[lld] [lld][ELF] Add --why-live flag (inspired by Mach-O) (PR #127112)

[Fangrui Song via llvm-commits]

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@@ -201,15 +235,71 @@ void MarkLive<ELFT>::enqueue(InputSectionBase *sec, uint64_t offset) {
     return;
   sec->partition = sec->partition ? 1 : partition;
 
+  if (!ctx.arg.whyLive.empty() && reason) {
+    if (sym) {
+      // If a specific symbol is referenced, that makes it alive. It may in turn
+      // make its section alive.
+      whyLive.try_emplace(sym, *reason);
+      whyLive.try_emplace(sec, sym);
+    } else {
+      // Otherwise, the reference generically makes the section live.
+      whyLive.try_emplace(sec, *reason);
+    }
+  }
+
   // Add input section to the queue.
   if (InputSection *s = dyn_cast<InputSection>(sec))
     queue.push_back(s);
 }
 
+// Print the stack of reasons that the given symbol is live.
+template <class ELFT> void MarkLive<ELFT>::printWhyLive(Symbol *s) const {
+  // Skip dead symbols. A symbol is dead if it belongs to a dead section.
+  if (auto *d = dyn_cast<Defined>(s)) {
+    auto *reason = dyn_cast_or_null<InputSectionBase>(d->section);
+    if (reason && !reason->isLive())
+      return;
+  }
+
+  auto msg = Msg(ctx);
+  msg << "live symbol: " << toStr(ctx, *s);
----------------
MaskRay wrote:

`<< s`

(`toStr` should be avoided if feasible)

https://github.com/llvm/llvm-project/pull/127112