8323159: Consider adding some text re. memory zeroing in Arena::allocate

Reviewed-by: mcimadamore, jvernee

Author: minborg

src/java.base/share/classes/java/lang/foreign/Arena.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2022, 2023, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2022, 2024, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -219,6 +219,9 @@ public interface Arena extends SegmentAllocator, AutoCloseable {
      * Segments allocated with the returned arena can be
      * {@linkplain MemorySegment#isAccessibleBy(Thread) accessed} by any thread.
      * Calling {@link #close()} on the returned arena will result in an {@link UnsupportedOperationException}.
+     * <p>
+     * Memory segments {@linkplain #allocate(long, long) allocated} by the returned arena
+     * are zero-initialized.
      *
      * @return a new arena that is managed, automatically, by the garbage collector
      */
@@ -231,6 +234,9 @@ static Arena ofAuto() {
      *          {@linkplain MemorySegment#isAccessibleBy(Thread) accessed} by any thread.
      *          Calling {@link #close()} on the returned arena will result in
      *          an {@link UnsupportedOperationException}.
+     * <p>
+     * Memory segments {@linkplain #allocate(long, long) allocated} by the returned arena
+     * are zero-initialized.
      */
     static Arena global() {
         class Holder {
@@ -243,6 +249,9 @@ class Holder {
      * {@return a new confined arena} Segments allocated with the confined arena can be
      *          {@linkplain MemorySegment#isAccessibleBy(Thread) accessed} by the thread
      *          that created the arena, the arena's <em>owner thread</em>.
+     * <p>
+     * Memory segments {@linkplain #allocate(long, long) allocated} by the returned arena
+     * are zero-initialized.
      */
     static Arena ofConfined() {
         return MemorySessionImpl.createConfined(Thread.currentThread()).asArena();
@@ -251,6 +260,9 @@ static Arena ofConfined() {
     /**
      * {@return a new shared arena} Segments allocated with the global arena can be
      *          {@linkplain MemorySegment#isAccessibleBy(Thread) accessed} by any thread.
+     * <p>
+     * Memory segments {@linkplain #allocate(long, long) allocated} by the returned arena
+     * are zero-initialized.
      */
     static Arena ofShared() {
         return MemorySessionImpl.createShared().asArena();

test/jdk/java/foreign/TestScope.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2023, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2023, 2024 Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -31,8 +31,11 @@
 import java.lang.foreign.Arena;
 import java.lang.foreign.MemorySegment;
 import java.lang.foreign.SymbolLookup;
+import java.lang.foreign.ValueLayout;
 import java.nio.ByteBuffer;
 import java.nio.IntBuffer;
+import java.util.HexFormat;
+import java.util.stream.LongStream;
 
 import static org.testng.Assert.*;
 
@@ -108,6 +111,30 @@ public void testSameLookupScope() {
         testDerivedBufferScope(segment1.reinterpret(10));
     }
 
+    @Test
+    public void testZeroedOfAuto() {
+        testZeroed(Arena.ofAuto());
+    }
+
+    @Test
+    public void testZeroedGlobal() {
+        testZeroed(Arena.global());
+    }
+
+    @Test
+    public void testZeroedOfConfined() {
+        try (Arena arena = Arena.ofConfined()) {
+            testZeroed(arena);
+        }
+    }
+
+    @Test
+    public void testZeroedOfShared() {
+        try (Arena arena = Arena.ofShared()) {
+            testZeroed(arena);
+        }
+    }
+
     void testDerivedBufferScope(MemorySegment segment) {
         ByteBuffer buffer = segment.asByteBuffer();
         MemorySegment.Scope expectedScope = segment.scope();
@@ -119,4 +146,14 @@ void testDerivedBufferScope(MemorySegment segment) {
         IntBuffer view = buffer.asIntBuffer();
         assertEquals(expectedScope, MemorySegment.ofBuffer(view).scope());
     }
+
+    private static final MemorySegment ZEROED_MEMORY = MemorySegment.ofArray(new byte[8102]);
+
+    void testZeroed(Arena arena) {
+        long byteSize = ZEROED_MEMORY.byteSize();
+        var segment = arena.allocate(byteSize, Long.BYTES);
+        long mismatch = ZEROED_MEMORY.mismatch(segment);
+        assertEquals(mismatch, -1);
+    }
+
 }