diff --git a/common_x86.h b/common_x86.h
index 4c17f3a04..4316318ec 100644
--- a/common_x86.h
+++ b/common_x86.h
@@ -360,5 +360,7 @@ REALNAME:
 // ffreep %st(0). 
 // Because Clang didn't support ffreep, we directly use the opcode.
 // Please check out http://www.sandpile.org/x86/opc_fpu.htm 
+#ifndef ffreep
 #define ffreep .byte 0xdf, 0xc0 #
 #endif
+#endif
diff --git a/common_x86_64.h b/common_x86_64.h
index e61e37e6b..7b6d11f7d 100644
--- a/common_x86_64.h
+++ b/common_x86_64.h
@@ -451,5 +451,7 @@ REALNAME:
 // ffreep %st(0). 
 // Because Clang didn't support ffreep, we directly use the opcode.
 // Please check out http://www.sandpile.org/x86/opc_fpu.htm 
+#ifndef ffreep
 #define ffreep .byte 0xdf, 0xc0 #
 #endif
+#endif