diff --git a/src/libCom/calc/calcPerform.c b/src/libCom/calc/calcPerform.c
index 4a073c774..40e173b61 100644
--- a/src/libCom/calc/calcPerform.c
+++ b/src/libCom/calc/calcPerform.c
@@ -25,6 +25,7 @@
 #include "postfix.h"
 #include "postfixPvt.h"
 
+
 static double calcRandom(void);
 static int cond_search(const char **ppinst, int match);
 
@@ -32,6 +33,11 @@ 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
  *
  * Evalutate the postfix expression
@@ -392,6 +398,10 @@ epicsShareFunc long
     return 0;
 }
 
+#if defined(_WIN32) && defined(_M_X64) && !defined(_MINGW)
+#  pragma optimize("", on)
+#endif
+
 epicsShareFunc long
 calcArgUsage(const char *pinst, unsigned long *pinputs, unsigned long *pstores)
 {