[ELF] Improve error message for relocations to symbols defined in discarded sections

Rather than report "undefined symbol: ", give more informative message
about the object file that defines the discarded section.

In particular, PR41133, if the section is a discarded COMDAT, print the
section group signature and the object file with the prevailing
definition. This is useful to track down some ODR issues.

We need to
* add `uint32_t DiscardedSecIdx` to Undefined for this feature.
* make ComdatGroups public and change its type to DenseMap<CachedHashStringRef, const InputFile *>

Reviewed By: ruiu

Differential Revision: https://reviews.llvm.org/D59649

llvm-svn: 361359

1 files changed, 6 insertions, 2 deletions

diff --git a/lld/ELF/SymbolTable.cpp b/lld/ELF/SymbolTable.cpp
index 7bad4f92a69..b6d1741f856 100644
--- a/lld/ELF/SymbolTable.cpp
+++ b/lld/ELF/SymbolTable.cpp
@@ -46,7 +46,7 @@ template <class ELFT> void SymbolTable::addCombinedLTOObject() {
     LTO->add(*F);
 
   for (InputFile *File : LTO->compile()) {
-    DenseSet<CachedHashStringRef> DummyGroups;
+    DenseMap<CachedHashStringRef, const InputFile *> DummyGroups;
     auto *Obj = cast<ObjFile<ELFT>>(File);
     Obj->parse(DummyGroups);
     for (Symbol *Sym : Obj->getGlobalSymbols())
@@ -135,7 +135,11 @@ Symbol *SymbolTable::addSymbol(const Symbol &New) {
 static void addUndefined(Symbol *Old, const Undefined &New) {
   // An undefined symbol with non default visibility must be satisfied
   // in the same DSO.
-  if (Old->isShared() && New.Visibility != STV_DEFAULT) {
+  //
+  // If this is a non-weak defined symbol in a discarded section, override the
+  // existing undefined symbol for better error message later.
+  if ((Old->isShared() && New.Visibility != STV_DEFAULT) ||
+      (Old->isUndefined() && New.Binding != STB_WEAK && New.DiscardedSecIdx)) {
     Old->replace(New);
     return;
   }