（p^q）和（p！=q）对于布尔人有什么有用的区别吗？

10 月，3 周 Questions & Answers 292

Java有两种检查两个布尔值是否不同的方法。您可以将它们与!=或^（xor）进行比较。当然，这两个操作符在所有情况下都会产生相同的结果。尽管如此，如在What's the difference between XOR and NOT-EQUAL-TO?中所讨论的那样，将它们都包括在内是有道理的。开发人员甚至可以根据上下文选择其中一个，有时“这两个布尔值中有一个是真的”读得更好，而其他时候“这两个布尔值不同”读得更好。所以，使用哪一种应该是品味和风格的问题

令我惊讶的是，javac并没有以相同的方式对待这些问题！考虑这个类：

class Test {
  public boolean xor(boolean p, boolean q) {
    return p ^ q;
  }
  public boolean inequal(boolean p, boolean q) {
    return p != q;
  }
}

显然，这两种方法具有相同的可见行为。但它们有不同的字节码：

$ javap -c Test
Compiled from "Test.java"
class Test {
  Test();
    Code:
       0: aload_0
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: return

  public boolean xor(boolean, boolean);
    Code:
       0: iload_1
       1: iload_2
       2: ixor
       3: ireturn

  public boolean inequal(boolean, boolean);
    Code:
       0: iload_1
       1: iload_2
       2: if_icmpeq     9
       5: iconst_1
       6: goto          10
       9: iconst_0
      10: ireturn
}

如果要我猜的话，我会说xor性能更好，因为它只返回比较的结果；再加上一个跳跃和一个额外的负载似乎是白费力气。但我没有猜测，而是使用Clojure的“criterium”基准测试工具对这两种方法的数十亿次调用进行了基准测试。虽然xor看起来快了一点，但我不太擅长统计，无法判断结果是否重要：

user=> (let [t (Test.)] (bench (.xor t true false)))
Evaluation count : 4681301040 in 60 samples of 78021684 calls.
             Execution time mean : 4.273428 ns
    Execution time std-deviation : 0.168423 ns
   Execution time lower quantile : 4.044192 ns ( 2.5%)
   Execution time upper quantile : 4.649796 ns (97.5%)
                   Overhead used : 8.723577 ns

Found 2 outliers in 60 samples (3.3333 %)
    low-severe   2 (3.3333 %)
 Variance from outliers : 25.4745 % Variance is moderately inflated by outliers
user=> (let [t (Test.)] (bench (.inequal t true false)))
Evaluation count : 4570766220 in 60 samples of 76179437 calls.
             Execution time mean : 4.492847 ns
    Execution time std-deviation : 0.162946 ns
   Execution time lower quantile : 4.282077 ns ( 2.5%)
   Execution time upper quantile : 4.813433 ns (97.5%)
                   Overhead used : 8.723577 ns

Found 2 outliers in 60 samples (3.3333 %)
    low-severe   2 (3.3333 %)
 Variance from outliers : 22.2554 % Variance is moderately inflated by outliers

在性能方面，有什么理由更喜欢写一个而不是另一个吗？在某种情况下，它们在实现上的差异使一个比另一个更合适？或者，有人知道为什么javac实现这两个相同的操作如此不同吗

¹当然，我不会鲁莽地使用这些信息进行微优化。我只是好奇这一切是如何运作的

# 1 楼答案

好吧，我将很快提供CPU如何翻译这一点并更新帖子，但与此同时，您看到的waaaay差异太小，无法关注

java中的字节码并不能表示一个方法的执行速度，有两个JIT编译器，一旦它们足够热，就会使这个方法看起来完全不同。另外javac已知在编译代码后只做很少的优化，真正的优化来自JIT

我已经用JMH进行了一些测试，只使用C1编译器，或者用GraalVM替换C2，或者根本不使用JIT。。。（下面有很多测试代码，您可以跳过它，只需查看结果，这是使用jdk-12btw完成的）。这段代码使用的是JMH，这是java世界中使用的事实上的微型基准测试工具（如果手工操作，则极易出错）

@Warmup(iterations = 10)
@OutputTimeUnit(TimeUnit.NANOSECONDS)
@Measurement(iterations = 2, time = 2, timeUnit = TimeUnit.SECONDS)
public class BooleanCompare {

    public static void main(String[] args) throws Exception {
        Options opt = new OptionsBuilder()
            .include(BooleanCompare.class.getName())
            .build();

        new Runner(opt).run();
    }

    @Benchmark
    @BenchmarkMode(Mode.AverageTime)
    @Fork(1)
    public boolean xor(BooleanExecutionPlan plan) {
        return plan.booleans()[0] ^ plan.booleans()[1];
    }

    @Benchmark
    @BenchmarkMode(Mode.AverageTime)
    @Fork(1)
    public boolean plain(BooleanExecutionPlan plan) {
        return plan.booleans()[0] != plan.booleans()[1];
    }

    @Benchmark
    @BenchmarkMode(Mode.AverageTime)
    @Fork(value = 1, jvmArgsAppend = "-Xint")
    public boolean xorNoJIT(BooleanExecutionPlan plan) {
        return plan.booleans()[0] != plan.booleans()[1];
    }

    @Benchmark
    @BenchmarkMode(Mode.AverageTime)
    @Fork(value = 1, jvmArgsAppend = "-Xint")
    public boolean plainNoJIT(BooleanExecutionPlan plan) {
        return plan.booleans()[0] != plan.booleans()[1];
    }

    @Benchmark
    @BenchmarkMode(Mode.AverageTime)
    @Fork(value = 1, jvmArgsAppend = "-XX:-TieredCompilation")
    public boolean xorC2Only(BooleanExecutionPlan plan) {
        return plan.booleans()[0] != plan.booleans()[1];
    }

    @Benchmark
    @BenchmarkMode(Mode.AverageTime)
    @Fork(value = 1, jvmArgsAppend = "-XX:-TieredCompilation")
    public boolean plainC2Only(BooleanExecutionPlan plan) {
        return plan.booleans()[0] != plan.booleans()[1];
    }

    @Benchmark
    @BenchmarkMode(Mode.AverageTime)
    @Fork(value = 1, jvmArgsAppend = "-XX:TieredStopAtLevel=1")
    public boolean xorC1Only(BooleanExecutionPlan plan) {
        return plan.booleans()[0] != plan.booleans()[1];
    }

    @Benchmark
    @BenchmarkMode(Mode.AverageTime)
    @Fork(value = 1, jvmArgsAppend = "-XX:TieredStopAtLevel=1")
    public boolean plainC1Only(BooleanExecutionPlan plan) {
        return plan.booleans()[0] != plan.booleans()[1];
    }

    @Benchmark
    @BenchmarkMode(Mode.AverageTime)
    @Fork(value = 1,
        jvmArgsAppend = {
            "-XX:+UnlockExperimentalVMOptions",
            "-XX:+EagerJVMCI",
            "-Dgraal.ShowConfiguration=info",
            "-XX:+UseJVMCICompiler",
            "-XX:+EnableJVMCI"
        })
    public boolean xorGraalVM(BooleanExecutionPlan plan) {
        return plan.booleans()[0] != plan.booleans()[1];
    }

    @Benchmark
    @BenchmarkMode(Mode.AverageTime)
    @Fork(value = 1,
        jvmArgsAppend = {
            "-XX:+UnlockExperimentalVMOptions",
            "-XX:+EagerJVMCI",
            "-Dgraal.ShowConfiguration=info",
            "-XX:+UseJVMCICompiler",
            "-XX:+EnableJVMCI"
        })
    public boolean plainGraalVM(BooleanExecutionPlan plan) {
        return plan.booleans()[0] != plan.booleans()[1];
    }

}

结果是：

BooleanCompare.plain         avgt    2    3.125          ns/op
BooleanCompare.xor           avgt    2    2.976          ns/op

BooleanCompare.plainC1Only   avgt    2    3.400          ns/op
BooleanCompare.xorC1Only     avgt    2    3.379          ns/op

BooleanCompare.plainC2Only   avgt    2    2.583          ns/op
BooleanCompare.xorC2Only     avgt    2    2.685          ns/op

BooleanCompare.plainGraalVM  avgt    2    2.980          ns/op
BooleanCompare.xorGraalVM    avgt    2    3.868          ns/op

BooleanCompare.plainNoJIT    avgt    2  243.348          ns/op
BooleanCompare.xorNoJIT      avgt    2  201.342          ns/op

我不是一个多才多艺的人，不会阅读汇编程序，尽管我有时喜欢这样做。。。以下是一些有趣的事情。如果我们这样做：

C1 compiler only with !=

/*
 * run many iterations of this with :
 *  java -XX:+UnlockDiagnosticVMOptions  
 *       -XX:TieredStopAtLevel=1  
 *       "-XX:CompileCommand=print,com/so/BooleanCompare.compare"  
 *       com.so.BooleanCompare
 */
public static boolean compare(boolean left, boolean right) {
    return left != right;
}

我们得到：

  0x000000010d1b2bc7: push   %rbp
  0x000000010d1b2bc8: sub    $0x30,%rsp  ;*iload_0 {reexecute=0 rethrow=0 return_oop=0}
                                         ; - com.so.BooleanCompare::compare@0 (line 22)

  0x000000010d1b2bcc: cmp    %edx,%esi
  0x000000010d1b2bce: mov    $0x0,%eax
  0x000000010d1b2bd3: je     0x000000010d1b2bde
  0x000000010d1b2bd9: mov    $0x1,%eax
  0x000000010d1b2bde: and    $0x1,%eax
  0x000000010d1b2be1: add    $0x30,%rsp
  0x000000010d1b2be5: pop    %rbp

对我来说，这段代码有点明显：将0放入eax，compare (edx, esi)->；如果不相等，则将1放入eax。返回eax & 1

C1 compiler with ^:

public static boolean compare(boolean left, boolean right) {
     return left ^ right;
}



  # parm0:    rsi       = boolean
  # parm1:    rdx       = boolean
  #           [sp+0x40]  (sp of caller)
  0x000000011326e5c0: mov    %eax,-0x14000(%rsp)
  0x000000011326e5c7: push   %rbp
  0x000000011326e5c8: sub    $0x30,%rsp   ;*iload_0 {reexecute=0 rethrow=0 return_oop=0}
                                          ; - com.so.BooleanCompare::compare@0 (line 22)

  0x000000011326e5cc: xor    %rdx,%rsi
  0x000000011326e5cf: and    $0x1,%esi
  0x000000011326e5d2: mov    %rsi,%rax
  0x000000011326e5d5: add    $0x30,%rsp
  0x000000011326e5d9: pop    %rbp

我真的不知道为什么这里需要and $0x1,%esi，否则我想这也相当简单

But if I enable C2 compiler, things are a lot more interesting.

/**
 * run with java
 * -XX:+UnlockDiagnosticVMOptions
 * -XX:CICompilerCount=2
 * -XX:-TieredCompilation
 * "-XX:CompileCommand=print,com/so/BooleanCompare.compare"
 * com.so.BooleanCompare
 */
public static boolean compare(boolean left, boolean right) {
    return left != right;
}



  # parm0:    rsi       = boolean
  # parm1:    rdx       = boolean
  #           [sp+0x20]  (sp of caller)
  0x000000011a2bbfa0: sub    $0x18,%rsp
  0x000000011a2bbfa7: mov    %rbp,0x10(%rsp)                

  0x000000011a2bbfac: xor    %r10d,%r10d
  0x000000011a2bbfaf: mov    $0x1,%eax
  0x000000011a2bbfb4: cmp    %edx,%esi
  0x000000011a2bbfb6: cmove  %r10d,%eax                     

  0x000000011a2bbfba: add    $0x10,%rsp
  0x000000011a2bbfbe: pop    %rbp

我甚至没有看到经典的尾声push ebp; mov ebp, esp; sub esp, x，而是通过以下方式看到一些非常不寻常的东西（至少对我而言）：

 sub    $0x18,%rsp
 mov    %rbp,0x10(%rsp)

 ....
 add    $0x10,%rsp
 pop    %rbp

再说一次，一个比我多才多艺的人，可以满怀希望地解释。否则它就像是生成的C1的更好版本：

xor    %r10d,%r10d // put zero into r10d
mov    $0x1,%eax   // put 1 into eax
cmp    %edx,%esi   // compare edx and esi
cmove  %r10d,%eax  // conditionally move the contents of r10d into eax

因为分支预测，AFAIKcmp/cmove比cmp/je好——至少我读到了这一点

XOR with C2 compiler:

public static boolean compare(boolean left, boolean right) {
    return left ^ right;
}



  0x000000010e6c9a20: sub    $0x18,%rsp
  0x000000010e6c9a27: mov    %rbp,0x10(%rsp)                

  0x000000010e6c9a2c: xor    %edx,%esi
  0x000000010e6c9a2e: mov    %esi,%eax
  0x000000010e6c9a30: and    $0x1,%eax
  0x000000010e6c9a33: add    $0x10,%rsp
  0x000000010e6c9a37: pop    %rbp

它看起来和编译器生成的C1几乎一样

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有 Java 编程相关的问题?

（p^q）和（p！=q）对于布尔人有什么有用的区别吗？

共 (1) 个答案

# 1 楼答案