[WebAssembly] Default to a reasonable signature in WebAssemblyAddMissingPrototypes

[sbc100]

Summary

Previously if we couldn't derive a prototype for a "no-prototype"
function from C we would leave it as is:

void foo(...)

With this change we instead give is an empty signature and remove
the "no-prototype" attribute.

This fixes the current wasm waterfall test failure.

[dschuff]

LGTM

llvm/lib/Target/WebAssembly/WebAssemblyAddMissingPrototypes.cpp

for (Function &F : M) {
     if (F.getFunctionType()->getNumParams() != 0)
       report_fatal_error(
           "Functions with 'no-prototype' attribute should not have params: " +
           F.getName());
 
     // Create a function prototype based on the first call site (first bitcast)
     // that we find.
     FunctionType *NewType = nullptr;
-    Function *NewF = nullptr;
     for (Use &U : F.uses()) {
       LLVM_DEBUG(dbgs() << "prototype-less use: " << F.getName() << "\n");
       LLVM_DEBUG(dbgs() << *U.getUser() << "\n");
       if (auto *BC = dyn_cast<BitCastOperator>(U.getUser())) {
         if (auto *DestType = dyn_cast<FunctionType>(
                 BC->getDestTy()->getPointerElementType())) {
           if (!NewType) {
             // Create a new function with the correct type
             NewType = DestType;
             LLVM_DEBUG(dbgs() << "found function type: " << *NewType << "\n");
-            NewF = Function::Create(NewType, F.getLinkage(), F.getName() + ".fixed_sig");
-            NewF->setAttributes(F.getAttributes());
-            NewF->removeFnAttr("no-prototype");
-            Replacements.emplace_back(&F, NewF);
           } else if (NewType != DestType) {
             errs() << "warning: prototype-less function used with "
                       "conflicting signatures: "
                    << F.getName() << "\n";
             LLVM_DEBUG(dbgs() << "  " << *DestType << "\n");
             LLVM_DEBUG(dbgs() << "  "<<  *NewType << "\n");
           }
         }
       }
     }
 
     if (!NewType) {
       LLVM_DEBUG(
           dbgs() << "could not derive a function prototype from usage: " +
                         F.getName() + "\n");
-      continue;
+      // We could not derive a type for this function.  In this case strip
+      // the isVarArg and make it a simple zero-arg function.  This has more
+      // chance of being correct.  The current signature of (...) is illegal in
+      // C since it doesn't have any arguments before the "...", we this at
+      // least makes it possible for this symbol to be resolved by the linker.
+      NewType = FunctionType::get(F.getFunctionType()->getReturnType(), false);
     }
+
+    Function *NewF =
+        Function::Create(NewType, F.getLinkage(), F.getName() + ".fixed_sig");
+    NewF->setAttributes(F.getAttributes());
+    NewF->removeFnAttr("no-prototype");
+    Replacements.emplace_back(&F, NewF);
   }
 
   for (auto &Pair : Replacements) {
     Function *OldF = Pair.first;
     Function *NewF = Pair.second;
     std::string Name = OldF->getName();
     M.getFunctionList().push_back(NewF);
     OldF->replaceAllUsesWith(
         ConstantExpr::getPointerBitCastOrAddrSpaceCast(NewF, OldF->getType()));
     OldF->eraseFromParent();
     NewF->setName(Name);
   }
 
   return !Replacements.empty();
 }

llvm/test/CodeGen/WebAssembly/add-prototypes.ll

 ; CHECK: call void @func_param(i64 (...)* bitcast (i64 (i32)* @foo to i64 (...)*))
 define void @as_paramater() {
   call void @func_param(i64 (...)* @foo)
   ret void
 }
 
 declare void @func_param(i64 (...)*)
 
+; CHECK: declare void @func_not_called()
+declare void @func_not_called(...) #1
+
 ; CHECK: declare extern_weak i64 @foo(i32)
 declare extern_weak i64 @foo(...) #1
 
 ; CHECK-NOT: attributes {{.*}} = { {{.*}}"no-prototype"{{.*}} }
 attributes #1 = { "no-prototype" }