unsigned overflow in find_last_elem (found by some tier6 tests)

openjdk · Nov 8, 2020 · fde1fc5 · fde1fc5
1 parent e46634f
commit fde1fc5
Showing 1 changed file with 4 additions and 2 deletions.
diff --git a/src/hotspot/share/opto/regmask.hpp b/src/hotspot/share/opto/regmask.hpp
@@ -178,8 +178,10 @@ class RegMask {
   // Get highest-numbered register from mask, or BAD if mask is empty.
   OptoReg::Name find_last_elem() const {
     assert(valid_watermarks(), "sanity");
-    for (unsigned i = _hwm; i >= _lwm; i--) {
-      uintptr_t bits = _RM_UP[i];
+    // Careful not to overflow if _lwm == 0
+    unsigned i = _hwm + 1;
+    while (i > _lwm) {
+      uintptr_t bits = _RM_UP[--i];
       if (bits) {
         return OptoReg::Name((i<<_LogWordBits) + find_highest_bit(bits));
       }