LTO: Do summary-based prevailing symbol resolution at --lto-O0.

[pcc]

Summary

Prevailing symbol resolution is necessary for correctness. Without
this we can end up dropping a referenced linkonce symbol from the link.

[tejohnson]

LGTM

llvm/lib/LTO/LTO.cpp

for (auto &Res : GlobalResolutions) {
         continue;
       auto GUID = GlobalValue::getGUID(
           GlobalValue::dropLLVMManglingEscape(Res.second.IRName));
       // Mark exported unless index-based analysis determined it to be dead.
       if (!DeadSymbols.count(GUID))
         ExportedGUIDs.insert(GUID);
     }
 
-    auto isPrevailing = [&](GlobalValue::GUID GUID,
-                            const GlobalValueSummary *S) {
-      return ThinLTO.PrevailingModuleForGUID[GUID] == S->modulePath();
-    };
     auto isExported = [&](StringRef ModuleIdentifier, GlobalValue::GUID GUID) {
       const auto &ExportList = ExportLists.find(ModuleIdentifier);
       return (ExportList != ExportLists.end() &&
               ExportList->second.count(GUID)) ||
              ExportedGUIDs.count(GUID);
     };
     thinLTOInternalizeAndPromoteInIndex(ThinLTO.CombinedIndex, isExported);
+  }
 
-    auto recordNewLinkage = [&](StringRef ModuleIdentifier,
-                                GlobalValue::GUID GUID,
-                                GlobalValue::LinkageTypes NewLinkage) {
-      ResolvedODR[ModuleIdentifier][GUID] = NewLinkage;
-    };
-
-    thinLTOResolveWeakForLinkerInIndex(ThinLTO.CombinedIndex, isPrevailing,
-                                       recordNewLinkage);
-  }
+  auto isPrevailing = [&](GlobalValue::GUID GUID,
+                          const GlobalValueSummary *S) {
+    return ThinLTO.PrevailingModuleForGUID[GUID] == S->modulePath();
+  };
 
   std::unique_ptr<ThinBackendProc> BackendProc =
       ThinLTO.Backend(Conf, ThinLTO.CombinedIndex, ModuleToDefinedGVSummaries,
                       AddStream, Cache);
 
   // Task numbers start at ParallelCodeGenParallelismLevel if an LTO
   // module is present, as tasks 0 through ParallelCodeGenParallelismLevel-1
   // are reserved for parallel code generation partitions.

llvm/test/LTO/Resolution/X86/linkonce.ll

+; RUN: opt -module-summary -o %t %s
+; RUN: llvm-lto2 run %t -O0 -r %t,foo,px -o %t2
+; RUN: llvm-nm %t2.0 | FileCheck %s
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+; CHECK: W foo
+define linkonce_odr void @foo() {
+  ret void
+}