Optimize Class.descriptorString() and Class.getCanonicalName0()

Hello, from discussion in https://github.com/openjdk/jdk/pull/3464 it appears, that in j.l.Class expressions like

String str = baseName.replace('.', '/') + '/' + name;

are not compiled into invokedynamic-based code, but into one using StringBuilder.

This happens due to some bootstraping issues. Currently the bytecode for the last (most often used) branch of Class.descriptorString() looks like

public sb()Ljava/lang/String;
   L0
    LINENUMBER 21 L0
    NEW java/lang/StringBuilder
    DUP
    INVOKESPECIAL java/lang/StringBuilder.<init> ()V
    ASTORE 1
   L1
    LINENUMBER 23 L1
    ALOAD 1
    LDC "a"
    INVOKEVIRTUAL java/lang/StringBuilder.append (Ljava/lang/String;)Ljava/lang/StringBuilder;
    POP
   L2
    LINENUMBER 24 L2
    ALOAD 1
    LDC "b"
    INVOKEVIRTUAL java/lang/StringBuilder.append (Ljava/lang/String;)Ljava/lang/StringBuilder;
    POP
   L3
    LINENUMBER 26 L3
    ALOAD 1
    INVOKEVIRTUAL java/lang/StringBuilder.toString ()Ljava/lang/String;
    ARETURN

Here the StringBuilder is created with default constructor and then expands if necessary while appending.

This can be improved by manually allocating StringBuilder of exact size. The benchmark demonstrates measurable improvement:

@State(Scope.Benchmark)
@BenchmarkMode(Mode.AverageTime)
@OutputTimeUnit(TimeUnit.NANOSECONDS)
@Fork(jvmArgsAppend = {"-Xms2g", "-Xmx2g"})
public class ClassDescriptorStringBenchmark {

    private final Class<?> clazzWithShortDescriptor = Object.class;
    private final Class<?> clazzWithLongDescriptor = getClass();

    @Benchmark
    public String descriptorString_short() {
        return clazzWithShortDescriptor.descriptorString();
    }

    @Benchmark
    public String descriptorString_long() {
        return clazzWithLongDescriptor.descriptorString();
    }
}

original
                                               Mode Score Error Units
descriptorString_long avgt 74.835 ± 0.262 ns/op
descriptorString_short avgt 43.822 ± 0.788 ns/op
descriptorString_long:·gc.alloc.rate.norm avgt 504.010 ± 0.001 B/op
descriptorString_short:·gc.alloc.rate.norm avgt 208.004 ± 0.001 B/op

patched
                                               Mode Score Error Units
descriptorString_long avgt 42.864 ± 0.160 ns/op
descriptorString_short avgt 27.255 ± 0.381 ns/op
descriptorString_long:·gc.alloc.rate.norm avgt 224.005 ± 0.001 B/op
descriptorString_short:·gc.alloc.rate.norm avgt 120.002 ± 0.001 B/op

Same can be done also for Class.isHidden() branch in Class.descriptorString() and for Class.getCanonicalName0()