Make the null region optional.

Author: christianhaeubl

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

@@ -431,7 +431,7 @@ public int getPreferredAddressSpaceAlignment() {
     @Fold
     @Override
     public int getImageHeapOffsetInAddressSpace() {
-        if (SubstrateOptions.SpawnIsolates.getValue() && CommittedMemoryProvider.get().guaranteesHeapPreferredAddressSpaceAlignment()) {
+        if (SubstrateOptions.SpawnIsolates.getValue() && SubstrateOptions.UseNullRegion.getValue() && CommittedMemoryProvider.get().guaranteesHeapPreferredAddressSpaceAlignment()) {
             /*
              * The image heap will be mapped in a way that there is a memory protected gap between
              * the heap base and the start of the image heap. The gap won't need any memory in the
@@ -445,7 +445,7 @@ public int getImageHeapOffsetInAddressSpace() {
     @Fold
     @Override
     public int getImageHeapNullRegionSize() {
-        if (SubstrateOptions.SpawnIsolates.getValue() && !CommittedMemoryProvider.get().guaranteesHeapPreferredAddressSpaceAlignment()) {
+        if (SubstrateOptions.SpawnIsolates.getValue() && SubstrateOptions.UseNullRegion.getValue() && !CommittedMemoryProvider.get().guaranteesHeapPreferredAddressSpaceAlignment()) {
             /*
              * Prepend a single null page to the image heap so that there is a memory protected gap
              * between the heap base and the start of the image heap. The null page is placed

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

@@ -24,6 +24,7 @@
  */
 package com.oracle.svm.core.genscavenge;
 
+import com.oracle.svm.core.config.ConfigurationValues;
 import com.oracle.svm.core.heap.Heap;
 import com.oracle.svm.core.image.ImageHeap;
 import com.oracle.svm.core.image.ImageHeapLayoutInfo;
@@ -34,7 +35,9 @@ public class LinearImageHeapLayouter extends AbstractImageHeapLayouter<LinearIma
     private final int nullRegionSize;
 
     public LinearImageHeapLayouter(ImageHeapInfo heapInfo, long startOffset, int nullRegionSize) {
+        assert startOffset >= 0;
         assert startOffset == 0 || startOffset >= Heap.getHeap().getImageHeapOffsetInAddressSpace() : "must be relative to the heap base";
+        assert nullRegionSize >= 0;
         this.heapInfo = heapInfo;
         this.startOffset = startOffset;
         this.nullRegionSize = nullRegionSize;
@@ -52,8 +55,8 @@ protected LinearImageHeapPartition createPartition(String name, boolean contains
 
     @Override
     protected ImageHeapLayoutInfo doLayout(ImageHeap imageHeap) {
-        long beginOffset = startOffset;
-        beginOffset += nullRegionSize;
+        long beginOffset = startOffset + spaceReservedForNull();
+        assert beginOffset >= ConfigurationValues.getObjectLayout().getAlignment() : "Zero designates null";
         LinearImageHeapAllocator allocator = new LinearImageHeapAllocator(beginOffset);
         for (LinearImageHeapPartition partition : getPartitions()) {
             partition.allocateObjects(allocator);
@@ -62,6 +65,13 @@ protected ImageHeapLayoutInfo doLayout(ImageHeap imageHeap) {
         return createLayoutInfo(startOffset, getWritablePrimitive().getStartOffset());
     }
 
+    private long spaceReservedForNull() {
+        if (startOffset == 0 && nullRegionSize == 0) {
+            return ConfigurationValues.getObjectLayout().getAlignment();
+        }
+        return nullRegionSize;
+    }
+
     /**
      * Store which objects are at the boundaries of the image heap partitions. Here, we also merge
      * the read-only reference partition with the read-only relocatable partition.

substratevm/src/com.oracle.svm.core/src/com/oracle/svm/core/SubstrateOptions.java

@@ -428,6 +428,9 @@ public static boolean useLLVMBackend() {
     @Option(help = "When set to true, the image generator verifies that the image heap does not contain a home directory as a substring", type = User)//
     public static final HostedOptionKey<Boolean> DetectUserDirectoriesInImageHeap = new HostedOptionKey<>(false);
 
+    @Option(help = "Determines if a null region is present between the heap base and the image heap.", type = Expert)//
+    public static final HostedOptionKey<Boolean> UseNullRegion = new HostedOptionKey<>(true);
+
     @Option(help = "The interval in minutes between watchdog checks (0 disables the watchdog)", type = OptionType.Expert)//
     public static final HostedOptionKey<Integer> DeadlockWatchdogInterval = new HostedOptionKey<>(10);
     @Option(help = "Exit the image builder VM after printing call stacks", type = OptionType.Expert)//