StringBuilder buffer allocation support in compiler

[Laszlo Hornyak]

Hi,

Given this sample code

    public static String sayHello(String name, int age) {
        return "Hello " + name + ", this is your " + age + "th birthday";
    }

The compiler will generate this bytecode:
       0: new           #11                 // class java/lang/StringBuilder
       3: dup
       4: invokespecial #12                 // Method
java/lang/StringBuilder."<init>":()V
       7: ldc           #19                 // String Hello
       9: invokevirtual #15                 // Method
java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      12: aload_0
      13: invokevirtual #15                 // Method
java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      16: ldc           #20                 // String , this is your
      18: invokevirtual #15                 // Method
java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      21: iload_1
      22: invokevirtual #13                 // Method
java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
      25: ldc           #21                 // String th birthday
      27: invokevirtual #15                 // Method
java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      30: invokevirtual #17                 // Method
java/lang/StringBuilder.toString:()Ljava/lang/String;
      33: areturn

The most interesting line is:
       4: invokespecial #12                 // Method
java/lang/StringBuilder."<init>":()V


Since all the string literals are given for the compiler, it could give an
estimation on the required buffer size to the StringBuilder constructor. In
this example the string literals already take 32 characters, plus a string
representation of an integer takes up to 12 characters. The String size is
0 to Integer.MAX_VALUE, but 16 might be a good default. In this case a
constructor call could be generated to allocate a buffer with 50 characters
and some (possibly all) buffer re-allocation could be saved in the
subsequent append calls.
The performance gain seems to be significant since the subsequent buffer
allocations can take up to 30-40 percent of the execution time.
Is there a specific reason for the java compiler not to support
pre-allocation of StringBuilder buffer based on string literals and
parameters?

Thank you,
Laszlo


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