[lld/COFF] Fix -start-lib / -end-lib more after reviews.llvm.org/D116434 (#124294)

This is a follow-up to #120452 in a way.

Since lld/COFF does not yet insert all defined in an obj file before all
undefineds (ELF and MachO do this, see #67445 and things linked from
there), it's possible that:

1. We add an obj file a.obj
2. a.obj contains an undefined that's in b.obj, causing b.obj to be
added
3. b.obj contains an undefined that's in a part of a.obj that's not yet
in the symbol table, causing a recursive load of a.obj, which adds the
symbols in there twice, leading to duplicate symbol errors.

For normal archives, `ArchiveFile::addMember()` has a `seen` check to
prevent this. For start-lib lazy objects, we can just check if the
archive is still lazy at the recursive call.

This bug is similar to issue #59162.

(Eventually, we'll probably want to do what the MachO and ELF ports do.)

Includes a test that caused duplicate symbol diagnostics before this
code change.

Author: nico

lld/COFF/InputFiles.cpp

@@ -151,6 +151,8 @@ void ArchiveFile::addMember(const Archive::Symbol &sym) {
                                  toCOFFString(symtab.ctx, sym));
 
   // Return an empty buffer if we have already returned the same buffer.
+  // FIXME: Remove this once we resolve all defineds before all undefineds in
+  //        ObjFile::initializeSymbols().
   if (!seen.insert(c.getChildOffset()).second)
     return;
 

lld/COFF/SymbolTable.cpp

@@ -56,6 +56,10 @@ static void forceLazy(Symbol *s) {
   }
   case Symbol::Kind::LazyObjectKind: {
     InputFile *file = cast<LazyObject>(s)->file;
+    // FIXME: Remove this once we resolve all defineds before all undefineds in
+    //        ObjFile::initializeSymbols().
+    if (!file->lazy)
+      return;
     file->lazy = false;
     file->symtab.ctx.driver.addFile(file);
     break;

lld/test/COFF/start-lib.ll

@@ -173,3 +173,72 @@ target triple = "x86_64-pc-windows-msvc"
 define void @baz() {
   ret void
 }
+
+
+; Check cycles between symbols in two /start-lib files.
+; If the links succeed and does not emit duplicate symbol diagnostics,
+; that's enough.
+
+; RUN: llc -filetype=obj %t.dir/main3.ll -o %t-main3.obj
+; RUN: llc -filetype=obj %t.dir/cycle1.ll -o %t-cycle1.obj
+; RUN: llc -filetype=obj %t.dir/cycle2.ll -o %t-cycle2.obj
+; RUN: opt -thinlto-bc %t.dir/main3.ll -o %t-main3.bc
+; RUN: opt -thinlto-bc %t.dir/cycle1.ll -o %t-cycle1.bc
+; RUN: opt -thinlto-bc %t.dir/cycle2.ll -o %t-cycle2.bc
+
+; RUN: lld-link -out:%t3.exe -entry:main \
+; RUN:     %t-main3.obj %t-cycle1.obj %t-cycle2.obj
+; RUN: lld-link -out:%t3.exe -entry:main \
+; RUN:     %t-main3.obj /start-lib %t-cycle1.obj %t-cycle2.obj /end-lib
+; RUN: lld-link -out:%t3.exe -entry:main \
+; RUN:     /start-lib %t-cycle1.obj %t-cycle2.obj /end-lib %t-main3.obj
+
+; RUN: lld-link -out:%t3.exe -entry:main \
+; RUN:     %t-main3.bc %t-cycle1.bc %t-cycle2.bc
+; RUN: lld-link -out:%t3.exe -entry:main \
+; RUN:     %t-main3.bc /start-lib %t-cycle1.bc %t-cycle2.bc /end-lib
+; RUN: lld-link -out:%t3.exe -entry:main \
+; RUN:     /start-lib %t-cycle1.bc %t-cycle2.bc /end-lib %t-main3.bc
+
+#--- main3.ll
+
+target datalayout = "e-m:w-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-pc-windows-msvc"
+
+declare void @foo1()
+
+define void @main() {
+  call void () @foo1()
+  ret void
+}
+
+#--- cycle1.ll
+
+target datalayout = "e-m:w-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-pc-windows-msvc"
+
+declare void @bar()
+
+define void @foo1() {
+  ; cycle1.ll pulls in cycle2.ll for bar(), and cycle2.ll then pulls in
+  ; cycle1.ll again for foo2().
+  call void () @bar()
+  ret void
+}
+
+define void @foo2() {
+  ret void
+}
+
+
+#--- cycle2.ll
+
+target datalayout = "e-m:w-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-pc-windows-msvc"
+
+declare void @foo2()
+
+define void @bar() {
+  call void () @foo2()
+  ret void
+}