[ASAN] Add the binder globals on Darwin to llvm.compiler.used to avoid LTO stripping

[zaks.anna]

LGTM, however, you should probably add someone who regularly works on LLVM IR to review this as well.

Also, please, add commit message describing what you are doing and why.

Thanks!
Anna.

llvm/lib/Transforms/Instrumentation/AddressSanitizer.cpp

if (ShouldUseMachOGlobalsSection()) {
     RegisteredFlag = new GlobalVariable(
         M, IntptrTy, false, GlobalVariable::CommonLinkage,
         ConstantInt::get(IntptrTy, 0), kAsanGlobalsRegisteredFlagName);
 
     // We also emit a structure which binds the liveness of the global
     // variable to the metadata struct.
     StructType *LivenessTy = StructType::get(IntptrTy, IntptrTy, nullptr);
 
+    // Keep the list of "Liveness" GV created to be added to llvm.compiler.used
+    SmallVector<Constant *, 16> LivenessGlobals;
+    LivenessGlobals.reserve(n);
+
     for (size_t i = 0; i < n; i++) {
       GlobalVariable *Metadata = new GlobalVariable(
           M, GlobalStructTy, false, GlobalVariable::InternalLinkage,
           Initializers[i], "");
       Metadata->setSection("__DATA,__asan_globals,regular");
       Metadata->setAlignment(1); // don't leave padding in between
 
       auto LivenessBinder = ConstantStruct::get(LivenessTy,
           Initializers[i]->getAggregateElement(0u),
           ConstantExpr::getPointerCast(Metadata, IntptrTy),
           nullptr);
+
+      // Recover the name of the variable this global is pointing to
+      StringRef GVName =
+          Initializers[i]->getAggregateElement(0u)->getOperand(0)->getName();
+
       GlobalVariable *Liveness = new GlobalVariable(
-          M, LivenessTy, false, GlobalVariable::InternalLinkage,
-          LivenessBinder, "");
+          M, LivenessTy, false, GlobalVariable::InternalLinkage, LivenessBinder,
+          Twine("__asan_binder_") + GVName);
       Liveness->setSection("__DATA,__asan_liveness,regular,live_support");
+      LivenessGlobals.push_back(
+          ConstantExpr::getBitCast(Liveness, IRB.getInt8PtrTy()));
+    }
+
+    if (!LivenessGlobals.empty()) {
+      // 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
+      // current
+      // API does not expose access to the section for each symbol.
+      if (GlobalVariable *LLVMUsed =
+              M.getGlobalVariable("llvm.compiler.used")) {
+        ConstantArray *Inits = cast<ConstantArray>(LLVMUsed->getInitializer());
+        for (auto &V : Inits->operands())
+          LivenessGlobals.push_back(cast<Constant>(&V));
+        LLVMUsed->eraseFromParent();
+      }
+      llvm::ArrayType *ATy =
+          llvm::ArrayType::get(IRB.getInt8PtrTy(), LivenessGlobals.size());
+      auto *LLVMUsed = new llvm::GlobalVariable(
+          M, ATy, false, llvm::GlobalValue::AppendingLinkage,
+          llvm::ConstantArray::get(ATy, LivenessGlobals), "llvm.compiler.used");
+      LLVMUsed->setSection("llvm.metadata");
     }
   } else {
     // On all other platfoms, we just emit an array of global metadata
     // structures.
     ArrayType *ArrayOfGlobalStructTy = ArrayType::get(GlobalStructTy, n);
     AllGlobals = new GlobalVariable(
         M, ArrayOfGlobalStructTy, false, GlobalVariable::InternalLinkage,
         ConstantArray::get(ArrayOfGlobalStructTy, Initializers), "");

llvm/test/Instrumentation/AddressSanitizer/global_metadata_darwin.ll

 
 ; Test that there is the flag global variable:
 ; CHECK: @__asan_globals_registered = common global i64 0
 
 ; Find the metadata for @global:
 ; CHECK: [[METADATA:@[0-9]+]] = internal global {{.*}} @global {{.*}} section "__DATA,__asan_globals,regular", align 1
 
 ; Find the liveness binder for @global and its metadata:
-; CHECK: @{{[0-9]+}} = internal global {{.*}} @global {{.*}} [[METADATA]] {{.*}} section "__DATA,__asan_liveness,regular,live_support"
+; CHECK: @__asan_binder_global = internal global {{.*}} @global {{.*}} [[METADATA]] {{.*}} section "__DATA,__asan_liveness,regular,live_support"
+
+; The binder has to be inserted to llvm.compiler.used to avoid being stripped
+; during LTO.
+; CHECK: @llvm.compiler.used {{.*}} @__asan_binder_global {{.*}} section "llvm.metadata"
+
 
 ; Test that __asan_register_image_globals is invoked from the constructor:
 ; CHECK-LABEL: define internal void @asan.module_ctor
 ; CHECK-NOT: ret
 ; CHECK: call void @__asan_register_image_globals(i64 ptrtoint (i64* @__asan_globals_registered to i64))
 ; CHECK: ret
 
 ; Test that __asan_unregister_image_globals is invoked from the destructor:
 ; CHECK-LABEL: define internal void @asan.module_dtor
 ; CHECK-NOT: ret
 ; CHECK: call void @__asan_unregister_image_globals(i64 ptrtoint (i64* @__asan_globals_registered to i64))
 ; CHECK: ret