RFR: 8352316: More MergeStoreBench

[Shaojin Wen]

On Wed, 19 Mar 2025 03:28:59 GMT, Shaojin Wen <swen at openjdk.org> wrote:

> Added performance tests related to String.getBytes/String.getChars/StringBuilder.append/System.arraycopy in constant scenarios to verify whether MergeStore works

I'm a developer of fastjson2. According to third-party benchmarks from https://github.com/fabienrenaud/java-json-benchmark, our library demonstrates the best performance. I would like to contribute some of these optimization techniques to OpenJDK, ideally by having C2 (the JIT compiler) directly support them.

Below is an example related to this PR. We have a JavaBean that needs to be serialized to a JSON string:


* JavaBean

class Bean {
	public int value;
}


* Target JSON Output

{"value":123}


* CodeGen-Generated JSONSerializer
fastjson2 uses ASM to generate a serializer class like the following. The methods writeNameValue0, writeNameValue1, and writeNameValue2 are candidate implementations. Among them, writeNameValue2 is the fastest when the field name length is 8, as it leverages UNSAFE.putLong for direct memory operations:


class BeanJSONSerializer {
	private static final String name = ""value":";
	private static final byte[] nameBytes = name.getBytes();
	private satic final long nameLong = UNSAFE.getLong(nameBytes, ARRAY_BYTE_BASE_OFFSET);	

	int writeNameValue0(byte[] bytes, int off, int value) {
		name.getBytes(0, 8, bytes, off);
		off += 8;
		return writeInt32(bytes, off, value);
	}

	int writeNameValue1(byte[] bytes, int off, int value) {
		System.arraycopy(nameBytes, 0, bytes, off, 8);
		off += 8;
		return writeInt32(bytes, off, value);
	}


	int writeNameValue2(byte[] bytes, int off, int value) {
		UNSAFE.putLong(bytes, ARRAY_BYTE_BASE_OFFSET + off, nameLong);
		off += 8;
		return writeInt32(bytes, off, value);
	}
}


We propose that the C2 compiler could optimize cases where the field name length is 4 or 8 bytes by automatically using direct memory operations similar to writeNameValue2. This would eliminate the need for manual unsafe operations in user code and improve serialization performance for common patterns.

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PR Comment: https://git.openjdk.org/jdk/pull/24108#issuecomment-2753046348