[asan] Put ctor/dtor in comdat.

When possible, put ASan ctor/dtor in comdat.

The only reason not to is global registration, which can be
TU-specific. This is not the case when there are no instrumented
globals. This is also limited to ELF targets, because MachO does
not have comdat, and COFF linkers may GC comdat constructors.

The benefit of this is a lot less __asan_init() calls: one per DSO
instead of one per TU. It's also necessary for the upcoming
gc-sections-for-globals change on Linux, where multiple references to
section start symbols trigger quadratic behaviour in gold linker.

This is a second re-land of r298756. This time with a flag to disable
the whole thing to avoid a bug in the gold linker:
  https://sourceware.org/bugzilla/show_bug.cgi?id=19002

llvm-svn: 301586

llvm/lib/Transforms/Instrumentation/AddressSanitizer.cpp

@@ -270,6 +270,13 @@ static cl::opt<bool>
                             "code stripping of globals"),
                    cl::Hidden, cl::init(true));
 
+// This is on by default even though there is a bug in gold:
+// https://sourceware.org/bugzilla/show_bug.cgi?id=19002
+static cl::opt<bool>
+    ClWithComdat("asan-with-comdat",
+                 cl::desc("Place ASan constructors in comdat sections"),
+                 cl::Hidden, cl::init(true));
+
 // Debug flags.
 static cl::opt<int> ClDebug("asan-debug", cl::desc("debug"), cl::Hidden,
                             cl::init(0));
@@ -607,7 +614,7 @@ public:
 private:
   void initializeCallbacks(Module &M);
 
-  bool InstrumentGlobals(IRBuilder<> &IRB, Module &M);
+  bool InstrumentGlobals(IRBuilder<> &IRB, Module &M, bool *CtorComdat);
   void InstrumentGlobalsCOFF(IRBuilder<> &IRB, Module &M,
                              ArrayRef<GlobalVariable *> ExtendedGlobals,
                              ArrayRef<Constant *> MetadataInitializers);
@@ -647,6 +654,9 @@ private:
   Function *AsanUnregisterGlobals;
   Function *AsanRegisterImageGlobals;
   Function *AsanUnregisterImageGlobals;
+
+  Function *AsanCtorFunction = nullptr;
+  Function *AsanDtorFunction = nullptr;
 };
 
 // Stack poisoning does not play well with exception handling.
@@ -1431,8 +1441,13 @@ void AddressSanitizerModule::poisonOneInitializer(Function &GlobalInit,
 void AddressSanitizerModule::createInitializerPoisonCalls(
     Module &M, GlobalValue *ModuleName) {
   GlobalVariable *GV = M.getGlobalVariable("llvm.global_ctors");
+  if (!GV)
+    return;
+
+  ConstantArray *CA = dyn_cast<ConstantArray>(GV->getInitializer());
+  if (!CA)
+    return;
 
-  ConstantArray *CA = cast<ConstantArray>(GV->getInitializer());
   for (Use &OP : CA->operands()) {
     if (isa<ConstantAggregateZero>(OP)) continue;
     ConstantStruct *CS = cast<ConstantStruct>(OP);
@@ -1636,11 +1651,10 @@ AddressSanitizerModule::CreateMetadataGlobal(Module &M, Constant *Initializer,
 }
 
 IRBuilder<> AddressSanitizerModule::CreateAsanModuleDtor(Module &M) {
-  Function *AsanDtorFunction =
+  AsanDtorFunction =
       Function::Create(FunctionType::get(Type::getVoidTy(*C), false),
                        GlobalValue::InternalLinkage, kAsanModuleDtorName, &M);
   BasicBlock *AsanDtorBB = BasicBlock::Create(*C, "", AsanDtorFunction);
-  appendToGlobalDtors(M, AsanDtorFunction, kAsanCtorAndDtorPriority);
 
   return IRBuilder<>(ReturnInst::Create(*C, AsanDtorBB));
 }
@@ -1756,7 +1770,10 @@ void AddressSanitizerModule::InstrumentGlobalsWithMetadataArray(
 // This function replaces all global variables with new variables that have
 // trailing redzones. It also creates a function that poisons
 // redzones and inserts this function into llvm.global_ctors.
-bool AddressSanitizerModule::InstrumentGlobals(IRBuilder<> &IRB, Module &M) {
+// Sets *CtorComdat to true if the global registration code emitted into the
+// asan constructor is comdat-compatible.
+bool AddressSanitizerModule::InstrumentGlobals(IRBuilder<> &IRB, Module &M, bool *CtorComdat) {
+  *CtorComdat = false;
   GlobalsMD.init(M);
 
   SmallVector<GlobalVariable *, 16> GlobalsToChange;
@@ -1766,7 +1783,10 @@ bool AddressSanitizerModule::InstrumentGlobals(IRBuilder<> &IRB, Module &M) {
   }
 
   size_t n = GlobalsToChange.size();
-  if (n == 0) return false;
+  if (n == 0) {
+    *CtorComdat = true;
+    return false;
+  }
 
   auto &DL = M.getDataLayout();
 
@@ -1938,17 +1958,36 @@ bool AddressSanitizerModule::runOnModule(Module &M) {
   if (CompileKernel)
     return false;
 
-  Function *AsanCtorFunction;
+  // Create a module constructor. A destructor is created lazily because not all
+  // platforms, and not all modules need it.
   std::tie(AsanCtorFunction, std::ignore) = createSanitizerCtorAndInitFunctions(
       M, kAsanModuleCtorName, kAsanInitName, /*InitArgTypes=*/{},
       /*InitArgs=*/{}, kAsanVersionCheckName);
-  appendToGlobalCtors(M, AsanCtorFunction, kAsanCtorAndDtorPriority);
 
+  bool CtorComdat = true;
   bool Changed = false;
   // TODO(glider): temporarily disabled globals instrumentation for KASan.
   if (ClGlobals) {
     IRBuilder<> IRB(AsanCtorFunction->getEntryBlock().getTerminator());
-    Changed |= InstrumentGlobals(IRB, M);
+    Changed |= InstrumentGlobals(IRB, M, &CtorComdat);
+  }
+
+  // Put the constructor and destructor in comdat if both
+  // (1) global instrumentation is not TU-specific
+  // (2) target is ELF.
+  if (ClWithComdat && TargetTriple.isOSBinFormatELF() && CtorComdat) {
+    AsanCtorFunction->setComdat(M.getOrInsertComdat(kAsanModuleCtorName));
+    appendToGlobalCtors(M, AsanCtorFunction, kAsanCtorAndDtorPriority,
+                        AsanCtorFunction);
+    if (AsanDtorFunction) {
+      AsanDtorFunction->setComdat(M.getOrInsertComdat(kAsanModuleDtorName));
+      appendToGlobalDtors(M, AsanDtorFunction, kAsanCtorAndDtorPriority,
+                          AsanDtorFunction);
+    }
+  } else {
+    appendToGlobalCtors(M, AsanCtorFunction, kAsanCtorAndDtorPriority);
+    if (AsanDtorFunction)
+      appendToGlobalDtors(M, AsanDtorFunction, kAsanCtorAndDtorPriority);
   }
 
   return Changed;

llvm/test/Instrumentation/AddressSanitizer/instrument_global.ll

@@ -6,8 +6,8 @@ target triple = "x86_64-unknown-linux-gnu"
 ; If a global is present, __asan_[un]register_globals should be called from
 ; module ctor/dtor
 
-; CHECK: llvm.global_ctors
 ; CHECK: @__asan_gen_ = private constant [8 x i8] c"<stdin>\00", align 1
+; CHECK: llvm.global_ctors
 ; CHECK: llvm.global_dtors
 
 ; Test that we don't instrument global arrays with static initializer

llvm/test/Instrumentation/AddressSanitizer/no-globals.ll

@@ -0,0 +1,12 @@
+; A module with no asan-instrumented globals has no asan destructor, and has an asan constructor in a comdat.
+; RUN: opt -mtriple=x86_64-unknown-linux-gnu < %s -asan -asan-module -asan-with-comdat=1 -S | FileCheck %s
+
+define void @f() {
+  ret void
+}
+
+; CHECK-NOT: @llvm.global_dtors
+; CHECK: @llvm.global_ctors = appending global [1 x { i32, void ()*, i8* }] [{ i32, void ()*, i8* } { i32 1, void ()* @asan.module_ctor, i8* bitcast (void ()* @asan.module_ctor to i8*) }]
+; CHECK-NOT: @llvm.global_dtors
+; CHECK: define internal void @asan.module_ctor() comdat
+; CHECK-NOT: @llvm.global_dtors