Handle section vs global name conflict.

This is a fix for PR26941.

When there is both a section and a global definition with the same
name, the global wins.

Section symbols are not added to the symbol table; section references
are left undefined and fixed up in the object writer unless they've
been satisfied by some other definition.

llvm-svn: 264649

llvm/lib/MC/ELFObjectWriter.cpp

@@ -375,9 +375,24 @@ uint64_t ELFObjectWriter::SymbolValue(const MCSymbol &Sym,
 
 void ELFObjectWriter::executePostLayoutBinding(MCAssembler &Asm,
                                                const MCAsmLayout &Layout) {
+  // Section symbols are used as definitions for undefined symbols with matching
+  // names. If there are multiple sections with the same name, the first one is
+  // used.
+  for (const MCSection &Sec : Asm) {
+    const MCSymbol *Begin = Sec.getBeginSymbol();
+    if (!Begin)
+      continue;
+
+    const MCSymbol *Alias = Asm.getContext().lookupSymbol(Begin->getName());
+    if (!Alias || !Alias->isUndefined())
+      continue;
+
+    Renames.insert(
+        std::make_pair(cast<MCSymbolELF>(Alias), cast<MCSymbolELF>(Begin)));
+  }
+
   // The presence of symbol versions causes undefined symbols and
   // versions declared with @@@ to be renamed.
-
   for (const MCSymbol &A : Asm.symbols()) {
     const auto &Alias = cast<MCSymbolELF>(A);
     // Not an alias.