diff --git a/lld/wasm/Driver.cpp b/lld/wasm/Driver.cpp
@@ -280,6 +280,12 @@ void LinkerDriver::addFile(StringRef path) {
   case file_magic::wasm_object:
     files.push_back(createObjectFile(mbref));
     break;
+  case file_magic::unknown:
+    if (mbref.getBuffer().starts_with("#STUB\n")) {
+      files.push_back(make<StubFile>(mbref));
+      break;
+    }
+    [[fallthrough]];
   default:
     error("unknown file type: " + mbref.getBufferIdentifier());
   }
@@ -450,6 +456,7 @@ static void readConfigs(opt::InputArgList &args) {
       parseCachePruningPolicy(args.getLastArgValue(OPT_thinlto_cache_policy)),
       "--thinlto-cache-policy: invalid cache policy");
   config->unresolvedSymbols = getUnresolvedSymbolPolicy(args);
+  config->whyExtract = args.getLastArgValue(OPT_why_extract);
   errorHandler().verbose = args.hasArg(OPT_verbose);
   LLVM_DEBUG(errorHandler().verbose = true);
 
@@ -631,7 +638,7 @@ static const char *getReproduceOption(opt::InputArgList &args) {
 }
 
 // Force Sym to be entered in the output. Used for -u or equivalent.
-static Symbol *handleUndefined(StringRef name) {
+static Symbol *handleUndefined(StringRef name, const char *option) {
   Symbol *sym = symtab->find(name);
   if (!sym)
     return nullptr;
@@ -640,8 +647,11 @@ static Symbol *handleUndefined(StringRef name) {
   // eliminate it. Mark the symbol as "used" to prevent it.
   sym->isUsedInRegularObj = true;
 
-  if (auto *lazySym = dyn_cast<LazySymbol>(sym))
+  if (auto *lazySym = dyn_cast<LazySymbol>(sym)) {
     lazySym->fetch();
+    if (!config->whyExtract.empty())
+      config->whyExtractRecords.emplace_back(option, sym->getFile(), *sym);
+  }
 
   return sym;
 }
@@ -653,8 +663,31 @@ static void handleLibcall(StringRef name) {
 
   if (auto *lazySym = dyn_cast<LazySymbol>(sym)) {
     MemoryBufferRef mb = lazySym->getMemberBuffer();
-    if (isBitcode(mb))
+    if (isBitcode(mb)) {
+      if (!config->whyExtract.empty())
+        config->whyExtractRecords.emplace_back("<libcall>", sym->getFile(),
+                                               *sym);
       lazySym->fetch();
+    }
+  }
+}
+
+static void writeWhyExtract() {
+  if (config->whyExtract.empty())
+    return;
+
+  std::error_code ec;
+  raw_fd_ostream os(config->whyExtract, ec, sys::fs::OF_None);
+  if (ec) {
+    error("cannot open --why-extract= file " + config->whyExtract + ": " +
+          ec.message());
+    return;
+  }
+
+  os << "reference\textracted\tsymbol\n";
+  for (auto &entry : config->whyExtractRecords) {
+    os << std::get<0>(entry) << '\t' << toString(std::get<1>(entry)) << '\t'
+       << toString(std::get<2>(entry)) << '\n';
   }
 }
 
@@ -807,6 +840,53 @@ static void createOptionalSymbols() {
     WasmSym::tlsBase = createOptionalGlobal("__tls_base", false);
 }
 
+static void processStubLibraries() {
+  log("-- processStubLibraries");
+  for (auto &stub_file : symtab->stubFiles) {
+    LLVM_DEBUG(llvm::dbgs()
+               << "processing stub file: " << stub_file->getName() << "\n");
+    for (auto [name, deps]: stub_file->symbolDependencies) {
+      auto* sym = symtab->find(name);
+      if (!sym || !sym->isUndefined() || !sym->isUsedInRegularObj ||
+          sym->forceImport) {
+        LLVM_DEBUG(llvm::dbgs() << "stub not in needed: " << name << "\n");
+        continue;
+      }
+      // The first stub library to define a given symbol sets this and
+      // definitions in later stub libraries are ignored.
+      sym->forceImport = true;
+      if (sym->traced)
+        message(toString(stub_file) + ": importing " + name);
+      else
+        LLVM_DEBUG(llvm::dbgs()
+                   << toString(stub_file) << ": importing " << name << "\n");
+      for (const auto dep : deps) {
+        auto* needed = symtab->find(dep);
+        if (!needed) {
+          error(toString(stub_file) + ": undefined symbol: " + dep +
+                ". Required by " + toString(*sym));
+        } else if (needed->isUndefined()) {
+          error(toString(stub_file) +
+                ": undefined symbol: " + toString(*needed) +
+                ". Required by " + toString(*sym));
+        } else {
+          LLVM_DEBUG(llvm::dbgs()
+                     << "force export: " << toString(*needed) << "\n");
+          needed->forceExport = true;
+          needed->isUsedInRegularObj = true;
+          if (auto *lazy = dyn_cast<LazySymbol>(needed)) {
+            lazy->fetch();
+            if (!config->whyExtract.empty())
+              config->whyExtractRecords.emplace_back(stub_file->getName(),
+                                                     sym->getFile(), *sym);
+          }
+        }
+      }
+    }
+  }
+  log("-- done processStubLibraries");
+}
+
 // Reconstructs command line arguments so that so that you can re-run
 // the same command with the same inputs. This is for --reproduce.
 static std::string createResponseFile(const opt::InputArgList &args) {
@@ -1035,16 +1115,16 @@ void LinkerDriver::linkerMain(ArrayRef<const char *> argsArr) {
 
   // Handle the `--undefined <sym>` options.
   for (auto *arg : args.filtered(OPT_undefined))
-    handleUndefined(arg->getValue());
+    handleUndefined(arg->getValue(), "<internal>");
 
   // Handle the `--export <sym>` options
   // This works like --undefined but also exports the symbol if its found
   for (auto &iter : config->exportedSymbols)
-    handleUndefined(iter.first());
+    handleUndefined(iter.first(), "--export");
 
   Symbol *entrySym = nullptr;
   if (!config->relocatable && !config->entry.empty()) {
-    entrySym = handleUndefined(config->entry);
+    entrySym = handleUndefined(config->entry, "--entry");
     if (entrySym && entrySym->isDefined())
       entrySym->forceExport = true;
     else
@@ -1061,7 +1141,8 @@ void LinkerDriver::linkerMain(ArrayRef<const char *> argsArr) {
       !WasmSym::callCtors->isUsedInRegularObj &&
       WasmSym::callCtors->getName() != config->entry &&
       !config->exportedSymbols.count(WasmSym::callCtors->getName())) {
-    if (Symbol *callDtors = handleUndefined("__wasm_call_dtors")) {
+    if (Symbol *callDtors =
+            handleUndefined("__wasm_call_dtors", "<internal>")) {
       if (auto *callDtorsFunc = dyn_cast<DefinedFunction>(callDtors)) {
         if (callDtorsFunc->signature &&
             (!callDtorsFunc->signature->Params.empty() ||
@@ -1096,12 +1177,16 @@ void LinkerDriver::linkerMain(ArrayRef<const char *> argsArr) {
   if (errorCount())
     return;
 
+  writeWhyExtract();
+
   // Do link-time optimization if given files are LLVM bitcode files.
   // This compiles bitcode files into real object files.
   symtab->compileBitcodeFiles();
   if (errorCount())
     return;
 
+  processStubLibraries();
+
   createOptionalSymbols();
 
   // Resolve any variant symbols that were created due to signature

diff --git a/lld/wasm/InputFiles.cpp b/lld/wasm/InputFiles.cpp
@@ -12,6 +12,7 @@
 #include "InputElement.h"
 #include "OutputSegment.h"
 #include "SymbolTable.h"
+#include "lld/Common/Args.h"
 #include "lld/Common/CommonLinkerContext.h"
 #include "lld/Common/Reproduce.h"
 #include "llvm/Object/Binary.h"
@@ -678,6 +679,48 @@ Symbol *ObjFile::createUndefined(const WasmSymbol &sym, bool isCalledDirectly) {
   llvm_unreachable("unknown symbol kind");
 }
 
+
+StringRef strip(StringRef s) {
+  while (s.starts_with(" ")) {
+    s = s.drop_front();
+  }
+  while (s.ends_with(" ")) {
+    s = s.drop_back();
+  }
+  return s;
+}
+
+void StubFile::parse() {
+  bool first = false;
+
+  for (StringRef line : args::getLines(mb)) {
+    // File must begin with #STUB
+    if (first) {
+      assert(line == "#STUB\n");
+      first = false;
+    }
+
+    // Lines starting with # are considered comments
+    if (line.startswith("#"))
+      continue;
+
+    StringRef sym;
+    StringRef rest;
+    std::tie(sym, rest) = line.split(':');
+    sym = strip(sym);
+    rest = strip(rest);
+
+    symbolDependencies[sym] = {};
+
+    while (rest.size()) {
+      StringRef first;
+      std::tie(first, rest) = rest.split(',');
+      first = strip(first);
+      symbolDependencies[sym].push_back(first);
+    }
+  }
+}
+
 void ArchiveFile::parse() {
   // Parse a MemoryBufferRef as an archive file.
   LLVM_DEBUG(dbgs() << "Parsing library: " << toString(this) << "\n");

diff --git a/lld/wasm/InputFiles.h b/lld/wasm/InputFiles.h
@@ -47,6 +47,7 @@ class InputFile {
     SharedKind,
     ArchiveKind,
     BitcodeKind,
+    StubKind,
   };
 
   virtual ~InputFile() {}
@@ -183,6 +184,18 @@ class BitcodeFile : public InputFile {
   static bool doneLTO;
 };
 
+// Stub libray (See docs/WebAssembly.rst)
+class StubFile : public InputFile {
+public:
+  explicit StubFile(MemoryBufferRef m) : InputFile(StubKind, m) {}
+
+  static bool classof(const InputFile *f) { return f->kind() == StubKind; }
+
+  void parse();
+
+  llvm::DenseMap<StringRef, std::vector<StringRef>> symbolDependencies;
+};
+
 inline bool isBitcode(MemoryBufferRef mb) {
   return identify_magic(mb.getBuffer()) == llvm::file_magic::bitcode;
 }

diff --git a/lld/wasm/Options.td b/lld/wasm/Options.td
@@ -221,6 +221,8 @@ defm whole_archive: B<"whole-archive",
     "Force load of all members in a static library",
     "Do not force load of all members in a static library (default)">;
 
+def why_extract: JJ<"why-extract=">, HelpText<"Print to a file about why archive members are extracted">;
+
 defm check_features: BB<"check-features",
     "Check feature compatibility of linked objects (default)",
     "Ignore feature compatibility of linked objects">;

diff --git a/lld/wasm/Relocations.cpp b/lld/wasm/Relocations.cpp
@@ -33,9 +33,9 @@ static bool requiresGOTAccess(const Symbol *sym) {
 }
 
 static bool allowUndefined(const Symbol* sym) {
-  // Symbols with explicit import names are always allowed to be undefined at
+  // Symbols that are explicitly imported are always allowed to be undefined at
   // link time.
-  if (sym->importName)
+  if (sym->isImported())
     return true;
   if (isa<UndefinedFunction>(sym) && config->importUndefined)
     return true;

diff --git a/lld/wasm/SymbolTable.cpp b/lld/wasm/SymbolTable.cpp
@@ -39,6 +39,13 @@ void SymbolTable::addFile(InputFile *file) {
     return;
   }
 
+  // stub file
+  if (auto *f = dyn_cast<StubFile>(file)) {
+    f->parse();
+    stubFiles.push_back(f);
+    return;
+  }
+
   if (config->trace)
     message(toString(file));
 
@@ -524,6 +531,9 @@ Symbol *SymbolTable::addUndefinedFunction(StringRef name,
       lazy->signature = sig;
     } else {
       lazy->fetch();
+      if (!config->whyExtract.empty())
+        config->whyExtractRecords.emplace_back(toString(file), s->getFile(),
+                                               *s);
     }
   } else {
     auto existingFunction = dyn_cast<FunctionSymbol>(s);
@@ -748,7 +758,10 @@ void SymbolTable::addLazy(ArchiveFile *file, const Archive::Symbol *sym) {
   }
 
   LLVM_DEBUG(dbgs() << "replacing existing undefined\n");
+  const InputFile *oldFile = s->getFile();
   file->addMember(sym);
+  if (!config->whyExtract.empty())
+    config->whyExtractRecords.emplace_back(toString(oldFile), s->getFile(), *s);
 }
 
 bool SymbolTable::addComdat(StringRef name) {

diff --git a/lld/wasm/SymbolTable.h b/lld/wasm/SymbolTable.h
@@ -103,6 +103,7 @@ class SymbolTable {
   DefinedFunction *createUndefinedStub(const WasmSignature &sig);
 
   std::vector<ObjFile *> objectFiles;
+  std::vector<StubFile *> stubFiles;
   std::vector<SharedFile *> sharedFiles;
   std::vector<BitcodeFile *> bitcodeFiles;
   std::vector<InputFunction *> syntheticFunctions;

diff --git a/lld/wasm/Symbols.cpp b/lld/wasm/Symbols.cpp
@@ -220,6 +220,10 @@ void Symbol::setHidden(bool isHidden) {
     flags |= WASM_SYMBOL_VISIBILITY_DEFAULT;
 }
 
+bool Symbol::isImported() const {
+  return isUndefined() && (importName.has_value() || forceImport);
+}
+
 bool Symbol::isExported() const {
   if (!isDefined() || isLocal())
     return false;

diff --git a/lld/wasm/Symbols.h b/lld/wasm/Symbols.h
@@ -114,6 +114,7 @@ class Symbol {
   void setOutputSymbolIndex(uint32_t index);
 
   WasmSymbolType getWasmType() const;
+  bool isImported() const;
   bool isExported() const;
   bool isExportedExplicit() const;
 
@@ -135,7 +136,8 @@ class Symbol {
   Symbol(StringRef name, Kind k, uint32_t flags, InputFile *f)
       : name(name), file(f), symbolKind(k), referenced(!config->gcSections),
         requiresGOT(false), isUsedInRegularObj(false), forceExport(false),
-        canInline(false), traced(false), isStub(false), flags(flags) {}
+        forceImport(false), canInline(false), traced(false), isStub(false),
+        flags(flags) {}
 
   StringRef name;
   InputFile *file;
@@ -160,6 +162,8 @@ class Symbol {
   // -e/--export command line flag)
   bool forceExport : 1;
 
+  bool forceImport : 1;
+
   // 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.
@@ -656,6 +660,7 @@ T *replaceSymbol(Symbol *s, ArgT &&... arg) {
   T *s2 = new (s) T(std::forward<ArgT>(arg)...);
   s2->isUsedInRegularObj = symCopy.isUsedInRegularObj;
   s2->forceExport = symCopy.forceExport;
+  s2->forceImport = symCopy.forceImport;
   s2->canInline = symCopy.canInline;
   s2->traced = symCopy.traced;
   s2->referenced = symCopy.referenced;

diff --git a/lld/wasm/Writer.cpp b/lld/wasm/Writer.cpp
@@ -647,7 +647,7 @@ static bool shouldImport(Symbol *sym) {
   if (config->allowUndefinedSymbols.count(sym->getName()) != 0)
     return true;
 
-  return sym->importName.has_value();
+  return sym->isImported();
 }
 
 void Writer::calculateImports() {
@@ -1570,7 +1570,7 @@ void Writer::run() {
       sym->forceExport = true;
   }
 
-  // Delay reporting error about explicit exports until after
+  // Delay reporting errors about explicit exports until after
   // addStartStopSymbols which can create optional symbols.
   for (auto &name : config->requiredExports) {
     Symbol *sym = symtab->find(name);