re(rd());
  std::uniform_int_distribution<int> rand_int;
  std::uniform_real_distribution<double> rand_dbl;
  for(int i=0;i<n;++i)
    vec.emplace_back(rand_int(re), rand_int(re), rand_dbl(re));
  auto start = std::chrono::system_clock::now();
#ifdef SLOW
  {
    int sup = 0;
    for(int i=0;i<n;++i)
sup=my_max(sup,my_max(std::get<0>(vec[i]),std::get<1>(vec[i])));
    volatile int noopt0 = sup;
  }
#else
  {
    int sup = 0;
    for(int i=0;i<n;++i)
sup=my_max(my_max(sup,std::get<0>(vec[i])),std::get<1>(vec[i]));
    volatile int noopt1 = sup;
  }
#endif
  auto finish = std::chrono::system_clock::now();
  std::cout << std::chrono::duration_cast<std::chrono::microseconds>(finish -
start).count() << '\n';
}


Now reassoc1 turns the fast code into the slow code before the vectorizer can
detect the reduction chain :-(