basic array copy: less virtualization for illegal values

Author: davleopo

compiler/src/jdk.internal.vm.compiler/src/org/graalvm/compiler/replacements/nodes/BasicArrayCopyNode.java

@@ -254,6 +254,39 @@ public boolean isExact() {
         return false;
     }
 
+    /**
+     * When dealing with arrays of primitives it's possible to store a larger value into a smaller
+     * component type array. For example a long value can be written into a byte array by storing
+     * the larger value in the first entry and storing {@code Illegal} in the slots whose value
+     * comes from the long value. So a long in a byte array would be a long value followed by 7
+     * {@code Illegal} values. That also means such a write much be treated as a group by arraycopy
+     * so reads and writes can't be performed in the middle of the illegal values.
+     * <p>
+     * So to ensure that an arraycopy properly treats these kinds of writes as group, it's
+     * sufficient to check that the first value is not {@code Illegal} and that the next value after
+     * the end is not {@code Illegal} since {@code Illegal} values only appear in the tail of these
+     * groups.
+     */
+    private static boolean ensureIllegalValuesCanBeRepresented(int length, int srcPos, int srcLength, int dstPos, int dstLength, VirtualObjectNode src, VirtualObjectNode dst, VirtualizerTool tool) {
+        if (length <= 0) {
+            return true;
+        }
+        // check source
+        ValueNode firstSrcEntry = tool.getEntry(src, srcPos);
+        ValueNode followingSrcEntry = srcPos + length < srcLength ? tool.getEntry(src, srcPos + length) : null;
+        if (firstSrcEntry.isIllegalConstant() || followingSrcEntry != null && followingSrcEntry.isIllegalConstant()) {
+            return false;
+        }
+
+        // check dst
+        ValueNode firstDstEntry = tool.getEntry(dst, dstPos);
+        ValueNode followingDstEntry = dstPos + length < dstLength ? tool.getEntry(dst, dstPos + length) : null;
+        if (firstDstEntry.isIllegalConstant() || followingDstEntry != null && followingDstEntry.isIllegalConstant()) {
+            return false;
+        }
+        return true;
+    }
+
     @Override
     public void virtualize(VirtualizerTool tool) {
         ValueNode sourcePosition = tool.getAlias(getSourcePosition());
@@ -287,6 +320,9 @@ public void virtualize(VirtualizerTool tool) {
                     if (!checkEntryTypes(srcPosInt, len, srcVirtual, destVirtual.type().getComponentType(), tool)) {
                         return;
                     }
+                    if (!ensureIllegalValuesCanBeRepresented(len, srcPosInt, srcVirtual.entryCount(), destPosInt, destVirtual.entryCount(), srcVirtual, destVirtual, tool)) {
+                        return;
+                    }
                     if (srcVirtual == destVirtual && srcPosInt < destPosInt) {
                         // must copy backwards to avoid losing elements
                         for (int i = len - 1; i >= 0; i--) {