无法使 rustc 对包含范围循环使用 simd 指令

通过编写简单的程序并将其与相应的C++变体进行比较来学习Rust.

事实证明，简单循环在rust中的运行速度比在c++中慢3倍，并相应地使用rust c和g++/clang++编译.在研究了这个问题之后，它似乎与以下事实有关:与g++不同，rustc不发出avx指令.

我试着在rustc标志上增加-C target-feature=+avx个，但没有成功.

就我而言，rustc使用llvm后端，所以它应该能够发出它们，这让我想到了一个问题:在rust中使用avx被认为是不安全的，还是我缺少了一种正确的方法来强制rustc启用avx？

Below I provide program source code and other relevant information:

Rust程序和编译标志:

rustc -C opt-level=3

use std::time;

const N: i32 = 2000;

fn main() {
    println!("n: {}", N);
    let start = time::Instant::now();

    let mut count: i32 = 0;
    for a in 1..=N {
        for b in 1..a {
            for c in 1..=b {
                if a*a == b*b + c*c {
                    count += 1;
                }
            }
        }
    }

    let duration = start.elapsed();
    println!("found: {}", count);
    println!("elapsed: {:?}", duration);
}

输出:

n: 2000
found: 1981
elapsed: 1.382231558s

C++程序和编译标志:

g++ -O3

clang++ -O3

#include <iostream>
#include <chrono>

constexpr int N = 2000;
constexpr double MS_TO_SEC = 1e-3;

int main(int argc, char **argv) {
    std::cout << "n: " << N << std::endl;
    auto start = std::chrono::high_resolution_clock::now();

    int count = 0;
    for (int a = 1; a <= N; ++a) {
        for (int b = 1; b < a; ++b) {
            for (int c = 1; c <= b; ++c) {
                if (a*a == b*b + c*c) {
                    count += 1;
                }
            }
        }
    }

    auto elapsed = std::chrono::high_resolution_clock::now() - start;
    std::cout << "found: " << count << std::endl;
    std::cout << "elapsed: " 
              << double(std::chrono::duration_cast<std::chrono::milliseconds>(elapsed).count())*MS_TO_SEC 
              << "s" << std::endl;
}

输出:

n: 2000
found: 1981
elapsed: 0.419s

编译器版本:

g++ 9.4.0
clang++ 10.0.0
rustc 1.64.0-nightly

CPU信息:

Architecture:                    x86_64
CPU op-mode(s):                  32-bit, 64-bit
Byte Order:                      Little Endian
Address sizes:                   39 bits physical, 48 bits virtual
CPU(s):                          8
On-line CPU(s) list:             0-7
Thread(s) per core:              2
Core(s) per socket:              4
Socket(s):                       1
NUMA node(s):                    1
Vendor ID:                       GenuineIntel
CPU family:                      6
Model:                           60
Model name:                      Intel(R) Core(TM) i7-4710HQ CPU @ 2.50GHz
Stepping:                        3
CPU MHz:                         800.000
CPU max MHz:                     3500,0000
CPU min MHz:                     800,0000
BogoMIPS:                        4988.85
Virtualization:                  VT-x
L1d cache:                       128 KiB
L1i cache:                       128 KiB
L2 cache:                        1 MiB
L3 cache:                        6 MiB
NUMA node0 CPU(s):               0-7
Flags:                           fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe syscall nx pdpe1gb rdtscp lm constant_tsc arch_perfmon pebs
                                  bts rep_good nopl xtopology nonstop_tsc cpuid aperfmperf pni pclmulqdq dtes64 monitor ds_cpl vmx est tm2 ssse3 sdbg fma cx16 xtpr pdcm pcid sse4_1 sse4_2 x2apic movbe popcnt tsc_
                                 deadline_timer aes xsave avx f16c rdrand lahf_lm abm cpuid_fault epb invpcid_single pti ssbd ibrs ibpb stibp tpr_shadow vnmi flexpriority ept vpid ept_ad fsgsbase tsc_adjust bmi1 
                                 avx2 smep bmi2 erms invpcid xsaveopt dtherm ida arat pln pts md_clear flush_l1d

编辑:

找到的解决方案: