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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * 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).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
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package com.oracle.svm.core.genscavenge;

import java.util.ArrayList;
import java.util.List;

import org.graalvm.compiler.core.common.NumUtil;
import org.graalvm.word.UnsignedWord;
import org.graalvm.word.WordFactory;

import com.oracle.svm.core.config.ConfigurationValues;
import com.oracle.svm.core.image.ImageHeap;
import com.oracle.svm.core.image.ImageHeapObject;
import com.oracle.svm.core.image.ImageHeapPartition;
import com.oracle.svm.core.util.UnsignedUtils;
import com.oracle.svm.core.util.VMError;

class ChunkedImageHeapAllocator {
    
A pseudo-partition for filler objects, see FillerObjectDummyPartition. /** A pseudo-partition for filler objects, see {@link FillerObjectDummyPartition}. */
    private static final ImageHeapPartition FILLERS_DUMMY_PARTITION = new FillerObjectDummyPartition();

    abstract static class Chunk {
        private final long begin;
        private final long endOffset;
        private boolean writable;

        Chunk(long begin, long endOffset, boolean writable) {
            this.begin = begin;
            this.endOffset = endOffset;
            this.writable = writable;
        }

        public long getBegin() {
            return begin;
        }

        public long getEnd() {
            return begin + endOffset;
        }

        public long getEndOffset() {
            return endOffset;
        }

        public abstract long getTopOffset();

        public void setWritable() {
            writable = true;
        }

        public boolean isWritable() {
            return writable;
        }
    }

    static final class UnalignedChunk extends Chunk {
        UnalignedChunk(long begin, long endOffset, boolean writable) {
            super(begin, endOffset, writable);
        }

        @Override
        public long getTopOffset() {
            return getEndOffset();
        }
    }

    final class AlignedChunk extends Chunk {
        private final List<ImageHeapObject> objects = new ArrayList<>();
        private long topOffset = alignedChunkObjectsOffset;

        AlignedChunk(long begin) {
            super(begin, alignedChunkSize, false);
        }

        public long allocate(ImageHeapObject obj, boolean writable) {
            long size = obj.getSize();
            VMError.guarantee(size <= getUnallocatedBytes(), "Object of size " + size + " does not fit in the chunk's remaining bytes");
            long objStart = getTop();
            topOffset += size;
            objects.add(obj);
            if (writable) {
                // Writable objects need a writable card table, so make the entire chunk writable
                setWritable();
            }
            return objStart;
        }

        public boolean tryAlignTop(int multiple) {
            long padding = computePadding(getTop(), multiple);
            if (padding != 0 && padding < minimumObjectSize) {
                /*
                 * Cannot fit a filler object into the remaining padding space, so need to go up to
                 * the next alignment multiple.
                 */
                padding = padding + multiple;
            }
            if (padding > getUnallocatedBytes()) {
                return false;
            }
            allocateFiller(padding);
            assert getTop() % multiple == 0;
            return true;
        }

        private void allocateFiller(long size) {
            if (size != 0) {
                ImageHeapObject filler = imageHeap.addFillerObject(NumUtil.safeToInt(size));
                if (filler == null) {
                    throw VMError.shouldNotReachHere("Failed to leave enough space for a filler object: " + size + " byte remaining");
                }
                filler.setHeapPartition(FILLERS_DUMMY_PARTITION);
                long location = allocate(filler, false);
                filler.setOffsetInPartition(location);
            }
        }

        public List<ImageHeapObject> getObjects() {
            return objects;
        }

        @Override
        public long getTopOffset() {
            return topOffset;
        }

        public long getTop() {
            return getBegin() + topOffset;
        }

        public long getUnallocatedBytes() {
            return getEndOffset() - topOffset;
        }
    }

    private final ImageHeap imageHeap;
    private final int alignedChunkSize;
    private final int alignedChunkAlignment;
    private final int alignedChunkObjectsOffset;
    private final int unalignedChunkObjectsOffset;

    private long position;

    private final ArrayList<UnalignedChunk> unalignedChunks = new ArrayList<>();
    private final ArrayList<AlignedChunk> alignedChunks = new ArrayList<>();
    private AlignedChunk currentAlignedChunk;

    final int minimumObjectSize;

    ChunkedImageHeapAllocator(ImageHeap imageHeap, long position) {
        this.imageHeap = imageHeap;
        this.alignedChunkSize = UnsignedUtils.safeToInt(HeapPolicy.getAlignedHeapChunkSize());
        this.alignedChunkAlignment = UnsignedUtils.safeToInt(HeapPolicy.getAlignedHeapChunkAlignment());
        this.alignedChunkObjectsOffset = UnsignedUtils.safeToInt(AlignedHeapChunk.getObjectsStartOffset());
        this.unalignedChunkObjectsOffset = UnsignedUtils.safeToInt(UnalignedHeapChunk.getObjectStartOffset());

        this.position = position;

        /* Cache to prevent frequent lookups of the object layout from ImageSingletons. */
        minimumObjectSize = ConfigurationValues.getObjectLayout().getMinimumObjectSize();
    }

    public long getPosition() {
        return (currentAlignedChunk != null) ? currentAlignedChunk.getTop() : position;
    }

    public void alignBetweenChunks(int multiple) {
        assert currentAlignedChunk == null;
        allocateRaw(computePadding(position, multiple));
    }

    public long allocateUnalignedChunkForObject(ImageHeapObject obj, boolean writable) {
        assert currentAlignedChunk == null;
        UnsignedWord objSize = WordFactory.unsigned(obj.getSize());
        long chunkSize = UnalignedHeapChunk.getChunkSizeForObject(objSize).rawValue();
        long chunkBegin = allocateRaw(chunkSize);
        unalignedChunks.add(new UnalignedChunk(chunkBegin, chunkSize, writable));
        return chunkBegin + unalignedChunkObjectsOffset;
    }

    public void maybeStartAlignedChunk() {
        if (currentAlignedChunk == null) {
            startNewAlignedChunk();
        }
    }

    public void startNewAlignedChunk() {
        finishAlignedChunk();
        alignBetweenChunks(alignedChunkAlignment);
        long chunkBegin = allocateRaw(alignedChunkSize);
        currentAlignedChunk = new AlignedChunk(chunkBegin);
        alignedChunks.add(currentAlignedChunk);
    }

    public long getRemainingBytesInAlignedChunk() {
        return currentAlignedChunk.getUnallocatedBytes();
    }

    public long allocateObjectInAlignedChunk(ImageHeapObject obj, boolean writable) {
        return currentAlignedChunk.allocate(obj, writable);
    }

    public void alignInAlignedChunk(int multiple) {
        if (!currentAlignedChunk.tryAlignTop(multiple)) {
            startNewAlignedChunk();
            boolean aligned = currentAlignedChunk.tryAlignTop(multiple);
            VMError.guarantee(aligned, "Cannot align to " + multiple + " bytes within an aligned chunk's object area");
        }
    }

    public void finishAlignedChunk() {
        currentAlignedChunk = null;
    }

    public List<AlignedChunk> getAlignedChunks() {
        return alignedChunks;
    }

    public List<UnalignedChunk> getUnalignedChunks() {
        return unalignedChunks;
    }

    private long allocateRaw(long size) {
        assert currentAlignedChunk == null;
        long begin = position;
        position += size;
        return begin;
    }

    private static long computePadding(long offset, int alignment) {
        long remainder = offset % alignment;
        return (remainder == 0) ? 0 : (alignment - remainder);
    }
}

A pseudo-partition for filler objects, which does not really exist at runtime, in any statistics, or otherwise. Necessary because like all other ImageHeapObjects, filler objects must be assigned to a partition, the start offset of which is used to compute their absolute locations. 

For filler objects in the middle of a partition (between genuine objects of that partition), it
would be acceptable to assign them to their enclosing partition. However, filler objects that are
inserted between partitions for alignment purposes are problematic because if they were assigned
to either partition, they would either be out of the partition's boundaries, or they would change
those boundaries, which would make them useless because that's exactly why they are needed there.
/**
 * A pseudo-partition for filler objects, which does not really exist at runtime, in any statistics,
 * or otherwise. Necessary because like all other {@link ImageHeapObject}s, filler objects must be
 * assigned to a partition, the start offset of which is used to compute their absolute locations.
 * <p>
 * For filler objects in the middle of a partition (between genuine objects of that partition), it
 * would be acceptable to assign them to their enclosing partition. However, filler objects that are
 * inserted between partitions for alignment purposes are problematic because if they were assigned
 * to either partition, they would either be out of the partition's boundaries, or they would change
 * those boundaries, which would make them useless because that's exactly why they are needed there.
 */
final class FillerObjectDummyPartition implements ImageHeapPartition {
    
Zero so that the partition-relative offsets of filler objects are always their absolute
locations.
/**
     * Zero so that the partition-relative offsets of filler objects are always their absolute
     * locations.
     */
    @Override
    public long getStartOffset() {
        return 0;
    }

    @Override
    public String getName() {
        throw VMError.shouldNotReachHere();
    }

    @Override
    public long getSize() {
        throw VMError.shouldNotReachHere();
    }
}