LowerTypeTests: Give imported symbols a type with size 0 so that they are not assumed not to alias.

It is possible for both a base and a derived class to be satisfied
with a unique vtable. If a program contains casts of the same pointer
to both of those types, the CFI checks will be lowered to this
(with ThinLTO):

if (p != &__typeid_base_global_addr)
  trap();
if (p != &__typeid_derived_global_addr)
  trap();

The optimizer may then use the first condition combined
with the assumption that __typeid_base_global_addr and
__typeid_derived_global_addr may not alias to optimize away the second
comparison, resulting in an unconditional trap.

This patch fixes the bug by giving imported globals the type [0 x i8]*,
which prevents the optimizer from assuming that they do not alias.

Differential Revision: https://reviews.llvm.org/D38873

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@315753 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 6 insertions, 2 deletions

diff --git a/lib/Transforms/IPO/LowerTypeTests.cpp b/lib/Transforms/IPO/LowerTypeTests.cpp
index 027f426649c..ab205f4285e 100644
--- a/lib/Transforms/IPO/LowerTypeTests.cpp
+++ b/lib/Transforms/IPO/LowerTypeTests.cpp
@@ -259,6 +259,7 @@ class LowerTypeTestsModule {
   IntegerType *Int1Ty = Type::getInt1Ty(M.getContext());
   IntegerType *Int8Ty = Type::getInt8Ty(M.getContext());
   PointerType *Int8PtrTy = Type::getInt8PtrTy(M.getContext());
+  ArrayType *Int8Arr0Ty = ArrayType::get(Type::getInt8Ty(M.getContext()), 0);
   IntegerType *Int32Ty = Type::getInt32Ty(M.getContext());
   PointerType *Int32PtrTy = PointerType::getUnqual(Int32Ty);
   IntegerType *Int64Ty = Type::getInt64Ty(M.getContext());
@@ -803,10 +804,13 @@ LowerTypeTestsModule::importTypeId(StringRef TypeId) {
   TIL.TheKind = TTRes.TheKind;
 
   auto ImportGlobal = [&](StringRef Name) {
-    Constant *C =
-        M.getOrInsertGlobal(("__typeid_" + TypeId + "_" + Name).str(), Int8Ty);
+    // Give the global a type of length 0 so that it is not assumed not to alias
+    // with any other global.
+    Constant *C = M.getOrInsertGlobal(("__typeid_" + TypeId + "_" + Name).str(),
+                                      Int8Arr0Ty);
     if (auto *GV = dyn_cast<GlobalVariable>(C))
       GV->setVisibility(GlobalValue::HiddenVisibility);
+    C = ConstantExpr::getBitCast(C, Int8PtrTy);
     return C;
   };