StringBuilder buffer allocation support in compiler

[Vitaly Davidovich]

Hi Laszlo,

I believe server jit compiler has an optimization pass that will attempt at
fusing this type of pattern into one string allocation -- see
src/share/vm/opto/stringopts.[h|c]pp in the sources.  Have you tried
benchmarking this after jit compiler gets a crack at it?


On Thu, Jun 5, 2014 at 3:27 PM, Laszlo Hornyak <laszlo.hornyak at gmail.com>
wrote:

> 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
>
>
> --
>
> EOF
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.openjdk.java.net/pipermail/compiler-dev/attachments/20140605/2f78a9ef/attachment.html>