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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
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package com.oracle.svm.core.jfr.traceid;

import org.graalvm.compiler.api.replacements.Fold;
import org.graalvm.nativeimage.ImageSingletons;
import org.graalvm.nativeimage.Platform;
import org.graalvm.nativeimage.Platforms;

import com.oracle.svm.core.Uninterruptible;
import com.oracle.svm.core.thread.VMOperation;

/**
 * Class holding the current JFR epoch. JFR uses an epoch system to safely separate constant pool
 * entries between adjacent chunks. Used to get the current or previous epoch and switch from one
 * epoch to another across an uninterruptible safepoint operation.
 */
public class JfrTraceIdEpoch {
    private static final long EPOCH_0_BIT = 0b01;
    private static final long EPOCH_1_BIT = 0b10;

    private boolean epoch;

    @Fold
    public static JfrTraceIdEpoch getInstance() {
        return ImageSingletons.lookup(JfrTraceIdEpoch.class);
    }

    @Platforms(Platform.HOSTED_ONLY.class)
    public JfrTraceIdEpoch() {
    }

    @Uninterruptible(reason = "Called from uninterruptible code.", mayBeInlined = true)
    public void changeEpoch() {
        assert VMOperation.isInProgressAtSafepoint();
        epoch = !epoch;
    }

    @Uninterruptible(reason = "Called from uninterruptible code.", mayBeInlined = true)
    long thisEpochBit() {
        return epoch ? EPOCH_1_BIT : EPOCH_0_BIT;
    }

    @Uninterruptible(reason = "Called from uninterruptible code.", mayBeInlined = true)
    long previousEpochBit() {
        return epoch ? EPOCH_0_BIT : EPOCH_1_BIT;
    }

    @Uninterruptible(reason = "Called from uninterruptible code.", mayBeInlined = true)
    public boolean currentEpoch() {
        return epoch;
    }

    @Uninterruptible(reason = "Called from uninterruptible code.", mayBeInlined = true)
    public boolean previousEpoch() {
        return !epoch;
    }
}
