[lld] [lld][ELF] Demote symbols in discarded sections to Undefined. (PR #68777)

[Fangrui Song via llvm-commits]

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@@ -3061,6 +3061,37 @@ void LinkerDriver::link(opt::InputArgList &args) {
     script->addOrphanSections();
   }
 
+  // Demote symbols defined relative to input sections that are discarded by
+  // /DISCARD/ so that relocations referencing them will get reported.
+  if (script->seenDiscard) {
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MaskRay wrote:

It's a corner case but I think is worth testing. The demotion logic applies `--gc-sections` as well but we only run the code when `/DISCARD/` is seen, so there will be some behavior differences.

I have updated `lld/test/ELF/gc-sections-tls.s` in https://github.com/MaskRay/llvm-project/tree/lld-symbols-in-discard  to test this.

Actually, we probably should do #69295 so that all non-local symbols in discarded sections are demoted, regardless of whether `/DISCARD/` is seen? @smithp35 

https://github.com/llvm/llvm-project/pull/68777