diff -Naur Python-3.6.5-orig/Lib/distutils/cygwinccompiler.py Python-3.6.5/Lib/distutils/cygwinccompiler.py
--- Python-3.6.5-orig/Lib/distutils/cygwinccompiler.py	2018-04-16 09:54:17.150783500 +0300
+++ Python-3.6.5/Lib/distutils/cygwinccompiler.py	2018-04-16 09:54:48.545070000 +0300
@@ -305,9 +305,9 @@
             raise CCompilerError(
                 'Cygwin gcc cannot be used with --compiler=mingw32')
 
-        self.set_executables(compiler='gcc -O -Wall',
-                             compiler_so='gcc -mdll -O -Wall',
-                             compiler_cxx='g++ -O -Wall',
+        self.set_executables(compiler='gcc -O2 -Wall',
+                             compiler_so='gcc -mdll -O2 -Wall',
+                             compiler_cxx='g++ -O2 -Wall',
                              linker_exe='gcc',
                              linker_so='%s %s %s'
                                         % (self.linker_dll, shared_option,