[LTO] Fix the interaction between linker redefined symbols and ThinLTO

This is the same as r304719 but for ThinLTO.
The substantial difference is that in this case we don't have
whole visibility, just the summary.
In the LTO case, when we got the resolution for the input file we
could just see if the linker told us whether a symbol was linker
redefined (using --wrap or --defsym) and switch the linkage directly
for the GV.

Here, we have the summary. So, we record that the linkage changed
from <whatever it was> to $weakany to prevent IPOs across this symbol
boundaries and actually just switch the linkage at FunctionImport time.

This patch should also fixes the lld bits (as all the scaffolding for
communicating if a symbol is linker redefined should be there & should
be the same), but I'll make sure to add some tests there as well.

Fixes PR33192.

Differential Revision:  https://reviews.llvm.org/D35064

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@307303 91177308-0d34-0410-b5e6-96231b3b80d8

lib/LTO/LTO.cpp

@@ -665,6 +665,15 @@ Error LTO::addThinLTO(BitcodeModule BM, ArrayRef<InputFile::Symbol> Syms,
         auto GUID = GlobalValue::getGUID(GlobalValue::getGlobalIdentifier(
             Sym.getIRName(), GlobalValue::ExternalLinkage, ""));
         ThinLTO.PrevailingModuleForGUID[GUID] = BM.getModuleIdentifier();
+
+        // For linker redefined symbols (via --wrap or --defsym) we want to
+        // switch the linkage to `weak` to prevent IPOs from happening.
+        // Find the summary in the module for this very GV and record the new
+        // linkage so that we can switch it when we import the GV.
+        if (Res.LinkerRedefined)
+          if (auto S = ThinLTO.CombinedIndex.findSummaryInModule(
+                  GUID, BM.getModuleIdentifier()))
+            S->setLinkage(GlobalValue::WeakAnyLinkage);
       }
     }
   }

lib/Transforms/IPO/FunctionImport.cpp

@@ -537,8 +537,6 @@ void llvm::thinLTOResolveWeakForLinkerModule(
   };
 
   auto updateLinkage = [&](GlobalValue &GV) {
-    if (!GlobalValue::isWeakForLinker(GV.getLinkage()))
-      return;
     // See if the global summary analysis computed a new resolved linkage.
     const auto &GS = DefinedGlobals.find(GV.getGUID());
     if (GS == DefinedGlobals.end())
@@ -546,6 +544,16 @@ void llvm::thinLTOResolveWeakForLinkerModule(
     auto NewLinkage = GS->second->linkage();
     if (NewLinkage == GV.getLinkage())
       return;
+
+    // Switch the linkage to weakany if asked for, e.g. we do this for
+    // linker redefined symbols (via --wrap or --defsym).
+    if (NewLinkage == GlobalValue::WeakAnyLinkage) {
+      GV.setLinkage(NewLinkage);
+      return;
+    }
+
+    if (!GlobalValue::isWeakForLinker(GV.getLinkage()))
+      return;
     // Check for a non-prevailing def that has interposable linkage
     // (e.g. non-odr weak or linkonce). In that case we can't simply
     // convert to available_externally, since it would lose the

test/LTO/Resolution/X86/linker-redef-thin.ll

@@ -0,0 +1,16 @@
+; RUN: opt -module-summary %s -o %t.o
+; RUN: llvm-lto2 run -o %t1.o %t.o -r %t.o,patatino,pr
+; RUN: llvm-readobj -t %t1.o.0 | FileCheck %s
+
+; CHECK: Name: patatino
+; CHECK-NEXT: Value:
+; CHECK-NEXT: Size:
+; CHECK-NEXT: Binding: Weak
+; CHECK-NEXT: Type: Function
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+define void @patatino() {
+  ret void
+}