RFR: 8254078: DataOutputStream is very slow post-disabling of Biased Locking

[Andrew Haley]

On Wed, 7 Oct 2020 14:06:27 GMT, Roger Riggs <rriggs at openjdk.org> wrote:

> There is a pre-existing race condition on use of the writeBuffer.
> It is allocated per DataOutputStream but is unsynchronized.
> Only the write(byte) and write(byte[], off, len) methods are synchronized.
> Will that be/become a problem?

Hard to say. There isn't a guarantee in the specification. After my patch, writeInt is

    public final void writeInt(int v) throws IOException {
        writeBuffer[0] = (byte)(v >>> 24);
        writeBuffer[1] = (byte)(v >>> 16);
        writeBuffer[2] = (byte)(v >>>  8);
        writeBuffer[3] = (byte)(v >>>  0);
        out.write(writeBuffer, 0, 4);
        incCount(4);
    }

whereas writeLong() is
    public final void writeLong(long v) throws IOException {
        writeBuffer[0] = (byte)(v >>> 56);
        writeBuffer[1] = (byte)(v >>> 48);
        writeBuffer[2] = (byte)(v >>> 40);
        writeBuffer[3] = (byte)(v >>> 32);
        writeBuffer[4] = (byte)(v >>> 24);
        writeBuffer[5] = (byte)(v >>> 16);
        writeBuffer[6] = (byte)(v >>>  8);
        writeBuffer[7] = (byte)(v >>>  0);
        out.write(writeBuffer, 0, 8);
        incCount(8);
    }

I know there's a history of Java programmers writing to the implementation, not the specification, but ISTM that if
writeLong() can be written this way so can writeInt().

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PR: https://git.openjdk.java.net/jdk/pull/542