[lld] [LLD][COFF] Detect weak reference cycles. (PR #104463)

[Alexandre Ganea via llvm-commits]

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@@ -126,9 +127,14 @@ DefinedImportThunk::DefinedImportThunk(COFFLinkerContext &ctx, StringRef name,
 
 Defined *Undefined::getWeakAlias() {
   // A weak alias may be a weak alias to another symbol, so check recursively.
-  for (Symbol *a = weakAlias; a; a = cast<Undefined>(a)->weakAlias)
+  SmallSet<Symbol *, 4> weakChain;
+  for (Symbol *a = weakAlias; a; a = cast<Undefined>(a)->weakAlias) {
+    if (weakChain.contains(a))
+      break; // We have a cycle.
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aganea wrote:

Instead of returning null then emitting "error: undefined symbol:" wouldn't it be nicer to say something like "can't resolve weak symbol because there's a cycle", and then "here's the cycle chain..." (with corresponding ref. obj files)?

https://github.com/llvm/llvm-project/pull/104463