ModelSim SE User's Manual - Electrical and Computer Engineering

Compiling Verilog files UM-121
The following code fragment and compiler invocation show how two different modules
that have the same name can be instantiated within the same design:
module top;
`uselib dir=/h/vendorA libext=.v
NAND2 u1(n1, n2, n3);
`uselib dir=/h/vendorB libext=.v
NAND2 u2(n4, n5, n6);
endmodule
vlog -compile_uselibs top
This allows the NAND2 module to have different definitions in the vendorA and vendorB
libraries.
`uselib is persistent
As mentioned above, the appearance of a `uselib directive in the source code explicitly
defines how instantiations that follow it are resolved. This may result in unexpected
consequences. For example, consider the following compile command:
vlog -compile_uselibs dut.v srtr.v
Assume that dut.v contains a `uselib directive. Since srtr.v is compiled after dut.v, the
`uselib directive is still in effect. When srtr is loaded it is using the `uselib directive from
dut.v to decide where to locate modules. If this is not what you intend, then you need to put
an empty `uselib at the end of dut.v to "close" the previous `uselib statement.
ModelSim SE User’s Manual