RFR: 8266622: Optimize Class.descriptorString() and Class.getCanonicalName0()

[Сергей Цыпанов]

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
-Xint
                                               Mode     Score     Error   Units
descriptorString_long                          avgt  6326.478 ± 107.251   ns/op
descriptorString_short                         avgt  5220.729 ± 103.545   ns/op
descriptorString_long:·gc.alloc.rate.norm      avgt   528.089 ±   0.021    B/op
descriptorString_short:·gc.alloc.rate.norm     avgt   232.036 ±   0.015    B/op

-XX:TieredStopAtLevel=1
                                               Mode      Score    Error   Units
descriptorString_long                          avgt    230.223 ±  1.254   ns/op
descriptorString_short                         avgt    164.255 ±  0.755   ns/op
descriptorString_long:·gc.alloc.rate.norm      avgt    528.046 ±  0.002    B/op
descriptorString_short:·gc.alloc.rate.norm     avgt    232.022 ±  0.001    B/op

full
                                               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
-Xint
                                               Mode      Score     Error   Units
descriptorString_long                          avgt   4485.994 ±  60.173   ns/op
descriptorString_short                         avgt   3949.965 ± 278.143   ns/op
descriptorString_long:·gc.alloc.rate.norm      avgt    336.051 ±   0.004    B/op
descriptorString_short:·gc.alloc.rate.norm     avgt    184.027 ±   0.010    B/op

-XX:TieredStopAtLevel=1
                                               Mode        Score    Error   Units
descriptorString_long                          avgt      185.774 ±  1.100   ns/op
descriptorString_short                         avgt      135.338 ±  1.066   ns/op
descriptorString_long:·gc.alloc.rate.norm      avgt      336.030 ±  0.001    B/op
descriptorString_short:·gc.alloc.rate.norm     avgt      184.019 ±  0.001    B/op

full
                                               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()

-------------

Commit messages:
 - 8266622: Optimize Class.descriptorString() and Class.getCanonicalName0()

Changes: https://git.openjdk.java.net/jdk/pull/3903/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3903&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8266622
  Stats: 19 lines in 1 file changed: 15 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3903.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3903/head:pull/3903

PR: https://git.openjdk.java.net/jdk/pull/3903