[WebAssembly] Don't remove function signatures in ThinLTOBitcodeWriter

llvm/lib/Transforms/IPO/ThinLTOBitcodeWriter.cpp

@@ -178,28 +178,29 @@ void simplifyExternals(Module &M) {
   FunctionType *EmptyFT =
       FunctionType::get(Type::getVoidTy(M.getContext()), false);
 
-  for (Function &F : llvm::make_early_inc_range(M)) {
-    if (F.isDeclaration() && F.use_empty()) {
+  for (Function &F : llvm::make_early_inc_range(M))
+    if (F.isDeclaration() && F.use_empty())
       F.eraseFromParent();
-      continue;
-    }
 
-    if (!F.isDeclaration() || F.getFunctionType() == EmptyFT ||
-        // Changing the type of an intrinsic may invalidate the IR.
-        F.getName().starts_with("llvm."))
-      continue;
+  // Wasm should preserve function signatures for the linker.
+  if (!Triple(M.getTargetTriple()).isWasm()) {
+    for (Function &F : llvm::make_early_inc_range(M)) {
+      if (!F.isDeclaration() || F.getFunctionType() == EmptyFT ||
+          // Changing the type of an intrinsic may invalidate the IR.
+          F.getName().starts_with("llvm."))
+        continue;
 
-    Function *NewF =
-        Function::Create(EmptyFT, GlobalValue::ExternalLinkage,
-                         F.getAddressSpace(), "", &M);
-    NewF->copyAttributesFrom(&F);
-    // Only copy function attribtues.
-    NewF->setAttributes(AttributeList::get(M.getContext(),
-                                           AttributeList::FunctionIndex,
-                                           F.getAttributes().getFnAttrs()));
-    NewF->takeName(&F);
-    F.replaceAllUsesWith(NewF);
-    F.eraseFromParent();
+      Function *NewF = Function::Create(EmptyFT, GlobalValue::ExternalLinkage,
+                                        F.getAddressSpace(), "", &M);
+      NewF->copyAttributesFrom(&F);
+      // Only copy function attribtues.
+      NewF->setAttributes(AttributeList::get(M.getContext(),
+                                             AttributeList::FunctionIndex,
+                                             F.getAttributes().getFnAttrs()));
+      NewF->takeName(&F);
+      F.replaceAllUsesWith(NewF);
+      F.eraseFromParent();
+    }
   }
 
   for (GlobalIFunc &I : llvm::make_early_inc_range(M.ifuncs())) {

llvm/test/ThinLTO/WebAssembly/lit.local.cfg

@@ -0,0 +1,2 @@
+if not "WebAssembly" in config.root.targets:
+    config.unsupported = True

llvm/test/ThinLTO/WebAssembly/thinlto-split-lto-unit.ll

@@ -0,0 +1,29 @@
+; RUN: opt -thinlto-bc -thinlto-split-lto-unit %s -o %t.o
+; RUN: llvm-modextract -b -n 0 %t.o -o - | llvm-dis | FileCheck %s --check-prefix=UNIT0
+; RUN: llvm-modextract -b -n 1 %t.o -o - | llvm-dis | FileCheck %s --check-prefix=UNIT1
+
+target datalayout = "e-m:e-p:32:32-p10:8:8-p20:8:8-i64:64-n32:64-S128-ni:1:10:20"
+target triple = "wasm32-unknown-unknown"
+
+; After split LTO unit generation, @test_gv will move to UNIT1 (because it has a
+; type metadata), and because it refers to @test, @test will be promoted by
+; adding its module ID hash to its name, and .lto_set_conditional directive will
+; be written in Unit 0.
+
+; UNIT0: module asm ".lto_set_conditional test,test.{{[0-9a-f]+}}"
+; UNIT0: define hidden i32 @test.{{[0-9a-f]+}}()
+
+; Unit 1 will contain @test's declaration. The normal ThinLTO split bitcode
+; writing removes the signatures from the split unit's declaration, but in Wasm
+; you should not do this in order to avoid function signature mismatch in
+; linker.
+
+; UNIT1: declare hidden i32 @test.{{[0-9a-f]+}}()
+
+@test_gv = constant ptr @test, align 4, !type !0
+
+define internal i32 @test() {
+  ret i32 0
+}
+
+!0 = !{i64 0, !{}}