[ELF] Avoid promoting an undefined weak entry symbol to global.

Without this patch, lld emits "error: undefined symbol: _start"
if it encountered only weak references to that symbol.

llvm-svn: 314790

lld/ELF/Driver.cpp

@@ -1034,8 +1034,7 @@ template <class ELFT> void LinkerDriver::link(opt::InputArgList &Args) {
   // If an entry symbol is in a static archive, pull out that file now
   // to complete the symbol table. After this, no new names except a
   // few linker-synthesized ones will be added to the symbol table.
-  if (Symtab->find(Config->Entry))
-    Symtab->addUndefined<ELFT>(Config->Entry);
+  Symtab->fetchIfLazy<ELFT>(Config->Entry);
 
   // Return if there were name resolution errors.
   if (ErrorCount)

lld/ELF/SymbolTable.cpp

@@ -581,17 +581,22 @@ void SymbolTable::addLazyObject(StringRef Name, LazyObjFile &Obj) {
     addFile<ELFT>(F);
 }
 
+// If we already saw this symbol, force loading its file.
+template <class ELFT> void SymbolTable::fetchIfLazy(StringRef Name) {
+  if (SymbolBody *B = find(Name)) {
+    // Mark the symbol not to be eliminated by LTO
+    // even if it is a bitcode symbol.
+    B->symbol()->IsUsedInRegularObj = true;
+    if (auto *L = dyn_cast_or_null<Lazy>(B))
+      if (InputFile *File = L->fetch())
+        addFile<ELFT>(File);
+  }
+}
+
 // Process undefined (-u) flags by loading lazy symbols named by those flags.
 template <class ELFT> void SymbolTable::scanUndefinedFlags() {
   for (StringRef S : Config->Undefined)
-    if (SymbolBody *B = find(S)) {
-      // Mark the symbol not to be eliminated by LTO
-      // even if it is a bitcode symbol.
-      B->symbol()->IsUsedInRegularObj = true;
-      if (auto *L = dyn_cast_or_null<Lazy>(B))
-        if (InputFile *File = L->fetch())
-          addFile<ELFT>(File);
-    }
+    fetchIfLazy<ELFT>(S);
 }
 
 // This function takes care of the case in which shared libraries depend on
@@ -874,6 +879,11 @@ template void SymbolTable::addShared<ELF64BE>(StringRef, SharedFile<ELF64BE> *,
                                               const typename ELF64BE::Sym &,
                                               const typename ELF64BE::Verdef *);
 
+template void SymbolTable::fetchIfLazy<ELF32LE>(StringRef);
+template void SymbolTable::fetchIfLazy<ELF32BE>(StringRef);
+template void SymbolTable::fetchIfLazy<ELF64LE>(StringRef);
+template void SymbolTable::fetchIfLazy<ELF64BE>(StringRef);
+
 template void SymbolTable::scanUndefinedFlags<ELF32LE>();
 template void SymbolTable::scanUndefinedFlags<ELF32BE>();
 template void SymbolTable::scanUndefinedFlags<ELF64LE>();

lld/ELF/SymbolTable.h

@@ -84,6 +84,7 @@ public:
                                    uint8_t Visibility, bool CanOmitFromDynSym,
                                    InputFile *File);
 
+  template <class ELFT> void fetchIfLazy(StringRef Name);
   template <class ELFT> void scanUndefinedFlags();
   template <class ELFT> void scanShlibUndefined();
   void scanVersionScript();

lld/test/ELF/weak-entry.s

@@ -0,0 +1,13 @@
+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t
+# RUN: ld.lld %t -o %tout
+# RUN: llvm-nm %tout | FileCheck %s
+
+# CHECK:      w _start
+# CHECK-NEXT: T foo
+
+.global foo
+.weak _start
+.text
+foo:
+	.dc.a _start