diff --git a/const_mask_extractor.cpp b/const_mask_extractor.cpp
new file mode 100644
index 0000000..63f8f9e
--- /dev/null
+++ b/const_mask_extractor.cpp
@@ -0,0 +1,71 @@
+/* Check cf5-opt.vim defs.
+VIM: let g:lcppflags="-std=c++11 -O2 -pthread"
+VIM: let g:wcppflags="/O2 /EHsc /DWIN32"
+VIM-: let g:cppflags=g:Iboost.g:Itbb
+VIM-: let g:ldflags=g:Lboost.g:Ltbb.g:tbbmalloc.g:tbbmproxy
+VIM-: let g:ldlibpath=g:Bboost.g:Btbb
+VIM-: let g:argv=""
+VIM-: let g:cf5output=0
+*/
+#include <iostream>
+#include <iomanip>
+#include <exception>
+
+#define HI_BIT_MASK( m, b ) (!((m)&unsigned(~0)<<b))
+#define ENUM_SHIFT( m ) (32-(\
+	 HI_BIT_MASK(m,0)+HI_BIT_MASK(m,1)+HI_BIT_MASK(m,2)+HI_BIT_MASK(m,3)\
+	+HI_BIT_MASK(m,4)+HI_BIT_MASK(m,5)+HI_BIT_MASK(m,6)+HI_BIT_MASK(m,7)\
+	+HI_BIT_MASK(m,8)+HI_BIT_MASK(m,9)+HI_BIT_MASK(m,10)+HI_BIT_MASK(m,11)\
+	+HI_BIT_MASK(m,12)+HI_BIT_MASK(m,13)+HI_BIT_MASK(m,14)+HI_BIT_MASK(m,15)\
+	+HI_BIT_MASK(m,16)+HI_BIT_MASK(m,17)+HI_BIT_MASK(m,18)+HI_BIT_MASK(m,19)\
+	+HI_BIT_MASK(m,20)+HI_BIT_MASK(m,21)+HI_BIT_MASK(m,22)+HI_BIT_MASK(m,23)\
+	+HI_BIT_MASK(m,24)+HI_BIT_MASK(m,25)+HI_BIT_MASK(m,26)+HI_BIT_MASK(m,27)\
+	+HI_BIT_MASK(m,28)+HI_BIT_MASK(m,29)+HI_BIT_MASK(m,30)+HI_BIT_MASK(m,31)))
+#define ENUM_MASK(m) (~(~(unsigned long long)0<<ENUM_SHIFT(m)))
+
+void test( int i )
+{
+	char t[ENUM_SHIFT(2345)];
+	std::cout << std::setw(10) << i 
+		<< " ==> mask:" << std::setw(10) << ENUM_MASK( i ) 
+		<< " shift: " << std::setw(10) << ENUM_SHIFT( i ) 
+		<< std::endl;
+}
+
+int main ( void )
+{try{
+
+	std::cout << std::hex << std::showbase;
+	test( 0x00000000 );
+	test( 0x00000001 );
+	test( 0x00000010 );
+	test( 0x00000100 );
+	test( 0x00001000 );
+	test( 0x00010000 );
+	test( 0x00100000 );
+	test( 0x01000000 );
+	test( 0x10000000 );
+	test( 0x80000000 );
+	test( 0x80008000 );
+	test( 0x00100010 );
+	test( 0x000040a0 );
+	test( 0x000040a0 );
+	test( 0x00c04fa0 );
+	test( 0xffffffff );
+	test( 0xcccccccc );
+
+	return 0;
+}
+catch ( const std::exception& e )
+{
+	std::cerr << std::endl
+			<< "std::exception(\"" << e.what() << "\")." << std::endl;
+	return 2;
+}
+catch ( ... )
+{
+	std::cerr  << std::endl
+			<< "unknown exception." << std::endl;
+	return 1;
+}}
+