In COFF, an undefined symbol can have up to one alternative name. If a symbol
is resolved by its regular name, then it's linked normally. If a symbol is not
found in any input files, all references to the regular name are resolved using
the alternative name. If the alternative name is not found, it's a link error.
This mechanism is called "weak externals".
To support this mechanism, I added a new member function fallback() to undefined
atom. If an undefined atom has the second name, fallback() returns a new undefined
atom that should be used instead of the original one to resolve undefines. If it
does not have the second name, the function returns nullptr.
Differential Revision: http://llvm-reviews.chandlerc.com/D1550
llvm-svn: 190625
* Moves enum Scope from DefinedAtom.h to Atom.h
* Adds scope method to AbsoluteAtom class
* Updates YAML to print scope of AbsoluteAtoms
* Update Native Reader/Writer to handle this new, "attribute"
* Adds testcase
Reviewed and approved by Nick Kledzik
llvm-svn: 166189
now Reader and Writer subclasses for each file format. Each Reader and
Writer subclass defines an "options" class which controls how that Reader
or Writer operates.
llvm-svn: 157774
