Minor cleanups and documentation changes.

Author: christianhaeubl

substratevm/src/com.oracle.svm.core.genscavenge/src/com/oracle/svm/core/genscavenge/ObjectHeaderImpl.java

@@ -90,7 +90,7 @@ public final class ObjectHeaderImpl extends ObjectHeader {
         numAlignmentBits = CodeUtil.log2(ConfigurationValues.getObjectLayout().getAlignment());
         int numMinimumReservedBits = 3;
         VMError.guarantee(numMinimumReservedBits <= numAlignmentBits, "Minimum set of reserved bits must be provided by object alignment");
-        if (isIdentityHasFieldOptional()) {
+        if (isIdentityHashFieldOptional()) {
             VMError.guarantee(ReferenceAccess.singleton().haveCompressedReferences(), "Ensures hubs (at the start of the image heap) remain addressable");
             numReservedBits = numMinimumReservedBits + 2;
             VMError.guarantee(numReservedBits <= numAlignmentBits || hasShift(),
@@ -160,9 +160,9 @@ public void initializeHeaderOfNewObject(Pointer objectPointer, Word encodedHub,
     @Override
     public boolean hasOptionalIdentityHashField(Word header) {
         if (GraalDirectives.inIntrinsic()) {
-            ReplacementsUtil.staticAssert(isIdentityHasFieldOptional(), "use only when hashcode fields are optional");
+            ReplacementsUtil.staticAssert(isIdentityHashFieldOptional(), "use only when hashcode fields are optional");
         } else {
-            VMError.guarantee(isIdentityHasFieldOptional(), "use only when hashcode fields are optional");
+            VMError.guarantee(isIdentityHashFieldOptional(), "use only when hashcode fields are optional");
         }
 
         UnsignedWord inFieldState = IDHASH_STATE_IN_FIELD.shiftLeft(IDHASH_STATE_SHIFT);
@@ -172,7 +172,7 @@ public boolean hasOptionalIdentityHashField(Word header) {
     @Uninterruptible(reason = "Called from uninterruptible code.", mayBeInlined = true)
     void setIdentityHashInField(Object o) {
         assert VMOperation.isGCInProgress();
-        VMError.guarantee(isIdentityHasFieldOptional());
+        VMError.guarantee(isIdentityHashFieldOptional());
         UnsignedWord oldHeader = readHeaderFromObject(o);
         UnsignedWord inFieldState = IDHASH_STATE_IN_FIELD.shiftLeft(IDHASH_STATE_SHIFT);
         UnsignedWord newHeader = oldHeader.and(IDHASH_STATE_BITS.not()).or(inFieldState);
@@ -198,9 +198,9 @@ void setIdentityHashInField(Object o) {
     @Override
     public void setIdentityHashFromAddress(Pointer ptr, Word currentHeader) {
         if (GraalDirectives.inIntrinsic()) {
-            ReplacementsUtil.staticAssert(isIdentityHasFieldOptional(), "use only when hashcode fields are optional");
+            ReplacementsUtil.staticAssert(isIdentityHashFieldOptional(), "use only when hashcode fields are optional");
         } else {
-            assert isIdentityHasFieldOptional() : "use only when hashcode fields are optional";
+            assert isIdentityHashFieldOptional() : "use only when hashcode fields are optional";
             assert !hasIdentityHashFromAddress(currentHeader) : "must not already have a hashcode";
         }
 
@@ -222,9 +222,9 @@ public boolean hasIdentityHashFromAddress(Word header) {
     @Uninterruptible(reason = "Called from uninterruptible code.", mayBeInlined = true)
     static boolean hasIdentityHashFromAddressInline(Word header) {
         if (GraalDirectives.inIntrinsic()) {
-            ReplacementsUtil.staticAssert(isIdentityHasFieldOptional(), "use only when hashcode fields are optional");
+            ReplacementsUtil.staticAssert(isIdentityHashFieldOptional(), "use only when hashcode fields are optional");
         } else {
-            assert isIdentityHasFieldOptional();
+            assert isIdentityHashFieldOptional();
         }
 
         UnsignedWord fromAddressState = IDHASH_STATE_FROM_ADDRESS.shiftLeft(IDHASH_STATE_SHIFT);
@@ -317,7 +317,7 @@ public long encodeAsImageHeapObjectHeader(ImageHeapObject obj, long hubOffsetFro
                 assert obj.getPartition() instanceof FillerObjectDummyPartition;
             }
         }
-        if (isIdentityHasFieldOptional()) {
+        if (isIdentityHashFieldOptional()) {
             header |= (IDHASH_STATE_IN_FIELD.rawValue() << IDHASH_STATE_SHIFT);
         }
         return header;
@@ -435,7 +435,7 @@ static boolean hasShift() {
     }
 
     @Fold
-    static boolean isIdentityHasFieldOptional() {
+    static boolean isIdentityHashFieldOptional() {
         return ConfigurationValues.getObjectLayout().isIdentityHashFieldOptional();
     }
 }

substratevm/src/com.oracle.svm.core/src/com/oracle/svm/core/config/ObjectLayout.java

@@ -49,8 +49,10 @@
  * {@link #isIdentityHashFieldInObjectHeader()}).</li>
  * <li>At a type specific offset, potentially outside the object header (see
  * {@link #isIdentityHashFieldAtTypeSpecificOffset()}).</li>
- * <li>Outside the object header, at a type and object state specific offset (see
- * {@link #isIdentityHashFieldOptional()}).</li>
+ * <li>Outside the object header, at a type- or object-specific offset (see
+ * {@link #isIdentityHashFieldOptional()}). Note that the field is not part of every object. When an
+ * object needs the field, the object is resized during garbage collection to accommodate the
+ * field.</li>
  * </ol>
  */
 public final class ObjectLayout {
@@ -72,7 +74,6 @@ public ObjectLayout(SubstrateTargetDescription target, int referenceSize, int ob
         assert CodeUtil.isPowerOf2(objectAlignment) : objectAlignment;
         assert arrayLengthOffset % Integer.BYTES == 0;
         assert hubOffset < firstFieldOffset && hubOffset < arrayLengthOffset : hubOffset;
-        assert identityHashMode == IdentityHashMode.OBJECT_HEADER || identityHashMode == IdentityHashMode.TYPE_SPECIFIC || identityHashMode == IdentityHashMode.OPTIONAL;
         assert (identityHashMode != IdentityHashMode.OPTIONAL && headerIdentityHashOffset > 0 && headerIdentityHashOffset < arrayLengthOffset && headerIdentityHashOffset % Integer.BYTES == 0) ||
                         (identityHashMode == IdentityHashMode.OPTIONAL && headerIdentityHashOffset == -1);
 
@@ -257,7 +258,7 @@ public enum IdentityHashMode {
         OBJECT_HEADER(0),
         /* At a type-specific offset (potentially outside the object header). */
         TYPE_SPECIFIC(1),
-        /* At a type and object-state specific offset (outside the object header). */
+        /* At a type- or object-specific offset (outside the object header). */
         OPTIONAL(2);
 
         final int value;

substratevm/src/com.oracle.svm.core/src/com/oracle/svm/core/hub/DynamicHub.java

@@ -632,7 +632,7 @@ public int getMonitorOffset() {
     /**
      * If possible, use {@link LayoutEncoding#getIdentityHashOffset(Object)} instead. If the hash
      * code field is optional, note that this method may return an offset that is outside the bounds
-     * of a freshly the object.
+     * of a newly allocated object.
      */
     @Uninterruptible(reason = "Called from uninterruptible code.", mayBeInlined = true)
     public int getIdentityHashOffset() {