do nothing in System.arraycopy if length <= 0

Previously, we risked segfaults by passing negative numbers to memcpy.

This commit also makes arraycopy throw an IndexOutOfBounds exception
instead of an ArrayStoreException if the specified offsets and lengths
would take us outside the bounds of one or both of the arrays, per the
Sun documentation.

src/builtin.cpp

@@ -594,25 +594,32 @@ Avian_java_lang_System_arraycopy
       if (LIKELY(elementSize)) {
         intptr_t sl = cast<uintptr_t>(src, BytesPerWord);
         intptr_t dl = cast<uintptr_t>(dst, BytesPerWord);
-        if (LIKELY(srcOffset >= 0 and srcOffset + length <= sl and
-                   dstOffset >= 0 and dstOffset + length <= dl))
-        {
-          uint8_t* sbody = &cast<uint8_t>(src, ArrayBody);
-          uint8_t* dbody = &cast<uint8_t>(dst, ArrayBody);
-          if (src == dst) {
-            memmove(dbody + (dstOffset * elementSize),
-                    sbody + (srcOffset * elementSize),
-                    length * elementSize);
+        if (LIKELY(length > 0)) {
+          if (LIKELY(srcOffset >= 0 and srcOffset + length <= sl and
+                     dstOffset >= 0 and dstOffset + length <= dl))
+          {
+            uint8_t* sbody = &cast<uint8_t>(src, ArrayBody);
+            uint8_t* dbody = &cast<uint8_t>(dst, ArrayBody);
+            if (src == dst) {
+              memmove(dbody + (dstOffset * elementSize),
+                      sbody + (srcOffset * elementSize),
+                      length * elementSize);
+            } else {
+              memcpy(dbody + (dstOffset * elementSize),
+                     sbody + (srcOffset * elementSize),
+                     length * elementSize);
+            }
+
+            if (classObjectMask(t, objectClass(t, dst))) {
+              mark(t, dst, ArrayBody + (dstOffset * BytesPerWord), length);
+            }
+
+            return;
           } else {
-            memcpy(dbody + (dstOffset * elementSize),
-                   sbody + (srcOffset * elementSize),
-                   length * elementSize);
+            t->exception = makeIndexOutOfBoundsException(t);
+            return;
           }
-
-          if (classObjectMask(t, objectClass(t, dst))) {
-            mark(t, dst, ArrayBody + (dstOffset * BytesPerWord), length);
-          }
-
+        } else {
           return;
         }
       }

src/machine.h

@@ -1759,6 +1759,12 @@ makeIllegalMonitorStateException(Thread* t)
   return makeIllegalMonitorStateException(t, 0, makeTrace(t), 0);
 }
 
+inline object
+makeIndexOutOfBoundsException(Thread* t)
+{
+  return makeIndexOutOfBoundsException(t, 0, makeTrace(t), 0);
+}
+
 inline object
 makeArrayIndexOutOfBoundsException(Thread* t, object message)
 {

vm.pro

@@ -38,6 +38,7 @@
 -keep public class java.lang.IllegalArgumentException
 -keep public class java.lang.IllegalMonitorStateException
 -keep public class java.lang.IllegalThreadStateException
+-keep public class java.lang.IndexOutOfBoundsException
 -keep public class java.lang.ArrayIndexOutOfBoundsException
 -keep public class java.lang.ArrayStoreException
 -keep public class java.lang.NegativeArraySizeException