If you don't want to use global variables just add the result of f to x and stop using the global variable, i.e.

   x += f()

3.9: 2.78

3.14: 3.86

3.14t: 3.91

This is a silly benchmark though. Look at pyperformance if you want something that might represent real script/application performance. Generally 3.14t is about 0.9x the performance of the default build. That depends on a lot of things though.