Store hash and mark bit in one field

The JVM and Native Image do not know that we only need 22 bits for the
hash, so the additional field for the mark bit unnecessarily increased
the object size across TruffleSqueak's object model.

src/de.hpi.swa.trufflesqueak/src/de/hpi/swa/trufflesqueak/model/AbstractSqueakObjectWithClassAndHash.java

@@ -13,6 +13,7 @@
 import de.hpi.swa.trufflesqueak.exceptions.SqueakExceptions;
 import de.hpi.swa.trufflesqueak.image.SqueakImageChunk;
 import de.hpi.swa.trufflesqueak.image.SqueakImageConstants;
+import de.hpi.swa.trufflesqueak.image.SqueakImageConstants.ObjectHeader;
 import de.hpi.swa.trufflesqueak.image.SqueakImageContext;
 import de.hpi.swa.trufflesqueak.image.SqueakImageWriter;
 import de.hpi.swa.trufflesqueak.interop.LookupMethodByStringNode;
@@ -21,18 +22,26 @@
 import de.hpi.swa.trufflesqueak.util.ObjectGraphUtils.ObjectTracer;
 
 public abstract class AbstractSqueakObjectWithClassAndHash extends AbstractSqueakObject {
-    public static final int IDENTITY_HASH_MASK = 0x400000 - 1;
+    public static final int SQUEAK_HASH_MASK = (1 << 22) - 1;
+    public static final int MARK_BIT = 1 << 24;
     /* Generate new hash if hash is 0 (see SpurMemoryManager>>#hashBitsOf:). */
     public static final int HASH_UNINITIALIZED = 0;
 
-    private int squeakHash;
+    /**
+     * Spur uses an 64-bit object header (see {@link ObjectHeader}). In TruffleSqueak, we only care
+     * about the hash, the class, and a few bits (e.g., isImmutable). Instead of storing the
+     * original object header, we directly reference the class, which avoids additional class table
+     * lookups. The 22-bit hash is stored in an {@code int} field, the remaining 10 bits are more
+     * than enough to encode additional information (e.g., marking state for {@code #allInstances}
+     * et al.). The JVM and GraalVM Native Image compress pointers by default, so these two fields
+     * can be represented by just one 64-bit word.
+     */
     private ClassObject squeakClass;
-    private boolean markingFlag;
+    private int squeahHashAndBits;
 
     // For special/well-known objects only.
     protected AbstractSqueakObjectWithClassAndHash(final SqueakImageContext image) {
-        squeakHash = HASH_UNINITIALIZED;
-        markingFlag = image.getCurrentMarkingFlag();
+        this(image, null);
     }
 
     protected AbstractSqueakObjectWithClassAndHash(final SqueakImageContext image, final int hash, final ClassObject klass) {
@@ -45,15 +54,17 @@ protected AbstractSqueakObjectWithClassAndHash(final SqueakImageContext image, f
     }
 
     private AbstractSqueakObjectWithClassAndHash(final boolean markingFlag, final int hash, final ClassObject klass) {
-        squeakHash = hash;
+        squeahHashAndBits = hash;
         squeakClass = klass;
-        this.markingFlag = markingFlag;
+        if (markingFlag) {
+            toggleMarkingFlag();
+        }
     }
 
     protected AbstractSqueakObjectWithClassAndHash(final AbstractSqueakObjectWithClassAndHash original) {
-        squeakHash = HASH_UNINITIALIZED;
+        squeahHashAndBits = original.squeahHashAndBits;
+        setSqueakHash(HASH_UNINITIALIZED);
         squeakClass = original.squeakClass;
-        markingFlag = original.markingFlag;
     }
 
     @Override
@@ -94,34 +105,47 @@ public final boolean hasFormatOf(final ClassObject other) {
     public final long getSqueakHash() {
         if (needsSqueakHash()) {
             /** Lazily initialize squeakHash and derive value from hashCode. */
-            squeakHash = System.identityHashCode(this) & IDENTITY_HASH_MASK;
+            initializeSqueakHash();
         }
-        return squeakHash;
+        return getSqueakHashValue();
     }
 
     public final long getSqueakHash(final BranchProfile needsHashProfile) {
         if (needsSqueakHash()) {
             /** Lazily initialize squeakHash and derive value from hashCode. */
             needsHashProfile.enter();
-            squeakHash = System.identityHashCode(this) & IDENTITY_HASH_MASK;
+            initializeSqueakHash();
         }
-        return squeakHash;
+        return getSqueakHashValue();
+    }
+
+    private long getSqueakHashValue() {
+        return squeahHashAndBits & SQUEAK_HASH_MASK;
     }
 
     public final boolean needsSqueakHash() {
-        return squeakHash == HASH_UNINITIALIZED;
+        return (squeahHashAndBits & SQUEAK_HASH_MASK) == HASH_UNINITIALIZED;
+    }
+
+    private void initializeSqueakHash() {
+        setSqueakHash(System.identityHashCode(this) & SQUEAK_HASH_MASK);
     }
 
     public final void setSqueakHash(final int newHash) {
-        squeakHash = newHash;
+        assert newHash <= SQUEAK_HASH_MASK;
+        squeahHashAndBits = (squeahHashAndBits & ~SQUEAK_HASH_MASK) + newHash;
     }
 
     public final boolean getMarkingFlag() {
-        return markingFlag;
+        return (squeahHashAndBits & MARK_BIT) != 0;
+    }
+
+    private void toggleMarkingFlag() {
+        squeahHashAndBits ^= MARK_BIT;
     }
 
     public final boolean isMarked(final boolean currentMarkingFlag) {
-        return markingFlag == currentMarkingFlag;
+        return getMarkingFlag() == currentMarkingFlag;
     }
 
     @Override
@@ -152,10 +176,10 @@ public final Object send(final SqueakImageContext image, final String selector,
      * @return <tt>false</tt> if already marked, <tt>true</tt> otherwise
      */
     public final boolean tryToMark(final boolean currentMarkingFlag) {
-        if (markingFlag == currentMarkingFlag) {
+        if (getMarkingFlag() == currentMarkingFlag) {
             return false;
         } else {
-            markingFlag = currentMarkingFlag;
+            toggleMarkingFlag();
             return true;
         }
     }

src/de.hpi.swa.trufflesqueak/src/de/hpi/swa/trufflesqueak/model/ClassObject.java

@@ -87,7 +87,6 @@ public SqueakImageContext getImage() {
 
     public long rehashForClassTable(final SqueakImageContext i) {
         final int newHash = i.getNextClassHash();
-        assert newHash < IDENTITY_HASH_MASK;
         setSqueakHash(newHash);
         return newHash;
     }

src/de.hpi.swa.trufflesqueak/src/de/hpi/swa/trufflesqueak/nodes/primitives/impl/MiscellaneousPrimitives.java

@@ -726,7 +726,7 @@ protected static final AbstractSqueakObject doCopy(final AbstractSqueakObjectWit
     public static final class PrimMaxIdentityHashNode extends AbstractSingletonPrimitiveNode {
         @Override
         public Object execute() {
-            return (long) AbstractSqueakObjectWithClassAndHash.IDENTITY_HASH_MASK;
+            return (long) AbstractSqueakObjectWithClassAndHash.SQUEAK_HASH_MASK;
         }
     }