This pass implements whole program optimization of virtual calls in cases
where we know (via bitset information) that the list of callee is fixed. This
includes the following:
- Single implementation devirtualization: if a virtual call has a single possible callee, replace all calls with a direct call to that callee.
- Virtual constant propagation: if the virtual function's return type is an integer <=64 bits and all possible callees are readnone, for each class and each list of constant arguments: evaluate the function, store the return value alongside the virtual table, and rewrite each virtual call as a load from the virtual table.
- Uniform return value optimization: if the conditions for virtual constant propagation hold and each function returns the same constant value, replace each virtual call with that constant.
- Unique return value optimization for i1 return values: if the conditions for virtual constant propagation hold and a single vtable's function returns 0, or a single vtable's function returns 1, replace each virtual call with a comparison of the vptr against that vtable's address.