[asan] Give global metadata private linkage.

[eugenis]

Summary

Internal linkage preserves names like "__asan_global_foo" which may
account to 2% of unstripped binary size.

[pcc]

lib/Transforms/Instrumentation/AddressSanitizer.cpp

1695

I suspect that private linkage isn't correct for darwin because the symbols need to be present in the object files for dead stripping to work, but I'll ask @kubamracek to comment on that.

[rnk]

lgtm

[eugenis]

Kept the linkage internal on MachO.

[pcc]

LGTM

[eugenis]

r299995

lib/Transforms/Instrumentation/AddressSanitizer.cpp

void AddressSanitizerModule::SetComdatForGlobalMetadata(
   Metadata->setComdat(G->getComdat());
 }
 
 // Create a separate metadata global and put it in the appropriate ASan
 // global registration section.
 GlobalVariable *
 AddressSanitizerModule::CreateMetadataGlobal(Module &M, Constant *Initializer,
                                              StringRef OriginalName) {
-  GlobalVariable *Metadata =
-      new GlobalVariable(M, Initializer->getType(), false,
-                         GlobalVariable::InternalLinkage, Initializer,
-                         Twine("__asan_global_") +
-                             GlobalValue::getRealLinkageName(OriginalName));
+  auto Linkage = TargetTriple.isOSBinFormatMachO()
+                     ? GlobalVariable::InternalLinkage
+                     : GlobalVariable::PrivateLinkage;
+  GlobalVariable *Metadata = new GlobalVariable(
+      M, Initializer->getType(), false, Linkage, Initializer,
+      Twine("__asan_global_") + GlobalValue::getRealLinkageName(OriginalName));
   Metadata->setSection(getGlobalMetadataSection());
   return Metadata;
 }
 
 IRBuilder<> AddressSanitizerModule::CreateAsanModuleDtor(Module &M) {
   Function *AsanDtorFunction =
       Function::Create(FunctionType::get(Type::getVoidTy(*C), false),
                        GlobalValue::InternalLinkage, kAsanModuleDtorName, &M);
GlobalVariable *Metadata =
         CreateMetadataGlobal(M, Initializer, G->getName());
 
     // On recent Mach-O platforms, we emit the global metadata in a way that
     // allows the linker to properly strip dead globals.
     auto LivenessBinder = ConstantStruct::get(
         LivenessTy, Initializer->getAggregateElement(0u),
         ConstantExpr::getPointerCast(Metadata, IntptrTy), nullptr);
     GlobalVariable *Liveness = new GlobalVariable(
         M, LivenessTy, false, GlobalVariable::InternalLinkage, LivenessBinder,
         Twine("__asan_binder_") + G->getName());
     Liveness->setSection("__DATA,__asan_liveness,regular,live_support");
     LivenessGlobals[i] = Liveness;
   }
 
   // Update llvm.compiler.used, adding the new liveness globals. This is
   // needed so that during LTO these variables stay alive. The alternative
   // would be to have the linker handling the LTO symbols, but libLTO

test/Instrumentation/AddressSanitizer/global_metadata_windows.ll

 ; Test that global metadata is placed in a separate section on Windows, and that
 ; it is in the same comdat group as the instrumented global. This ensures that
 ; linker dead stripping (/OPT:REF) works as intended.
 
 ; FIXME: Later we can use this to instrument linkonce odr string literals.
 
 ; RUN: opt < %s -asan -asan-module -S | FileCheck %s
 
 target datalayout = "e-m:w-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-pc-windows-msvc19.0.24215"
 
 $mystr = comdat any
 
 ; CHECK: $dead_global = comdat noduplicates
 ; CHECK: @dead_global = local_unnamed_addr global { i32, [60 x i8] } { i32 42, [60 x i8] zeroinitializer }, comdat, align 32
-; CHECK: @__asan_global_dead_global = internal global { {{.*}} }, section ".ASAN$GL", comdat($dead_global), align 64
+; CHECK: @__asan_global_dead_global = private global { {{.*}} }, section ".ASAN$GL", comdat($dead_global), align 64
 
 @dead_global = local_unnamed_addr global i32 42, align 4
 @mystr = linkonce_odr unnamed_addr constant [5 x i8] c"main\00", comdat, align 1
 
 ; Function Attrs: nounwind uwtable
 define i32 @main() local_unnamed_addr #0 {
 entry:
   %call = tail call i32 @puts(i8* getelementptr inbounds ([5 x i8], [5 x i8]* @mystr, i64 0, i64 0))