diff --git a/src/libCom/calc/calcPerform.c b/src/libCom/calc/calcPerform.c
index 2221f169f..e53d860e2 100644
--- a/src/libCom/calc/calcPerform.c
+++ b/src/libCom/calc/calcPerform.c
@@ -33,6 +33,10 @@ static int cond_search(const char **ppinst, int match);
 #define PI 3.14159265358979323
 #endif
 
+/* Turn off global optimization for 64-bit MSVC builds */
+#if defined(_WIN32) && defined(_M_X64) && !defined(_MINGW)
+#  pragma optimize("g", off)
+#endif
 
 /* calcPerform
  *
@@ -379,6 +383,9 @@ epicsShareFunc long
     *presult = *ptop;
     return 0;
 }
+#if defined(_WIN32) && defined(_M_X64) && !defined(_MINGW)
+#  pragma optimize("", on)
+#endif
 
 
 epicsShareFunc long