Global program checking (GPC), invoked by the -Xlistx option, does the following:
Enforces type-checking rules of Fortran more stringently than usual, especially between separately compiled routines
Enforces some portability restrictions needed to move programs between different machines or operating systems
Detects legal constructions that nevertheless might be suboptimal or error-prone
Reveals other potential bugs and obscurities
In particular, global checking reports problems such as:
Possible conflicts due to data type mismatches in common blocks between different subprograms
Usage problems