diff --git a/src/share/vm/gc_implementation/g1/g1SATBCardTableModRefBS.cpp b/src/share/vm/gc_implementation/g1/g1SATBCardTableModRefBS.cpp
index 9f20a920b74125998a3aa8a4ac6b616ee1998a8e..7382be27f8cdedbeebb12f83137b1bae2c211ba0 100644
--- a/src/share/vm/gc_implementation/g1/g1SATBCardTableModRefBS.cpp
+++ b/src/share/vm/gc_implementation/g1/g1SATBCardTableModRefBS.cpp
@@ -95,7 +95,15 @@ void G1SATBCardTableModRefBS::g1_mark_as_young(const MemRegion& mr) {
   jbyte *const first = byte_for(mr.start());
   jbyte *const last = byte_after(mr.last());
 
-  memset(first, g1_young_gen, last - first);
+  // Below we may use an explicit loop instead of memset() because on
+  // certain platforms memset() can give concurrent readers phantom zeros.
+  if (UseMemSetInBOT) {
+    memset(first, g1_young_gen, last - first);
+  } else {
+    for (jbyte* i = first; i < last; i++) {
+      *i = g1_young_gen;
+    }
+  }
 }
 
 #ifndef PRODUCT