[lld] [llvm] WIP: [LLD][COFF] Make unresolved symbol search behavior compliant with MSVC link.exe (PR #85290)

[via llvm-commits]

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@@ -562,25 +565,120 @@ std::pair<Symbol *, bool> SymbolTable::insert(StringRef name, InputFile *file) {
   return result;
 }
 
+static LazyIntrusiveNode *lazyNode(Symbol *s) {
+  if (auto *sym = dyn_cast<LazyArchive>(s))
+    return &sym->node;
+  if (auto *sym = dyn_cast<LazyObject>(s))
+    return &sym->node;
+  return nullptr;
+}
+
+static ArchiveFile *lazyParent(InputFile *f) {
+  if (!f)
+    return nullptr;
+  if (auto *obj = dyn_cast<ObjFile>(f))
+    return obj->parent;
+  if (auto *obj = dyn_cast<BitcodeFile>(f))
+    return obj->parent;
+  return nullptr;
+}
+
+static ArchiveFile *lazyArchive(Symbol *s) {
+  if (auto *sym = dyn_cast<LazyArchive>(s))
+    return sym->file;
+  if (auto *sym = dyn_cast<LazyObject>(s))
+    return lazyParent(sym->file);
+  return nullptr;
+}
+
+// The search behavior for undefined symbols is different when the OBJ
+// was pulled from an archive (LIB). This is documented here:
+// https://learn.microsoft.com/en-us/cpp/build/reference/link-input-files?view=msvc-170
+// "Object files on the command line are processed in the order they
+// appear on the command line. Libraries are searched in command line
+// order as well, with the following caveat: Symbols that are unresolved
+// when bringing in an object file from a library are searched for in
+// that library first, and then the following libraries from the command
+// line and /DEFAULTLIB (Specify default library) directives, and then
+// to any libraries at the beginning of the command line."
+static Symbol *searchArchiveSymbol(Symbol *s, ArchiveFile *pivot) {
+  auto &Alloc = getSpecificAllocSingleton<SymbolUnion>().Allocator;
+  Symbol *curr = s;
+  for (;;) {
+    if (lazyArchive(curr)->CmdLineIndex >= pivot->CmdLineIndex)
+      return curr;
+    uint32_t next = lazyNode(curr)->next;
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sylvain-audi wrote:

Same thing about potential null return value for `lazyNode`

https://github.com/llvm/llvm-project/pull/85290