Index: test/wasm/cxx-mangling.ll
===================================================================
--- test/wasm/cxx-mangling.ll
+++ test/wasm/cxx-mangling.ll
@@ -0,0 +1,66 @@
+; RUN: llc -filetype=obj %s -o %t.o
+; RUN: wasm-ld --demangle --check-signatures -o %t_demangle.wasm %t.o
+; RUN: obj2yaml %t_demangle.wasm | FileCheck %s
+; RUN: wasm-ld --no-demangle --check-signatures -o %t_nodemangle.wasm %t.o
+; RUN: obj2yaml %t_nodemangle.wasm | FileCheck %s
+
+target triple = "wasm32-unknown-unknown-wasm"
+
+; Check that the EXPORT name is still mangled, but that the "name" custom
+; section contains the unmangled name.
+
+define void @_Z3fooi(i32 %arg) {
+  ret void
+}
+
+declare extern_weak void @_Z3bari(i32 %arg)
+
+define void @_start() {
+  call void @_Z3fooi(i32 1)
+  call void @_Z3bari(i32 1)
+  ret void
+}
+
+; CHECK:        - Type:            EXPORT
+; CHECK-NEXT:     Exports:
+; CHECK-NEXT:       - Name:            memory
+; CHECK-NEXT:         Kind:            MEMORY
+; CHECK-NEXT:         Index:           0
+; CHECK-NEXT:       - Name:            __heap_base
+; CHECK-NEXT:         Kind:            GLOBAL
+; CHECK-NEXT:         Index:           1
+; CHECK-NEXT:       - Name:            __data_end
+; CHECK-NEXT:         Kind:            GLOBAL
+; CHECK-NEXT:         Index:           2
+; CHECK-NEXT:       - Name:            _start
+; CHECK-NEXT:         Kind:            FUNCTION
+; CHECK-NEXT:         Index:           3
+; CHECK-NEXT:       - Name:            _Z3fooi
+; CHECK-NEXT:         Kind:            FUNCTION
+; CHECK-NEXT:         Index:           2
+; CHECK-NEXT:   - Type:            CODE
+; CHECK-NEXT:     Functions:
+; CHECK-NEXT:       - Index:           0
+; CHECK-NEXT:         Locals:
+; CHECK-NEXT:         Body:            0B
+; CHECK-NEXT:       - Index:           1
+; CHECK-NEXT:         Locals:
+; CHECK-NEXT:         Body:            000B
+; CHECK-NEXT:       - Index:           2
+; CHECK-NEXT:         Locals:
+; CHECK-NEXT:         Body:            0B
+; CHECK-NEXT:       - Index:           3
+; CHECK-NEXT:         Locals:
+; CHECK-NEXT:         Body:            410110828080800041011081808080000B
+; CHECK-NEXT:   - Type:            CUSTOM
+; CHECK-NEXT:     Name:            name
+; CHECK-NEXT:     FunctionNames:
+; CHECK-NEXT:       - Index:           0
+; CHECK-NEXT:         Name:            __wasm_call_ctors
+; CHECK-NEXT:       - Index:           1
+; CHECK-NEXT:         Name:            'undefined function bar(int)'
+; CHECK-NEXT:       - Index:           2
+; CHECK-NEXT:         Name:            'foo(int)'
+; CHECK-NEXT:       - Index:           3
+; CHECK-NEXT:         Name:            _start
+; CHECK-NEXT: ...
Index: wasm/Driver.cpp
===================================================================
--- wasm/Driver.cpp
+++ wasm/Driver.cpp
@@ -17,6 +17,7 @@
 #include "lld/Common/Args.h"
 #include "lld/Common/ErrorHandler.h"
 #include "lld/Common/Memory.h"
+#include "lld/Common/Strings.h"
 #include "lld/Common/Threads.h"
 #include "lld/Common/Version.h"
 #include "llvm/ADT/Twine.h"
@@ -239,7 +240,10 @@
 
     // Add a synthetic dummy for weak undefined functions.  These dummies will
     // be GC'd if not used as the target of any "call" instructions.
-    StringRef StubName = Saver.save("undefined function " + toString(*Sym));
+    Optional<std::string> SymName = demangleItanium(Sym->getName());
+    StringRef StubName =
+        Saver.save("undefined function " +
+                   (SymName ? StringRef(*SymName) : Sym->getName()));
     SyntheticFunction *Func = make<SyntheticFunction>(Sig, StubName);
     Func->setBody(UnreachableFn);
     // Ensure it compares equal to the null pointer, and so that table relocs
Index: wasm/Writer.cpp
===================================================================
--- wasm/Writer.cpp
+++ wasm/Writer.cpp
@@ -534,13 +534,15 @@
   for (const Symbol *S : ImportedSymbols) {
     if (auto *F = dyn_cast<FunctionSymbol>(S)) {
       writeUleb128(Sub.OS, F->getFunctionIndex(), "func index");
-      writeStr(Sub.OS, F->getName(), "symbol name");
+      Optional<std::string> Name = demangleItanium(F->getName());
+      writeStr(Sub.OS, Name ? StringRef(*Name) : F->getName(), "symbol name");
     }
   }
   for (const InputFunction *F : InputFunctions) {
     if (!F->getName().empty()) {
       writeUleb128(Sub.OS, F->getFunctionIndex(), "func index");
-      writeStr(Sub.OS, F->getName(), "symbol name");
+      Optional<std::string> Name = demangleItanium(F->getName());
+      writeStr(Sub.OS, Name ? StringRef(*Name) : F->getName(), "symbol name");
     }
   }