[WebAssembly] Fix undefined weak function symbols in LTO builds

[sbc100]

Summary

Fixes PR40219

[ruiu]

LGTM

lld/trunk/test/wasm/lto/weak-undefined.ll

+; RUN: llvm-as %s -o %t.o
+; RUN: wasm-ld %t.o -o %t.wasm
+; RUN: obj2yaml %t.wasm | FileCheck %s
+
+; Test that undefined weak external functions are handled in the LTO case
+; We had a bug where stub function generation was failing because functions
+; that are in bitcode (pre-LTO) don't have signatures assigned.
+
+target datalayout = "e-m:e-p:32:32-i64:64-n32:64-S128"
+target triple = "wasm32-unknown-unknown"
+
+declare extern_weak i32 @foo()
+
+define void @_start() #0 {
+entry:
+    %call2 = call i32 @foo()
+    ret void
+}
+
+; CHECK: Name:            undefined function foo

lld/trunk/wasm/Driver.cpp

if (Sym && Sym->isDefined())
       Sym->ForceExport = true;
     else if (!Config->AllowUndefined)
       error(Twine("symbol exported via --export not found: ") +
             Arg->getValue());
   }
 
   Symbol *EntrySym = nullptr;
   if (!Config->Relocatable) {
-    // Add synthetic dummies for weak undefined functions.
-    handleWeakUndefines();
-
     if (!Config->Shared && !Config->Entry.empty()) {
       EntrySym = handleUndefined(Config->Entry);
       if (EntrySym && EntrySym->isDefined())
         EntrySym->ForceExport = true;
       else
         error("entry symbol not defined (pass --no-entry to supress): " +
               Config->Entry);
     }
 
     // Make sure we have resolved all symbols.
     if (!Config->AllowUndefined)
       Symtab->reportRemainingUndefines();
   }
 
   if (errorCount())
     return;
 
   // Do link-time optimization if given files are LLVM bitcode files.
   // This compiles bitcode files into real object files.
   Symtab->addCombinedLTOObject();
   if (errorCount())
     return;
 
+  // Add synthetic dummies for weak undefined functions.  Must happen
+  // after LTO otherwise functions may not yet have signatures.
+  if (!Config->Relocatable)
+    handleWeakUndefines();
+
   if (EntrySym)
     EntrySym->setHidden(false);
 
   if (errorCount())
     return;
 
   // Do size optimizations: garbage collection
   markLive();
 
   // Write the result to the file.
   writeResult();
 }