LinkerScript: Add evaluation of the EXTERN command

[meadori]

Summary

This patch implements evaluation of the GNU ld EXTERN command.

The parsing part was submitted in D8271.

[davide]

LGTM

include/lld/ReaderWriter/LinkerScript.h

 
 private:
   llvm::ArrayRef<const MemoryBlock *> _blocks;
 };
 
 /// Represents an extern command.
 class Extern : public Command {
 public:
+  typedef llvm::ArrayRef<StringRef>::const_iterator const_iterator;
+
   Extern(Parser &ctx,
          const SmallVectorImpl<StringRef> &symbols)
       : Command(ctx, Kind::Extern) {
     size_t numSymbols = symbols.size();
     StringRef *symbolsStart =
         getAllocator().Allocate<StringRef>(numSymbols);
     std::copy(std::begin(symbols), std::end(symbols), symbolsStart);
     _symbols = llvm::makeArrayRef(symbolsStart, numSymbols);
   }
 
   static bool classof(const Command *c) {
     return c->getKind() == Kind::Extern;
   }
 
   void dump(raw_ostream &os) const override;
+  const_iterator begin() const { return _symbols.begin(); }
+  const_iterator end() const { return _symbols.end(); }
 
 private:
   llvm::ArrayRef<StringRef> _symbols;
 };
 
 
 /// Stores the parse tree of a linker script.
 class LinkerScript {

lib/Driver/GnuLdDriver.cpp

for (const script::Command *c : script->_commands) {
     }
     if (auto *searchDir = dyn_cast<script::SearchDir>(c))
       if (!nostdlib)
         ctx.addSearchPath(searchDir->getSearchPath());
     if (auto *entry = dyn_cast<script::Entry>(c))
       ctx.setEntrySymbolName(entry->getEntryName());
     if (auto *output = dyn_cast<script::Output>(c))
       ctx.setOutputPath(output->getOutputFileName());
+    if (auto *externs = dyn_cast<script::Extern>(c)) {
+      for (auto symbol : *externs) {
+        ctx.addInitialUndefinedSymbol(symbol);
+      }
+    }
   }
   // Transfer ownership of the script to the linking context
   ctx.addLinkerScript(std::move(parser));
   return std::error_code();
 }
 
 bool GnuLdDriver::applyEmulation(llvm::Triple &triple,
                                  llvm::opt::InputArgList &args,

test/elf/linkerscript/Inputs/externs.ls

+/* A simple valid linker script used for testing the EXTERN command.
+ */
+EXTERN(_foo bar __baz)

test/elf/linkerscript/externs.objtxt

+# Check symbols defined with the EXTERN command are added as undefined
+# symbols.
+
+# RUN: lld -flavor gnu -target x86_64 -T %p/Inputs/externs.ls -r %s \
+# RUN:     --output-filetype=yaml | FileCheck %s
+
+defined-atoms:
+  - name:            main
+    scope:           global
+    content:         [ B8, 00, 00, 00, 00, C7, 44, 24, FC, 00, 00, 00, 00, C3 ]
+    alignment:       2^4
+    section-choice:  custom-required
+    section-name:    .text
+
+# CHECK: undefined-atoms:
+# CHECK:   - name:            _foo
+# CHECK:     can-be-null:     at-buildtime
+# CHECK:   - name:            bar
+# CHECK:     can-be-null:     at-buildtime
+# CHECK:   - name:            __baz
+# CHECK:     can-be-null:     at-buildtime