frame.hpp is a popular header file (included by 876 out of 1000 HotSpot.o file). It includes method.hpp, which in turn pulls in all the class metadata types such as instanceKlass.hpp, constantPool.hpp, etc.

We can replace the inclusion of method.hpp with a forward declaration of the Method class.

This reduces the number of .o files that include method.hpp from 882 to 586.

HotSpot build time is reduced by more than 2%.