From 983e2777d0b1574bcc5ef91fd962d61a25899c39 Mon Sep 17 00:00:00 2001
From: Peter Hofer <peter.hofer@oracle.com>
Date: Fri, 5 Jan 2024 18:18:18 +0100
Subject: [PATCH] Avoid needlessly rescanning objects.

---
 .../svm/core/genscavenge/AlignedHeapChunk.java     |  4 ++--
 .../svm/core/genscavenge/GreyObjectsWalker.java    |  6 +++++-
 .../com/oracle/svm/core/genscavenge/HeapChunk.java | 14 +++++++-------
 3 files changed, 14 insertions(+), 10 deletions(-)

diff --git a/substratevm/src/com.oracle.svm.core.genscavenge/src/com/oracle/svm/core/genscavenge/AlignedHeapChunk.java b/substratevm/src/com.oracle.svm.core.genscavenge/src/com/oracle/svm/core/genscavenge/AlignedHeapChunk.java
index e45c244257d0..8e45dbc58ced 100644
--- a/substratevm/src/com.oracle.svm.core.genscavenge/src/com/oracle/svm/core/genscavenge/AlignedHeapChunk.java
+++ b/substratevm/src/com.oracle.svm.core.genscavenge/src/com/oracle/svm/core/genscavenge/AlignedHeapChunk.java
@@ -149,8 +149,8 @@ static boolean walkObjects(AlignedHeader that, ObjectVisitor visitor) {
 
     @AlwaysInline("GC performance")
     @Uninterruptible(reason = "Called from uninterruptible code.", mayBeInlined = true)
-    static boolean walkObjectsInline(AlignedHeader that, ObjectVisitor visitor) {
-        return HeapChunk.walkObjectsFromInline(that, getObjectsStart(that), visitor);
+    static boolean walkObjectsFromInline(AlignedHeader that, Pointer start, ObjectVisitor visitor) {
+        return HeapChunk.walkObjectsFromInline(that, start, visitor);
     }
 
     @Fold
diff --git a/substratevm/src/com.oracle.svm.core.genscavenge/src/com/oracle/svm/core/genscavenge/GreyObjectsWalker.java b/substratevm/src/com.oracle.svm.core.genscavenge/src/com/oracle/svm/core/genscavenge/GreyObjectsWalker.java
index 09ad12fd3568..c0d70a40f52b 100644
--- a/substratevm/src/com.oracle.svm.core.genscavenge/src/com/oracle/svm/core/genscavenge/GreyObjectsWalker.java
+++ b/substratevm/src/com.oracle.svm.core.genscavenge/src/com/oracle/svm/core/genscavenge/GreyObjectsWalker.java
@@ -87,12 +87,15 @@ void walkGreyObjects() {
     @Uninterruptible(reason = "Called from uninterruptible code.", mayBeInlined = true)
     private void walkAlignedGreyObjects() {
         AlignedHeapChunk.AlignedHeader aChunk;
+        Pointer aStart;
         if (alignedHeapChunk.isNull() && alignedTop.isNull()) {
             /* If the snapshot is empty, then I have to walk from the beginning of the Space. */
             aChunk = space.getFirstAlignedHeapChunk();
+            aStart = (aChunk.isNonNull() ? AlignedHeapChunk.getObjectsStart(aChunk) : WordFactory.nullPointer());
         } else {
             /* Otherwise walk Objects that arrived after the snapshot. */
             aChunk = alignedHeapChunk;
+            aStart = alignedTop;
         }
         /* Visit Objects in the AlignedChunks. */
         GreyToBlackObjectVisitor visitor = GCImpl.getGCImpl().getGreyToBlackObjectVisitor();
@@ -100,10 +103,11 @@ private void walkAlignedGreyObjects() {
             AlignedHeapChunk.AlignedHeader lastChunk;
             do {
                 lastChunk = aChunk;
-                if (!AlignedHeapChunk.walkObjectsInline(aChunk, visitor)) {
+                if (!AlignedHeapChunk.walkObjectsFromInline(aChunk, aStart, visitor)) {
                     throw VMError.shouldNotReachHereAtRuntime();
                 }
                 aChunk = HeapChunk.getNext(aChunk);
+                aStart = (aChunk.isNonNull() ? AlignedHeapChunk.getObjectsStart(aChunk) : WordFactory.nullPointer());
             } while (aChunk.isNonNull());
 
             /* Move the scan point. */
diff --git a/substratevm/src/com.oracle.svm.core.genscavenge/src/com/oracle/svm/core/genscavenge/HeapChunk.java b/substratevm/src/com.oracle.svm.core.genscavenge/src/com/oracle/svm/core/genscavenge/HeapChunk.java
index 33d7acb9f0b6..84406410daf5 100644
--- a/substratevm/src/com.oracle.svm.core.genscavenge/src/com/oracle/svm/core/genscavenge/HeapChunk.java
+++ b/substratevm/src/com.oracle.svm.core.genscavenge/src/com/oracle/svm/core/genscavenge/HeapChunk.java
@@ -297,20 +297,20 @@ private static SignedWord offsetFromPointer(Header<?> that, PointerBase pointer)
     }
 
     @NeverInline("Not performance critical")
-    public static boolean walkObjectsFrom(Header<?> that, Pointer offset, ObjectVisitor visitor) {
-        return walkObjectsFromInline(that, offset, visitor);
+    public static boolean walkObjectsFrom(Header<?> that, Pointer start, ObjectVisitor visitor) {
+        return walkObjectsFromInline(that, start, visitor);
     }
 
     @AlwaysInline("GC performance")
     @Uninterruptible(reason = "Called from uninterruptible code.", mayBeInlined = true)
-    public static boolean walkObjectsFromInline(Header<?> that, Pointer startOffset, ObjectVisitor visitor) {
-        Pointer offset = startOffset;
-        while (offset.belowThan(getTopPointer(that))) { // crucial: top can move, so always re-read
-            Object obj = offset.toObject();
+    public static boolean walkObjectsFromInline(Header<?> that, Pointer start, ObjectVisitor visitor) {
+        Pointer p = start;
+        while (p.belowThan(getTopPointer(that))) { // crucial: top can move, so always re-read
+            Object obj = p.toObject();
             if (!callVisitor(visitor, obj)) {
                 return false;
             }
-            offset = offset.add(LayoutEncoding.getSizeFromObjectInlineInGC(obj));
+            p = p.add(LayoutEncoding.getSizeFromObjectInlineInGC(obj));
         }
         return true;
     }