[msan] Put msan constructor in a comdat.

MSan adds a constructor to each translation unit that calls
__msan_init, and does nothing else. The idea is to run __msan_init
before any instrumented code. This results in multiple constructors
and multiple .init_array entries in the final binary, one per
translation unit. This is absolutely unnecessary; one would be
enough.

This change moves the constructors to a comdat group in order to drop
the extra ones.

llvm-svn: 260632

1 files changed, 24 insertions, 13 deletions

diff --git a/llvm/lib/Transforms/Utils/ModuleUtils.cpp b/llvm/lib/Transforms/Utils/ModuleUtils.cpp
index 040ae963b34..609daad0f29 100644
--- a/llvm/lib/Transforms/Utils/ModuleUtils.cpp
+++ b/llvm/lib/Transforms/Utils/ModuleUtils.cpp
@@ -21,8 +21,8 @@
 
 using namespace llvm;
 
-static void appendToGlobalArray(const char *Array, 
-                                Module &M, Function *F, int Priority) {
+static void appendToGlobalArray(const char *Array, Module &M, Function *F,
+                                int Priority, Constant *Data) {
   IRBuilder<> IRB(M.getContext());
   FunctionType *FnTy = FunctionType::get(IRB.getVoidTy(), false);
 
@@ -31,15 +31,26 @@ static void appendToGlobalArray(const char *Array,
   SmallVector<Constant *, 16> CurrentCtors;
   StructType *EltTy;
   if (GlobalVariable *GVCtor = M.getNamedGlobal(Array)) {
-    // If there is a global_ctors array, use the existing struct type, which can
-    // have 2 or 3 fields.
     ArrayType *ATy = cast<ArrayType>(GVCtor->getValueType());
-    EltTy = cast<StructType>(ATy->getElementType());
+    StructType *OldEltTy = cast<StructType>(ATy->getElementType());
+    // Upgrade a 2-field global array type to the new 3-field format if needed.
+    if (Data && OldEltTy->getNumElements() < 3)
+      EltTy = StructType::get(IRB.getInt32Ty(), PointerType::getUnqual(FnTy),
+                              IRB.getInt8PtrTy(), nullptr);
+    else
+      EltTy = OldEltTy;
     if (Constant *Init = GVCtor->getInitializer()) {
       unsigned n = Init->getNumOperands();
       CurrentCtors.reserve(n + 1);
-      for (unsigned i = 0; i != n; ++i)
-        CurrentCtors.push_back(cast<Constant>(Init->getOperand(i)));
+      for (unsigned i = 0; i != n; ++i) {
+        auto Ctor = cast<Constant>(Init->getOperand(i));
+        if (EltTy != OldEltTy)
+          Ctor = ConstantStruct::get(
+              EltTy, Ctor->getAggregateElement((unsigned)0),
+              Ctor->getAggregateElement(1),
+              Constant::getNullValue(IRB.getInt8PtrTy()), nullptr);
+        CurrentCtors.push_back(Ctor);
+      }
     }
     GVCtor->eraseFromParent();
   } else {
@@ -54,7 +65,8 @@ static void appendToGlobalArray(const char *Array,
   CSVals[1] = F;
   // FIXME: Drop support for the two element form in LLVM 4.0.
   if (EltTy->getNumElements() >= 3)
-    CSVals[2] = llvm::Constant::getNullValue(IRB.getInt8PtrTy());
+    CSVals[2] = Data ? ConstantExpr::getPointerCast(Data, IRB.getInt8PtrTy())
+                     : Constant::getNullValue(IRB.getInt8PtrTy());
   Constant *RuntimeCtorInit =
       ConstantStruct::get(EltTy, makeArrayRef(CSVals, EltTy->getNumElements()));
 
@@ -70,12 +82,12 @@ static void appendToGlobalArray(const char *Array,
                            GlobalValue::AppendingLinkage, NewInit, Array);
 }
 
-void llvm::appendToGlobalCtors(Module &M, Function *F, int Priority) {
-  appendToGlobalArray("llvm.global_ctors", M, F, Priority);
+void llvm::appendToGlobalCtors(Module &M, Function *F, int Priority, Constant *Data) {
+  appendToGlobalArray("llvm.global_ctors", M, F, Priority, Data);
 }
 
-void llvm::appendToGlobalDtors(Module &M, Function *F, int Priority) {
-  appendToGlobalArray("llvm.global_dtors", M, F, Priority);
+void llvm::appendToGlobalDtors(Module &M, Function *F, int Priority, Constant *Data) {
+  appendToGlobalArray("llvm.global_dtors", M, F, Priority, Data);
 }
 
 GlobalVariable *
@@ -132,4 +144,3 @@ std::pair<Function *, Function *> llvm::createSanitizerCtorAndInitFunctions(
   }
   return std::make_pair(Ctor, InitFunction);
 }
-