Add a fallback mechanism for undefined atom.

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

1 files changed, 8 insertions, 1 deletions

diff --git a/lld/lib/Core/Resolver.cpp b/lld/lib/Core/Resolver.cpp
index d56f7d1067f..716f0d48d53 100644
--- a/lld/lib/Core/Resolver.cpp
+++ b/lld/lib/Core/Resolver.cpp
@@ -198,7 +198,7 @@ void Resolver::resolveUndefines() {
     undefineGenCount = _symbolTable.size();
     std::vector<const UndefinedAtom *> undefines;
     _symbolTable.undefines(undefines);
-    for ( const Atom *undefAtom : undefines ) {
+    for (const UndefinedAtom *undefAtom : undefines) {
       StringRef undefName = undefAtom->name();
       // load for previous undefine may also have loaded this undefine
       if (!_symbolTable.isDefined(undefName)) {
@@ -208,6 +208,13 @@ void Resolver::resolveUndefines() {
                                     false,  // dataSymbolOnly
                                     *this);
       }
+      // If the undefined symbol has an alternative name, try to resolve the
+      // symbol with the name to give it a second chance. This feature is used
+      // for COFF "weak external" symbol.
+      if (const UndefinedAtom *fallbackUndefAtom = undefAtom->fallback()) {
+        _symbolTable.addReplacement(undefAtom, fallbackUndefAtom);
+        _symbolTable.add(*fallbackUndefAtom);
+      }
     }
     // search libraries for overrides of common symbols
     if (searchArchives || searchSharedLibs) {