Avoid unnecessary ChunkedImageHeapPartition alignment from empty huge object partitions.

Author: peter-hofer

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

@@ -30,7 +30,6 @@
 import org.graalvm.nativeimage.ImageInfo;
 import org.graalvm.word.UnsignedWord;
 
-import com.oracle.svm.core.SubstrateOptions;
 import com.oracle.svm.core.SubstrateUtil;
 import com.oracle.svm.core.config.ConfigurationValues;
 import com.oracle.svm.core.genscavenge.ChunkedImageHeapAllocator.AlignedChunk;
@@ -96,7 +95,7 @@ public ChunkedImageHeapLayouter(ImageHeapInfo heapInfo, long startOffset) {
         this.partitions[WRITABLE_PATCHED] = new ChunkedImageHeapPartition("writablePatched", true, false, alignment, alignment);
         this.partitions[WRITABLE_REGULAR] = new ChunkedImageHeapPartition("writable", true, false, alignment, alignment);
         this.partitions[WRITABLE_HUGE] = new ChunkedImageHeapPartition("writableHuge", true, true, alignment, alignment);
-        this.partitions[READ_ONLY_HUGE] = new ChunkedImageHeapPartition("readOnlyHuge", false, true, alignment, SubstrateOptions.getPageSize());
+        this.partitions[READ_ONLY_HUGE] = new ChunkedImageHeapPartition("readOnlyHuge", false, true, alignment, alignment);
 
         this.heapInfo = heapInfo;
         this.startOffset = startOffset;
@@ -219,7 +218,9 @@ private ImageHeapLayoutInfo populateInfoObjects(int dynamicHubCount, int pageSiz
 
         long writableEnd = getWritableHuge().getStartOffset() + getWritableHuge().getSize();
         long writableSize = writableEnd - offsetOfFirstWritableAlignedChunk;
-        long imageHeapSize = getReadOnlyHuge().getStartOffset() + getReadOnlyHuge().getSize() - startOffset;
+        /* Aligning the end to the page size can be required for mapping into memory. */
+        long imageHeapEnd = NumUtil.roundUp(getReadOnlyHuge().getStartOffset() + getReadOnlyHuge().getSize(), pageSize);
+        long imageHeapSize = imageHeapEnd - startOffset;
         return new ImageHeapLayoutInfo(startOffset, imageHeapSize, offsetOfFirstWritableAlignedChunk, writableSize, getReadOnlyRelocatable().getStartOffset(), getReadOnlyRelocatable().getSize(),
                         getWritablePatched().getStartOffset(), getWritablePatched().getSize());
     }

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

@@ -82,6 +82,17 @@ void layout(ChunkedImageHeapAllocator allocator, ImageHeapLayouterControl contro
     }
 
     private void layoutInUnalignedChunks(ChunkedImageHeapAllocator allocator, ImageHeapLayouterControl control) {
+        if (objects.isEmpty()) {
+            /*
+             * Without objects, don't force finishing the current chunk and therefore committing
+             * space for the rest of it. Another partition might be able to continue filling it, or,
+             * if no more objects follow, we don't need to dedicate space in the image at all.
+             */
+            startOffset = allocator.getPosition();
+            endOffset = startOffset;
+            return;
+        }
+
         allocator.finishAlignedChunk();
         allocator.alignBetweenChunks(getStartAlignment());
         startOffset = allocator.getPosition();