diff -r bbd1da3f538f -r f3108e56b502 src/share/vm/gc_implementation/g1/g1GCPhaseTimes.cpp
--- a/src/share/vm/gc_implementation/g1/g1GCPhaseTimes.cpp	Wed Aug 01 04:19:22 2018 -0400
+++ b/src/share/vm/gc_implementation/g1/g1GCPhaseTimes.cpp	Thu Aug 02 03:54:51 2018 -0700
@@ -41,13 +41,13 @@
   int _cur;
 
   void vappend(const char* format, va_list ap)  ATTRIBUTE_PRINTF(2, 0) {
-    int res = vsnprintf(&_buffer[_cur], BUFFER_LEN - _cur, format, ap);
-    if (res != -1) {
-      _cur += res;
-    } else {
+    int res = os::vsnprintf(&_buffer[_cur], BUFFER_LEN - _cur, format, ap);
+    if (res > BUFFER_LEN) {
       DEBUG_ONLY(warning("buffer too small in LineBuffer");)
       _buffer[BUFFER_LEN -1] = 0;
       _cur = BUFFER_LEN; // vsnprintf above should not add to _buffer if we are called again
+    } else if (res != -1) {
+      _cur += res;
     }
   }