Update documentation.

Author: christianhaeubl

src/hotspot/share/jvmci/jvmciCompilerToVM.cpp

@@ -2355,11 +2355,14 @@ static jobject read_field_value(Handle obj, long displacement, jchar type_char,
   }
 
   // Perform basic sanity checks on the read.  Primitive reads are permitted to read outside the
-  // bounds of their fields but object reads must map exactly onto the underlying oop slot.
+  // bounds of their fields but object reads must map exactly onto the underlying oop slot. This
+  // also ensures that the load is naturally aligned, which is required because the code below
+  // uses volatile reads (see JDK-8275874).
   bool aligned = (displacement % basic_type_elemsize) == 0;
   if (!aligned) {
     JVMCI_THROW_MSG_NULL(IllegalArgumentException, "read is unaligned");
   }
+
   if (obj->is_array()) {
     // Disallow reading after the last element of an array
     size_t array_length = arrayOop(obj())->length();
@@ -2370,6 +2373,7 @@ static jobject read_field_value(Handle obj, long displacement, jchar type_char,
       JVMCI_THROW_MSG_NULL(IllegalArgumentException, "reading after last array element");
     }
   }
+
   if (basic_type == T_OBJECT) {
     if (obj->is_objArray()) {
       if (displacement < arrayOopDesc::base_offset_in_bytes(T_OBJECT)) {

src/jdk.internal.vm.ci/share/classes/jdk/vm/ci/meta/MemoryAccessProvider.java

@@ -35,9 +35,11 @@ public interface MemoryAccessProvider {
      * @param displacement the displacement within the object in bytes
      * @param bits the number of bits to read from memory
      * @return the read value encapsulated in a {@link JavaConstant} object of {@link JavaKind} kind
+     *         or {@code null} in case that the value could not be read
      * @throws IllegalArgumentException if the read is out of bounds of the object or {@code kind}
      *             is {@link JavaKind#Void} or not {@linkplain JavaKind#isPrimitive() primitive}
-     *             kind or {@code bits} is not 8, 16, 32 or 64, or the read is unaligned
+     *             kind or {@code bits} is not 8, 16, 32 or 64, or the value cannot be read safely
+     *             (e.g. unaligned volatile reads on AArch64)
      */
     JavaConstant readPrimitiveConstant(JavaKind kind, Constant base, long displacement, int bits) throws IllegalArgumentException;