With gcc 7.1.1 from Fedora 26 on x86-64 there are warnings about the potential usage of maybe uninitialized memory in src/hotspot/cpu/x86/assembler_x86.cpp and in src/hotspot/cpu/x86/interp_masm_x86.cpp.

The problems arises from the class RelocationHolder which has the private fields:
  enum { _relocbuf_size = 5 };                                                                                                                                                                                                                 
  void* _relocbuf[ _relocbuf_size ];

and the default constructor for RelocationHolder does not set the elements to _relocbuf to NULL. This is an optimization, RelocationHolder is used *a lot* and setting the elements of RelocationHolder::_relocbuf to NULL in the default constructor might result in a performance penalty. Instead, all users of RelocationHolder::_relocbuf take care to not use uninitialized memory.

The problem continues because the class Address in src/hotspot/cpu/x86/assembler_x86.hpp has a private field:
  RelocationHolder _rspec;
and the default constructor for Address does not initialize _rspec._relocbuf (most likely for performance reasons). The class Address also has a default copy constructor, which will copy all the elements of _rspec._relocbuf, which will result in a read of uninitialized memory. However, this is a benign usage of uninitialized memory, since we take no action based on the content of the uninitialized memory (it is just copied byte for byte).