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
