[lld][WebAssembly] Handle mixed strong and weak undefined symbols

[sbc100]

Summary

When there are both strong and weak references to an undefined
symbol ensure that the strong reference prevails in the output symbol
generating the correct error.

Test case copied from lld/test/ELF/weak-and-strong-undef.s

[dschuff]

lld/test/wasm/weak-and-strong-undef.s

2

string->strong

lld/wasm/SymbolTable.cpp

430

symbols->symbol

lld/test/wasm/Inputs/weak-undefined.s

+.functype foo () -> ()
+.weak foo

lld/test/wasm/weak-and-strong-undef.s

+# Test that when a symbol (foo) is both weakly and stronly undefined the
+# string undefined symbol alwaeys generates an error, whichever object
+# file is seen first.
+
+# RUN: llvm-mc -filetype=obj -triple=wasm32-unknown-unknown %s -o %t1.o
+# RUN: llvm-mc -filetype=obj -triple=wasm32-unknown-unknown %S/Inputs/weak-undefined.s -o %t2.o
+# RUN: not wasm-ld %t1.o %t2.o -o /dev/null 2>&1 | FileCheck %s
+# RUN: not wasm-ld %t2.o %t1.o -o /dev/null 2>&1 | FileCheck %s
+
+# CHECK: undefined symbol: foo
+
+_start:
+    .globl _start
+    .functype _start () -> ()
+    call foo
+    end_function
+
+.functype foo () -> ()

lld/wasm/SymbolTable.cpp

if (importModule) {
     if (!existing->importModule)
       existing->importModule = importModule;
     if (existing->importModule != importModule)
       error("import module mismatch for symbol: " + toString(*existing) +
             "\n>>> defined as " + *existing->importModule + " in " +
             toString(existing->getFile()) + "\n>>> defined as " +
             *importModule + " in " + toString(file));
   }
+
+  // Update symbol binding, if the existing symbols is weak
+  uint32_t binding = existing->flags & WASM_SYMBOL_BINDING_MASK;
+  if (existing->isWeak() && binding != WASM_SYMBOL_BINDING_WEAK) {
+    existing->flags = (existing->flags & ~WASM_SYMBOL_BINDING_MASK) | binding;
+  }
 }
 
 Symbol *SymbolTable::addUndefinedFunction(StringRef name,
                                           Optional<StringRef> importName,
                                           Optional<StringRef> importModule,
                                           uint32_t flags, InputFile *file,
                                           const WasmSignature *sig,
                                           bool isCalledDirectly) {
   LLVM_DEBUG(dbgs() << "addUndefinedFunction: " << name << " ["
                     << (sig ? toString(*sig) : "none")
-                    << "] IsCalledDirectly:" << isCalledDirectly << "\n");
+                    << "] IsCalledDirectly:" << isCalledDirectly 
+                    << " flags=0x" << utohexstr(flags) << "\n");
   assert(flags & WASM_SYMBOL_UNDEFINED);
 
   Symbol *s;
   bool wasInserted;
   std::tie(s, wasInserted) = insert(name, file);
   if (s->traced)
     printTraceSymbolUndefined(name, file);
 
   auto replaceSym = [&]() {
     replaceSymbol<UndefinedFunction>(s, name, importName, importModule, flags,
                                      file, sig, isCalledDirectly);
   };
 
   if (wasInserted)
     replaceSym();
   else if (auto *lazy = dyn_cast<LazySymbol>(s))
     lazy->fetch();
   else {
     auto existingFunction = dyn_cast<FunctionSymbol>(s);
     if (!existingFunction) {
       reportTypeError(s, file, WASM_SYMBOL_TYPE_FUNCTION);
       return s;
     }
-    auto *existingUndefined = dyn_cast<UndefinedFunction>(existingFunction);
     if (!existingFunction->signature && sig)
       existingFunction->signature = sig;
+    auto *existingUndefined = dyn_cast<UndefinedFunction>(existingFunction);
     if (isCalledDirectly && !signatureMatches(existingFunction, sig)) {
       // If the existing undefined functions is not called direcltly then let
       // this one take precedence.  Otherwise the existing function is either
       // direclty called or defined, in which case we need a function variant.
       if (existingUndefined && !existingUndefined->isCalledDirectly)
         replaceSym();
       else if (getFunctionVariant(s, sig, file, &s))
         replaceSym();

 // equivalent of an undefined function and also handleSymbolVariants for
 // undefined functions that don't match the signature of the definition.
 InputFunction *SymbolTable::replaceWithUnreachable(Symbol *sym,
                                                    const WasmSignature &sig,
                                                    StringRef debugName) {
   auto *func = make<SyntheticFunction>(sig, sym->getName(), debugName);
   func->setBody(unreachableFn);
   syntheticFunctions.emplace_back(func);
-  replaceSymbol<DefinedFunction>(sym, sym->getName(), sym->getFlags(), nullptr,
+  replaceSymbol<DefinedFunction>(sym, sym->getName(), sym->flags, nullptr,
                                  func);
   return func;
 }
 
 // For weak undefined functions, there may be "call" instructions that reference
 // the symbol. In this case, we need to synthesise a dummy/stub function that
 // will abort at runtime, so that relocations can still provided an operand to
 // the call instruction that passes Wasm validation.

lld/wasm/Symbols.h

public:
   }
 
   // Returns the symbol name.
   StringRef getName() const { return name; }
 
   // Returns the file from which this symbol was created.
   InputFile *getFile() const { return file; }
 
-  uint32_t getFlags() const { return flags; }
-
   InputChunk *getChunk() const;
 
   // Indicates that the section or import for this symbol will be included in
   // the final image.
   bool isLive() const;
 
   // Marks the symbol's InputChunk as Live, so that it will be included in the
   // final image.
uint32_t getGOTIndex() const {
     return gotIndex;
   }
 
   void setGOTIndex(uint32_t index);
   bool hasGOTIndex() const { return gotIndex != INVALID_INDEX; }
 
 protected:
   Symbol(StringRef name, Kind k, uint32_t flags, InputFile *f)
-      : name(name), file(f), flags(flags), symbolKind(k),
+      : name(name), file(f), symbolKind(k),
         referenced(!config->gcSections), requiresGOT(false),
         isUsedInRegularObj(false), forceExport(false), canInline(false),
-        traced(false) {}
+        traced(false), flags(flags) {}
 
   StringRef name;
   InputFile *file;
-  uint32_t flags;
   uint32_t outputSymbolIndex = INVALID_INDEX;
   uint32_t gotIndex = INVALID_INDEX;
   Kind symbolKind;
 
 public:
   bool referenced : 1;
 
   // True for data symbols that needs a dummy GOT entry.  Used for static
public:
 
   // False if LTO shouldn't inline whatever this symbol points to. If a symbol
   // is overwritten after LTO, LTO shouldn't inline the symbol because it
   // doesn't know the final contents of the symbol.
   bool canInline : 1;
 
   // True if this symbol is specified by --trace-symbol option.
   bool traced : 1;
+
+  uint32_t flags;
 };
 
 class FunctionSymbol : public Symbol {
 public:
   static bool classof(const Symbol *s) {
     return s->kind() == DefinedFunctionKind ||
            s->kind() == UndefinedFunctionKind;
   }

lld/wasm/SyntheticSections.cpp

void LinkingSection::writeBody() {
 
   if (!symtabEntries.empty()) {
     SubSection sub(WASM_SYMBOL_TABLE);
     writeUleb128(sub.os, symtabEntries.size(), "num symbols");
 
     for (const Symbol *sym : symtabEntries) {
       assert(sym->isDefined() || sym->isUndefined());
       WasmSymbolType kind = sym->getWasmType();
-      uint32_t flags = sym->getFlags();
+      uint32_t flags = sym->flags;
 
       writeU8(sub.os, kind, "sym kind");
       writeUleb128(sub.os, flags, "sym flags");
 
       if (auto *f = dyn_cast<FunctionSymbol>(sym)) {
         writeUleb128(sub.os, f->getFunctionIndex(), "index");
         if (sym->isDefined() || (flags & WASM_SYMBOL_EXPLICIT_NAME) != 0)
           writeStr(sub.os, sym->getName(), "sym name");