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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.genscavenge;

import org.graalvm.nativeimage.Platform;
import org.graalvm.nativeimage.Platforms;
import org.graalvm.word.Pointer;
import org.graalvm.word.WordFactory;

import com.oracle.svm.core.annotate.AlwaysInline;
import com.oracle.svm.core.annotate.NeverInline;
import com.oracle.svm.core.log.Log;
import com.oracle.svm.core.util.VMError;

Apply an ObjectVisitor to all the new Objects in a Space since a snapshot.
This knows that allocations take place from the last HeapChunks of the Space. And it knows (too
much about) that AlignedChunks have a top pointer.
/**
 * Apply an ObjectVisitor to all the new Objects in a Space since a snapshot.
 *
 * This knows that allocations take place from the last HeapChunks of the Space. And it knows (too
 * much about) that AlignedChunks have a top pointer.
 */
final class GreyObjectsWalker {
    
The Space that is being captured. /** The Space that is being captured. */
    private Space space;

    /* The top of the Space during capture. */
    private AlignedHeapChunk.AlignedHeader alignedHeapChunk;
    private Pointer alignedTop;
    private UnalignedHeapChunk.UnalignedHeader unalignedHeapChunk;

    @Platforms(Platform.HOSTED_ONLY.class)
    GreyObjectsWalker() {
    }

    
Take a snapshot of a Space, such that all Objects in the Space are now black, and any new
Objects in the Space will be grey, and can have an ObjectVisitor applied to them.
/**
     * Take a snapshot of a Space, such that all Objects in the Space are now black, and any new
     * Objects in the Space will be grey, and can have an ObjectVisitor applied to them.
     */
    void setScanStart(Space s) {
        Log trace = Log.noopLog().string("[Space.GreyObjectsWalker.setScanStart:").string("  s: ").string(s.getName());
        space = s;
        AlignedHeapChunk.AlignedHeader aChunk = s.getLastAlignedHeapChunk();
        alignedHeapChunk = aChunk;
        trace.string("  alignedHeapChunk: ").hex(alignedHeapChunk).string("  isNull: ").bool(aChunk.isNull());
        alignedTop = (aChunk.isNonNull() ? HeapChunk.getTopPointer(aChunk) : WordFactory.nullPointer());
        trace.string("  alignedTop: ").hex(alignedTop);
        unalignedHeapChunk = s.getLastUnalignedHeapChunk();
        trace.string("  unalignedChunkPointer: ").hex(unalignedHeapChunk).string("]").newline();
    }

    
Compare the snapshot to the current state of the Space to see if there are grey Objects. /** Compare the snapshot to the current state of the Space to see if there are grey Objects. */
    @AlwaysInline("GC performance")
    boolean haveGreyObjects() {
        return alignedHeapChunk.notEqual(space.getLastAlignedHeapChunk()) || alignedHeapChunk.isNonNull() && alignedTop.notEqual(HeapChunk.getTopPointer(alignedHeapChunk)) ||
                        unalignedHeapChunk.notEqual(space.getLastUnalignedHeapChunk());
    }

    @NeverInline("Split the GC into reasonable compilation units")
    void walkGreyObjects() {
        while (haveGreyObjects()) {
            walkAlignedGreyObjects();
            walkUnalignedGreyObjects();
        }
    }

    @AlwaysInline("GC performance")
    private void walkAlignedGreyObjects() {
        AlignedHeapChunk.AlignedHeader aChunk;
        if (alignedHeapChunk.isNull() && alignedTop.isNull()) {
            /* If the snapshot is empty, then I have to walk from the beginning of the Space. */
            aChunk = space.getFirstAlignedHeapChunk();
        } else {
            /* Otherwise walk Objects that arrived after the snapshot. */
            aChunk = alignedHeapChunk;
        }
        /* Visit Objects in the AlignedChunks. */
        GreyToBlackObjectVisitor visitor = GCImpl.getGCImpl().getGreyToBlackObjectVisitor();
        if (aChunk.isNonNull()) {
            AlignedHeapChunk.AlignedHeader lastChunk;
            do {
                lastChunk = aChunk;
                if (!AlignedHeapChunk.walkObjectsInline(aChunk, visitor)) {
                    throw VMError.shouldNotReachHere();
                }
                aChunk = HeapChunk.getNext(aChunk);
            } while (aChunk.isNonNull());

            /* Move the scan point. */
            alignedHeapChunk = lastChunk;
            alignedTop = HeapChunk.getTopPointer(lastChunk);
        }
    }

    @AlwaysInline("GC performance")
    private void walkUnalignedGreyObjects() {
        /* Visit the Objects in the UnalignedChunk after the snapshot UnalignedChunk. */
        UnalignedHeapChunk.UnalignedHeader uChunk;
        if (unalignedHeapChunk.isNull()) {
            uChunk = space.getFirstUnalignedHeapChunk();
        } else {
            uChunk = HeapChunk.getNext(unalignedHeapChunk);
        }
        GreyToBlackObjectVisitor visitor = GCImpl.getGCImpl().getGreyToBlackObjectVisitor();
        if (uChunk.isNonNull()) {
            UnalignedHeapChunk.UnalignedHeader lastChunk;
            do {
                lastChunk = uChunk;
                if (!UnalignedHeapChunk.walkObjectsInline(uChunk, visitor)) {
                    throw VMError.shouldNotReachHere();
                }
                uChunk = HeapChunk.getNext(uChunk);
            } while (uChunk.isNonNull());

            /* Move the scan point. */
            unalignedHeapChunk = lastChunk;
        }
    }
}