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

[Peter Smith 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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smithp35 wrote:

Could there be a case where the first match we see doesn't have a symbol, but a later chain of relocations would?

In that case we could prefer to update sec -> *reason to sec -> new-symbol

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