[lto] added getCOFFWeakExternalFallback

[inglorion]

Summary

This allows clients of the LTO API to determine the name of the fallback symbol for COFF weak externals.

[pcc]

include/llvm/LTO/LTO.h

156

I'd make this function specifically for COFF weak externals. No other format requires this feature during symbol resolution, as far as I know.

157

*refers to

159

Maybe assert that the symbol is a weak external?

161

This could be a bitcast or a gep, so I think you need to call stripPointerCasts on the result. You also need to mangle the symbol name, see how we set Name for example.

163

I don't think we can get here if the symbol is a valid weak external. I'd assert (via cast or otherwise) that this is the case.

[inglorion]

• renamed to getCOFFWeakExternalFallback to indicate this only works for COFF weak externals
• asserted that the symbol is a weak external
• added mangling

[inglorion]

(I'll update the title of the revision if you're happy with the new name.)

[pcc]

LGTM

[inglorion]

Updating title and summary.

include/llvm/LTO/LTO.h

public:
 
   public:
     Symbol(ArrayRef<ModuleSymbolTable::Symbol>::iterator I,
            const ModuleSymbolTable &SymTab, const InputFile *File)
         : I(I), SymTab(SymTab), File(File) {
       skip();
     }
 
+    /// For COFF weak externals, returns the name of the symbol that is used
+    /// as a fallback if the weak external remains undefined.
+    std::string getCOFFWeakExternalFallback() const {
+      assert((Flags & object::BasicSymbolRef::SF_Weak) &&
+             (Flags & object::BasicSymbolRef::SF_Indirect) &&
+             "symbol is not a weak external");
+      std::string Name;
+      raw_string_ostream OS(Name);
+      SymTab.printSymbolName(
+          OS,
+          cast<GlobalValue>(
+              cast<GlobalAlias>(getGV())->getAliasee()->stripPointerCasts()));
+      OS.flush();
+      return Name;
+    }
+
     /// Returns the mangled name of the global.
     StringRef getName() const { return Name; }
 
     uint32_t getFlags() const { return Flags; }
     GlobalValue::VisibilityTypes getVisibility() const {
       if (isGV())
         return getGV()->getVisibility();
       return GlobalValue::DefaultVisibility;