[lld][WebAssembly] Create optional internal symbols only after LTO ob…

…ject as been added

This is important for the cases where new symbols can be introduced
during LTO.   Specifically this happens for during TLS-lowering where
references to `__tls_base` can be introduced.

Fixes: emscripten-core/emscripten#12489

Differential Revision: https://reviews.llvm.org/D111171

lld/test/wasm/lto/tls.ll

@@ -0,0 +1,45 @@
+; Test that LTO objects build with TLS and threading support can be linked into
+; a single threaded binary.  Specifically the references to `__tls_base` that
+; can be generated at LTO-time need to trigger the creation of the internal/fake
+; `__tls_base` symbol in the linker.
+
+; RUN: llvm-as %s -o %t1.o
+; RUN: wasm-ld --export=tls_int --export=get_tls %t1.o -o %t
+; RUN: obj2yaml %t | FileCheck %s
+
+target datalayout = "e-m:e-p:32:32-i64:64-f128:64-n32:64-S128-ni:1:10:20"
+target triple = "wasm32-unknown-emscripten"
+
+@tls_int = dso_local thread_local global i32 99
+
+define i32 @get_tls() #0 {
+  %val = load i32, i32* @tls_int
+  ret i32 %val
+}
+
+define void @_start() #0 {
+  ret void
+}
+
+attributes #0 = { noinline nounwind optnone "target-features"="+atomics,+bulk-memory,+mutable-globals,+sign-ext" }
+
+;      CHECK:   - Type:            GLOBAL
+; CHECK-NEXT:     Globals:
+; CHECK-NEXT:       - Index:           0
+; CHECK-NEXT:         Type:            I32
+; CHECK-NEXT:         Mutable:         true
+; CHECK-NEXT:         InitExpr:
+; CHECK-NEXT:           Opcode:          I32_CONST
+; CHECK-NEXT:           Value:           66576
+; CHECK-NEXT:       - Index:           1
+; CHECK-NEXT:         Type:            I32
+; CHECK-NEXT:         Mutable:         false
+; CHECK-NEXT:         InitExpr:
+; CHECK-NEXT:           Opcode:          I32_CONST
+; CHECK-NEXT:           Value:           1024
+
+;      CHECK:     GlobalNames:
+; CHECK-NEXT:       - Index:           0
+; CHECK-NEXT:         Name:            __stack_pointer
+; CHECK-NEXT:       - Index:           1
+; CHECK-NEXT:         Name:            __tls_base

lld/wasm/Driver.cpp

@@ -947,8 +947,6 @@ void LinkerDriver::linkerMain(ArrayRef<const char *> argsArr) {
     }
   }
 
-  createOptionalSymbols();
-
   if (errorCount())
     return;
 
@@ -976,6 +974,8 @@ void LinkerDriver::linkerMain(ArrayRef<const char *> argsArr) {
   if (errorCount())
     return;
 
+  createOptionalSymbols();
+
   // Resolve any variant symbols that were created due to signature
   // mismatchs.
   symtab->handleSymbolVariants();

lld/wasm/SymbolTable.cpp

@@ -258,11 +258,11 @@ DefinedGlobal *SymbolTable::addSyntheticGlobal(StringRef name, uint32_t flags,
 
 DefinedGlobal *SymbolTable::addOptionalGlobalSymbol(StringRef name,
                                                     InputGlobal *global) {
-  LLVM_DEBUG(dbgs() << "addOptionalGlobalSymbol: " << name << " -> " << global
-                    << "\n");
   Symbol *s = find(name);
   if (!s || s->isDefined())
     return nullptr;
+  LLVM_DEBUG(dbgs() << "addOptionalGlobalSymbol: " << name << " -> " << global
+                    << "\n");
   syntheticGlobals.emplace_back(global);
   return replaceSymbol<DefinedGlobal>(s, name, WASM_SYMBOL_VISIBILITY_HIDDEN,
                                       nullptr, global);