為什么我的Rust程式運行速度比Java等價程式慢兩倍多？

我有一個程式可以找到所有小于或等于輸入的整數，這些數字可以表示為兩個立方體的總和，兩次，也就是拉馬努金數問題。

我是用 Java 和 Rust 撰寫的，但是，它在 Rust 中的運行速度是 Java 的兩倍多。

我可以做些什么來使其表現更好，或者以其他方式改進它？

防銹代碼：

use num_integer::Roots;
fn main() {
    let v = 984067;
    // let v = 87539319;
    for i in 1..=v {
        ramanujan(i)
    }
}
fn ramanujan(m: i32) {
    let maxcube = m.cbrt();
    let mut res1 = 0;
    let mut res2 = 0;
    let mut _res3 = 0;
    let mut _res4 = 0;
    for i in 1..=maxcube {
        for j in 1..=maxcube {
            if i * i * i   j * j * j == m {
                res1 = i;
                res2 = j;
                break;
            }
        }
    }
    for k in 1..=maxcube {
        for l in 1..=maxcube {
            if k == res1 || k == res2 || l == res1 || l == res2 {
                continue;
            }
            if k * k * k   l * l * l == m {
                _res3 = k;
                _res4 = l;
                break;
            }
        }
    }
    // if ((res1 * res1 * res1)   (res2 * res2 * res2) == m) && ((res3 * res3 * res3)   (res4 * res4 * res4) == m) {
    //     println!("{} is representable as the sums of two different sets of two cubes!\nThese values are {}, {}, and {}, {}.", m, res1, res2, res3, res4);
    // }
}

爪哇代碼：

public class Ramun {
    public static void main(String[] args) {
        int v = 984067;
        // int v = 87539319;
        for (int i = 1; i <= v; i  ) {
            ramanujan(i);
        }
    }

    public static void ramanujan(int m) {
        int maxcube = (int) Math.round(Math.cbrt(m));
        int res1 = 0, res2 = 0, res3 = 0, res4 = 0;
        for (int i = 1; i <= maxcube; i  ) {
            for (int j = 1; j <= maxcube; j  ) {
                if (((i * i * i)   (j * j * j)) == m) {
                    res1 = i;
                    res2 = j;
                    break;
                }
            }
        }
        for (int k = 1; k <= maxcube; k  ) {
            for (int l = 1; l <= maxcube; l  ) {
                if (k == res1 || k == res2 || l == res1 || l == res2)
                    continue;
                if (((k * k * k)   (l * l * l)) == m) {
                    res3 = k;
                    res4 = l;
                    break;
                }
            }
        }
        // if (((res1 * res1 * res1)   (res2 * res2 * res2) == m) && ((res3 * res3 * res3)   (res4 * res4 * res4) == m)) {
        //     System.out.printf("%d is representable as the sums of two different sets of two cubes!%nThese values are %d, %d, and %d, %d.%n", m, res1, res2, res3, res4);
        // }
    }
}

兩個程式的時間輸出

uj5u.com熱心網友回復：

問題在于RangeInclusive。它可能是昂貴的：

fn ramanujan(m: i32) {
    let maxcube = m.cbrt()   1; // we know it can't overflow
    let mut res1 = 0;
    let mut res2 = 0;
    let mut res3 = 0;
    let mut res4 = 0;

    for i in 1..maxcube {
        for j in 1..maxcube {
            if i * i * i   j * j * j == m {
                res1 = i;
                res2 = j;
                break;
            }
        }
    }

    for k in 1..maxcube {
        for l in 1..maxcube {
            if k == res1 || k == res2 || l == res1 || l == res2 {
                continue;
            }
            if k * k * k   l * l * l == m {
                res3 = k;
                res4 = l;
                break;
            }
        }
    }
}

結果：

From: 0.01s user 0.00s system 0% cpu 17.993 total
To: 0.00s user 0.01s system 0% cpu 3.494 total

向#45222添加評論以引起對該問題的關注。

看起來也for_each()允許更好的運行時（也就是說我不喜歡使用它，for回圈更自然并且應該具有相同的性能，在我看來，它應該被視為一個錯誤）：

fn ramanujan(m: i32) {
    let maxcube = m.cbrt();
    let mut res1 = 0;
    let mut res2 = 0;
    let mut res3 = 0;
    let mut res4 = 0;

    (1..=maxcube).for_each(|i| {
        (1..=maxcube).try_for_each(|j| {
            if i * i * i   j * j * j == m {
                res1 = i;
                res2 = j;
                ControlFlow::Break(())
            } else {
                ControlFlow::Continue(())
            }
        });
    });
    (1..=maxcube).for_each(|k| {
        (1..=maxcube).try_for_each(|l| {
            if k != res1 && k != res2 && l != res1 && l != res2 && k * k * k   l * l * l == m {
                res3 = k;
                res4 = l;
                ControlFlow::Break(())
            } else {
                ControlFlow::Continue(())
            }
        });
    });
}

0.00s user 0.01s system 0% cpu 4.029 total

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