RFR: 8187578: BitMap::reallocate should check if old_map is NULL

[Erik Helin]

Hi all,

I'm still trying to compile with gcc 7.1.1 and run into another small 
issue. BitMap::reallocate calls Copy::disjoint_words and there is a case 
in Copy::disjoint_words that might result in call to memcpy. The problem 
is that BitMap::reallocate does not check that the "from" argument to 
Copy::disjoint_words differs from NULL, and a call to memcpy with a NULL 
argument is undefined behavior.

Webrev:
http://cr.openjdk.java.net/~ehelin/8187578/00/

Patch:
--- old/src/hotspot/share/utilities/bitMap.cpp	2017-09-15 
14:47:21.471113699 +0200
+++ new/src/hotspot/share/utilities/bitMap.cpp	2017-09-15 
14:47:21.179112252 +0200
@@ -81,8 +81,10 @@
    if (new_size_in_words > 0) {
      map = allocator.allocate(new_size_in_words);

-    Copy::disjoint_words((HeapWord*)old_map, (HeapWord*) map,
-                         MIN2(old_size_in_words, new_size_in_words));
+    if (old_map != NULL) {
+      Copy::disjoint_words((HeapWord*)old_map, (HeapWord*) map,
+                           MIN2(old_size_in_words, new_size_in_words));
+    }

      if (new_size_in_words > old_size_in_words) {
        clear_range_of_words(map, old_size_in_words, new_size_in_words);

Thanks,
Erik