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

[Daniel Thornburgh 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);
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mysterymath wrote:

I tried this, but updating liveness reason beyond the first encountered can cause the liveness reason to become circular. The first encountered reason is immune to this, since it is part of the acyclic region of the reference graph close to the GC roots.

There's probably a way around this, but it seems like a big lift for the benefit, particularly in the first patch.

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