optimizations when translating java source code to bytecode

Question

Are there any optimizations, such as dead code elimination, involved when translating java source files to bytecodes?

One cool one it does (according to Josh Bloch, not that he'd know ;-) is that n*31 is turned into (n << 5) - n. — corsiKa
– corsiKa, Commented Feb 17, 2011 at 5:10

Stephen C · Accepted Answer · 2011-02-17 04:02:56Z

2

The standard Java compilers do few optimizations on the emitted bytecodes. I think that the reasoning is that unoptimized bytecodes will be easier for the HotSpot JIT compiler to optimize.

The links that @Mitch Wheat provided in comments above (particularly the 2nd one) date from the days when HotSpot JIT was new technology.

answered Feb 17, 2011 at 4:02

Stephen C

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Shambhu · Accepted Answer · 2016-03-05 07:58:25Z

While searching all source code optimization, i came across this question and though I am answering after long time this question asked.

After jdk1.7, String concatenation using plus [+] operator converted to StringBuilder append e.g

public static void main(String[] args) {
String s = new String("");
s = s+"new";
}

Converted to StringBuilder append as shown in bytecode

public static void main(java.lang.String[]);
    descriptor: ([Ljava/lang/String;)V
    flags: ACC_PUBLIC, ACC_STATIC
    Code:
      stack=3, locals=2, args_size=1
         0: new           #2                  // class java/lang/String
         3: dup
         4: ldc           #3                  // String
         6: invokespecial #4                  // Method java/lang/String."<init>":(Ljava/lang/String;)V
         9: astore_1
        10: new           #5                  // class java/lang/StringBuilder
        13: dup
        14: invokespecial #6                  // Method java/lang/StringBuilder."<init>":()V
        17: aload_1
        18: invokevirtual #7                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
        21: ldc           #8                  // String new
        23: invokevirtual #7                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
        26: invokevirtual #9                  // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
        29: astore_1
        30: return
      LineNumberTable:
        line 13: 0
        line 14: 10
        line 15: 30
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0      31     0  args   [Ljava/lang/String;
           10      21     1     s   Ljava/lang/String;
}

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