LTO: Link non-prevailing weak_odr or linkonce_odr globals into the combined module with available_externally linkage.

These linkages mean that the ultimately prevailing symbol will have the same
semantics as any non-prevailing copy of the symbol, so we are free to ignore
the linker's resolution.

Differential Revision: https://reviews.llvm.org/D29367

llvm-svn: 293865

1 files changed, 17 insertions, 4 deletions

diff --git a/llvm/lib/LTO/LTO.cpp b/llvm/lib/LTO/LTO.cpp
index 4d0c039891e..df19ded398d 100644
--- a/llvm/lib/LTO/LTO.cpp
+++ b/llvm/lib/LTO/LTO.cpp
@@ -446,6 +446,11 @@ Error LTO::addRegularLTO(BitcodeModule BM, const SymbolResolution *&ResI,
     if (GV.hasAppendingLinkage())
       Keep.push_back(&GV);
 
+  DenseSet<GlobalObject *> AliasedGlobals;
+  for (auto &GA : M.aliases())
+    if (GlobalObject *GO = GA.getBaseObject())
+      AliasedGlobals.insert(GO);
+
   for (const InputFile::Symbol &Sym :
        make_range(InputFile::symbol_iterator(SymTab.symbols().begin(), SymTab,
                                              nullptr),
@@ -471,13 +476,21 @@ Error LTO::addRegularLTO(BitcodeModule BM, const SymbolResolution *&ResI,
           GV->setLinkage(GlobalValue::WeakODRLinkage);
           break;
         }
-      } else if (GV->hasAvailableExternallyLinkage()) {
-        // We can link available_externally symbols even if they are
-        // non-prevailing.
+      } else if (isa<GlobalObject>(GV) &&
+                 (GV->hasLinkOnceODRLinkage() || GV->hasWeakODRLinkage() ||
+                  GV->hasAvailableExternallyLinkage()) &&
+                 !AliasedGlobals.count(cast<GlobalObject>(GV))) {
+        // Either of the above three types of linkage indicates that the
+        // chosen prevailing symbol will have the same semantics as this copy of
+        // the symbol, so we can link it with available_externally linkage. We
+        // only need to do this if the symbol is undefined.
         GlobalValue *CombinedGV =
             RegularLTO.CombinedModule->getNamedValue(GV->getName());
-        if (!CombinedGV || CombinedGV->isDeclaration())
+        if (!CombinedGV || CombinedGV->isDeclaration()) {
           Keep.push_back(GV);
+          GV->setLinkage(GlobalValue::AvailableExternallyLinkage);
+          cast<GlobalObject>(GV)->setComdat(nullptr);
+        }
       }
     }
     // Common resolution: collect the maximum size/alignment over all commons.