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

[Peter Smith via llvm-commits]

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@@ -119,17 +142,29 @@ void MarkLive<ELFT>::resolveReloc(InputSectionBase &sec, RelTy &rel,
     // group/SHF_LINK_ORDER rules (b) if the associated text section should be
     // discarded, marking the LSDA will unnecessarily retain the text section.
     if (!(fromFDE && ((relSec->flags & (SHF_EXECINSTR | SHF_LINK_ORDER)) ||
-                      relSec->nextInSectionGroup)))
-      enqueue(relSec, offset);
+                      relSec->nextInSectionGroup))) {
+      Symbol *canonicalSym = d;
+      if (TrackWhyLive && d->isSection()) {
+        if (Symbol *s = relSec->getEnclosingSymbol(offset))
+          canonicalSym = s;
+        else
+          canonicalSym = nullptr;
+      }
+      enqueue(relSec, offset, canonicalSym, reason);
+    }
     return;
   }
 
-  if (auto *ss = dyn_cast<SharedSymbol>(&sym))
-    if (!ss->isWeak())
+  if (auto *ss = dyn_cast<SharedSymbol>(&sym)) {
+    if (!ss->isWeak()) {
       cast<SharedFile>(ss->file)->isNeeded = true;
+      if (TrackWhyLive)
+        whyLive.try_emplace(&sym, reason);
+    }
+  }
 
   for (InputSectionBase *sec : cNamedSections.lookup(sym.getName()))
-    enqueue(sec, 0);
+    enqueue(sec, 0, nullptr, reason);
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smithp35 wrote:

I found the nullptr and std::nullopt callsites a bit more difficult to reason about, I had to keep referring back to the definition to remember what it meant.

Just thinking if there's an easy way to help out. One way would be to make a constant for nullptr like noSym and noObj (or noItem). Possibly a comment like nullptr /* no symbol */. This isn't a strong opinion.

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