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/compiler-rt/trunk@315753 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 25 insertions, 0 deletions

diff --git a/test/cfi/vtable-may-alias.cpp b/test/cfi/vtable-may-alias.cpp
new file mode 100644
index 000000000..f63b53b8c
--- /dev/null
+++ b/test/cfi/vtable-may-alias.cpp
@@ -0,0 +1,25 @@
+// RUN: %clangxx_cfi -o %t %s
+// RUN: %run %t
+
+// In this example, both __typeid_A_global_addr and __typeid_B_global_addr will
+// refer to the same address. Make sure that the compiler does not assume that
+// they do not alias.
+
+struct A {
+  virtual void f() = 0;
+};
+
+struct B : A {
+  virtual void f() {}
+};
+
+__attribute__((weak)) void foo(void *p) {
+  B *b = (B *)p;
+  A *a = (A *)b;
+  a->f();
+}
+
+int main() {
+  B b;
+  foo(&b);
+}