[CodeGenCXX] annotate a GEP to a derived class with 'inbounds' (PR35909)

The standard says:
[expr.static.cast] p11: "If the prvalue of type “pointer to cv1 B” points to a B 
that is actually a subobject of an object of type D, the resulting pointer points 
to the enclosing object of type D. Otherwise, the behavior is undefined."

Therefore, the GEP must be inbounds.

This should solve the failure to optimize away a null check shown in PR35909:
https://bugs.llvm.org/show_bug.cgi?id=35909 

Differential Revision: https://reviews.llvm.org/D42249


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@322950 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 27 insertions, 0 deletions

diff --git a/test/CodeGenCXX/derived-cast.cpp b/test/CodeGenCXX/derived-cast.cpp
new file mode 100644
index 0000000000..bf2b258c5e
--- /dev/null
+++ b/test/CodeGenCXX/derived-cast.cpp
@@ -0,0 +1,27 @@
+// RUN: %clang_cc1 -triple x86_64-unknown-unknown -emit-llvm %s -o - | FileCheck %s
+
+class A {
+    int a;
+};
+
+class B {
+    int b;
+public:
+    A *getAsA();
+};
+
+class X : public A, public B {
+    int x;
+};
+
+// PR35909 - https://bugs.llvm.org/show_bug.cgi?id=35909
+
+A *B::getAsA() {
+  return static_cast<X*>(this);
+
+  // CHECK-LABEL: define %class.A* @_ZN1B6getAsAEv
+  // CHECK: %[[THIS:.*]] = load %class.B*, %class.B**
+  // CHECK-NEXT: %[[BC:.*]] = bitcast %class.B* %[[THIS]] to i8*
+  // CHECK-NEXT: getelementptr inbounds i8, i8* %[[BC]], i64 -4
+}
+