--- a/graal/com.oracle.graal.nodes/src/com/oracle/graal/nodes/type/IntegerStamp.java	Sun Jun 23 21:44:15 2013 +0200
+++ b/graal/com.oracle.graal.nodes/src/com/oracle/graal/nodes/type/IntegerStamp.java	Sun Jun 23 21:44:35 2013 +0200
@@ -131,13 +131,14 @@
         IntegerStamp other = (IntegerStamp) otherStamp;
         if (lowerBound > other.upperBound || upperBound < other.lowerBound) {
             return true;
-        } else {
-            if ((mask & other.mask) == 0) {
-                // zero is the only common value if the masks don't overlap => check for non-zero
-                return lowerBound > 0 || upperBound < 0 || other.lowerBound > 0 || other.upperBound < 0;
-            }
-            return false;
+        } else if ((mask & other.mask) == 0 && (lowerBound > 0 || upperBound < 0 || other.lowerBound > 0 || other.upperBound < 0)) {
+            /*
+             * Zero is the only common value if the masks don't overlap. If one of the two values is
+             * less than or greater than zero, they are always distinct.
+             */
+            return true;
         }
+        return false;
     }
 
     @Override

--- a/mx/commands.py	Sun Jun 23 21:44:15 2013 +0200
+++ b/mx/commands.py	Sun Jun 23 21:44:35 2013 +0200
@@ -937,6 +937,11 @@
         vm(['-esa', '-version'])
         tasks.append(t.stop())
 
+        _vmbuild = 'fastdebug'
+        t = Task('BootstrapWithGCVerification:fastdebug')
+        vm(['-XX:+VerifyBeforeGC', '-version'])
+        tasks.append(t.stop())
+
         _vmbuild = 'product'
         t = Task('BootstrapWithRegisterPressure:product')
         vm(['-G:RegisterPressure=rbx,r11,r10,r14,xmm3,xmm11,xmm14', '-esa', '-version'])