为什么这个循环多次的程序在循环后有一个“println”时需要时间？

loops eclipse java println

2022-09-01 17:08:02

这是我正在尝试的小代码。此程序需要大量时间来执行。在运行时，如果我试图通过日食中的终止按钮杀死它，它会返回。我可以使用从终端杀死它。Terminate Failedkill -9 <PID>

但是，当我没有在程序的最后一行打印变量结果时（请检查代码的注释部分），程序会立即退出。

我想知道：

为什么在打印结果值时执行需要时间？
请注意，如果我不打印值，相同的循环会立即结束。
为什么日食不能杀死程序？

更新 1 ：JVM似乎在运行时（而不是编译时）优化代码。此线程很有帮助。

更新2：当我打印的值时，不起作用。只有工作。任何可能的原因？valuejstack <PID>jstack -F <PID>

    public class TestClient {

        private static void loop() {
            long value =0;

            for (int j = 0; j < 50000; j++) {
                for (int i = 0; i < 100000000; i++) {
                    value += 1;
                }
            }
            //When the value is being printed, the program 
            //is taking time to complete
            System.out.println("Done "+ value);

            //When the value is NOT being printed, the program 
            //completes immediately
            //System.out.println("Done ");
        }

        public static void main(String[] args) {
            loop();
        }
    }

答案 1

这是一个JIT编译器优化（不是Java编译器优化）。

如果比较 java 编译器为这两个版本生成的字节码，您将看到这两个版本都存在循环。

以下是使用 println 的反编译方法的外观：

private static void loop() {
    long value = 0L;

    for(int j = 0; j < '썐'; ++j) {
        for(int i = 0; i < 100000000; ++i) {
            ++value;
        }
    }

    System.out.println("Done " + value);
}

这是删除 println 时反编译方法的外观：

private static void loop() {
    long value = 0L;

    for(int j = 0; j < '썐'; ++j) {
        for(int i = 0; i < 100000000; ++i) {
            ++value;
        }
    }
}

如您所见，循环仍然存在。

但是，您可以使用以下 JVM 选项启用 JIT 编译器日志记录和程序集打印：

-XX:+UnlockDiagnosticVMOptions -XX:+LogCompilation -XX:+PrintAssembly

您可能还需要下载hsdis-amd64.dylib并放入您的工作目录（MacOS，HotSpot Java 8）

运行 TestClient 后，您应该会在控制台中看到 JIT 编译器生成的代码。在这里，我将仅发布输出的摘录。

不带打印的版本：

# {method} 'loop' '()V' in 'test/TestClient'
0x000000010e3c2500: callq  0x000000010dc1c202  ;   {runtime_call}
0x000000010e3c2505: data32 data32 nopw 0x0(%rax,%rax,1)
0x000000010e3c2510: sub    $0x18,%rsp
0x000000010e3c2517: mov    %rbp,0x10(%rsp)
0x000000010e3c251c: mov    %rsi,%rdi
0x000000010e3c251f: movabs $0x10dc760ec,%r10
0x000000010e3c2529: callq  *%r10              ;*iload_3
                                              ; - test.TestClient::loop@12 (line 9)
0x000000010e3c252c: add    $0x10,%rsp
0x000000010e3c2530: pop    %rbp
0x000000010e3c2531: test   %eax,-0x1c18537(%rip)        # 0x000000010c7aa000
                                              ;   {poll_return}
0x000000010e3c2537: retq

带打印的版本：

# {method} 'loop' '()V' in 'test/TestClient'
0x00000001092c36c0: callq  0x0000000108c1c202  ;   {runtime_call}
0x00000001092c36c5: data32 data32 nopw 0x0(%rax,%rax,1)
0x00000001092c36d0: mov    %eax,-0x14000(%rsp)
0x00000001092c36d7: push   %rbp
0x00000001092c36d8: sub    $0x10,%rsp
0x00000001092c36dc: mov    0x10(%rsi),%r13
0x00000001092c36e0: mov    0x8(%rsi),%ebp
0x00000001092c36e3: mov    (%rsi),%ebx
0x00000001092c36e5: mov    %rsi,%rdi
0x00000001092c36e8: movabs $0x108c760ec,%r10
0x00000001092c36f2: callq  *%r10
0x00000001092c36f5: jmp    0x00000001092c3740
0x00000001092c36f7: add    $0x1,%r13          ;*iload_3
                                              ; - test.TestClient::loop@12 (line 9)
0x00000001092c36fb: inc    %ebx               ;*iinc
                                              ; - test.TestClient::loop@22 (line 9)
0x00000001092c36fd: cmp    $0x5f5e101,%ebx
0x00000001092c3703: jl     0x00000001092c36f7  ;*if_icmpge
                                              ; - test.TestClient::loop@15 (line 9)
0x00000001092c3705: jmp    0x00000001092c3734
0x00000001092c3707: nopw   0x0(%rax,%rax,1)
0x00000001092c3710: mov    %r13,%r8           ;*iload_3
                                              ; - test.TestClient::loop@12 (line 9)
0x00000001092c3713: mov    %r8,%r13
0x00000001092c3716: add    $0x10,%r13         ;*ladd
                                              ; - test.TestClient::loop@20 (line 10)
0x00000001092c371a: add    $0x10,%ebx         ;*iinc
                                              ; - test.TestClient::loop@22 (line 9)
0x00000001092c371d: cmp    $0x5f5e0f2,%ebx
0x00000001092c3723: jl     0x00000001092c3710  ;*if_icmpge
                                              ; - test.TestClient::loop@15 (line 9)
0x00000001092c3725: add    $0xf,%r8           ;*ladd
                                              ; - test.TestClient::loop@20 (line 10)
0x00000001092c3729: cmp    $0x5f5e101,%ebx
0x00000001092c372f: jl     0x00000001092c36fb
0x00000001092c3731: mov    %r8,%r13           ;*iload_3
                                              ; - test.TestClient::loop@12 (line 9)
0x00000001092c3734: inc    %ebp               ;*iinc
                                              ; - test.TestClient::loop@28 (line 8)
0x00000001092c3736: cmp    $0xc350,%ebp
0x00000001092c373c: jge    0x00000001092c376c  ;*if_icmpge
                                              ; - test.TestClient::loop@7 (line 8)
0x00000001092c373e: xor    %ebx,%ebx
0x00000001092c3740: mov    %ebx,%r11d
0x00000001092c3743: inc    %r11d              ;*iload_3
                                              ; - test.TestClient::loop@12 (line 9)
0x00000001092c3746: mov    %r13,%r8
0x00000001092c3749: add    $0x1,%r8           ;*ladd
                                              ; - test.TestClient::loop@20 (line 10)
0x00000001092c374d: inc    %ebx               ;*iinc
                                              ; - test.TestClient::loop@22 (line 9)
0x00000001092c374f: cmp    %r11d,%ebx
0x00000001092c3752: jge    0x00000001092c3759  ;*if_icmpge
                                              ; - test.TestClient::loop@15 (line 9)
0x00000001092c3754: mov    %r8,%r13
0x00000001092c3757: jmp    0x00000001092c3746
0x00000001092c3759: cmp    $0x5f5e0f2,%ebx
0x00000001092c375f: jl     0x00000001092c3713
0x00000001092c3761: mov    %r13,%r10
0x00000001092c3764: mov    %r8,%r13
0x00000001092c3767: mov    %r10,%r8
0x00000001092c376a: jmp    0x00000001092c3729  ;*if_icmpge
                                              ; - test.TestClient::loop@7 (line 8)
0x00000001092c376c: mov    $0x24,%esi
0x00000001092c3771: mov    %r13,%rbp
0x00000001092c3774: data32 xchg %ax,%ax
0x00000001092c3777: callq  0x0000000109298f20  ; OopMap{off=188}
                                              ;*getstatic out
                                              ; - test.TestClient::loop@34 (line 13)
                                              ;   {runtime_call}
0x00000001092c377c: callq  0x0000000108c1c202  ;*getstatic out
                                              ; - test.TestClient::loop@34 (line 13)
                                              ;   {runtime_call}

此外，您还应该具有具有JIT编译器步骤的热点.log文件。以下是摘录：

<phase name='optimizer' nodes='114' live='77' stamp='0.100'>
            <phase name='idealLoop' nodes='115' live='67' stamp='0.100'>
                <loop_tree>
                    <loop idx='119' >
                        <loop idx='185' main_loop='185' >
                        </loop>
                    </loop>
                </loop_tree>
                <phase_done name='idealLoop' nodes='197' live='111' stamp='0.101'/>
            </phase>
            <phase name='idealLoop' nodes='197' live='111' stamp='0.101'>
                <loop_tree>
                    <loop idx='202' >
                        <loop idx='159' inner_loop='1' pre_loop='131' >
                        </loop>
                        <loop idx='210' inner_loop='1' main_loop='210' >
                        </loop>
                        <loop idx='138' inner_loop='1' post_loop='131' >
                        </loop>
                    </loop>
                </loop_tree>
                <phase_done name='idealLoop' nodes='221' live='113' stamp='0.101'/>
            </phase>
            <phase name='idealLoop' nodes='221' live='113' stamp='0.101'>
                <loop_tree>
                    <loop idx='202' >
                        <loop idx='159' inner_loop='1' pre_loop='131' >
                        </loop>
                        <loop idx='210' inner_loop='1' main_loop='210' >
                        </loop>
                        <loop idx='138' inner_loop='1' post_loop='131' >
                        </loop>
                    </loop>
                </loop_tree>
                <phase_done name='idealLoop' nodes='241' live='63' stamp='0.101'/>
            </phase>
            <phase name='ccp' nodes='241' live='63' stamp='0.101'>
                <phase_done name='ccp' nodes='241' live='63' stamp='0.101'/>
            </phase>
            <phase name='idealLoop' nodes='241' live='63' stamp='0.101'>
                <loop_tree>
                    <loop idx='202' inner_loop='1' >
                    </loop>
                </loop_tree>
                <phase_done name='idealLoop' nodes='253' live='56' stamp='0.101'/>
            </phase>
            <phase name='idealLoop' nodes='253' live='56' stamp='0.101'>
                <phase_done name='idealLoop' nodes='253' live='33' stamp='0.101'/>
            </phase>
            <phase_done name='optimizer' nodes='253' live='33' stamp='0.101'/>
        </phase>

您可以使用 JitWatch 工具进一步分析 JIT 编译器生成的热点.log文件 https://github.com/AdoptOpenJDK/jitwatch/wiki

JitWatch TriView for TestClient