From: Sanjay Patel <spatel@rotateright.com>
Date: Fri, 19 Jan 2018 15:14:51 +0000 (+0000)
Subject: [CodeGenCXX] annotate a GEP to a derived class with 'inbounds' (PR35909)
X-Git-Url: https://granicus.if.org/sourcecode?a=commitdiff_plain;h=3e53723a4808739b8cc707cd0c60c7d0e7d67080;p=clang

[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
---

diff --git a/lib/CodeGen/CGClass.cpp b/lib/CodeGen/CGClass.cpp
index a6915071ec..593d6152c8 100644
--- a/lib/CodeGen/CGClass.cpp
+++ b/lib/CodeGen/CGClass.cpp
@@ -406,8 +406,8 @@ CodeGenFunction::GetAddressOfDerivedClass(Address BaseAddr,
 
   // Apply the offset.
   llvm::Value *Value = Builder.CreateBitCast(BaseAddr.getPointer(), Int8PtrTy);
-  Value = Builder.CreateGEP(Value, Builder.CreateNeg(NonVirtualOffset),
-                            "sub.ptr");
+  Value = Builder.CreateInBoundsGEP(Value, Builder.CreateNeg(NonVirtualOffset),
+                                    "sub.ptr");
 
   // Just cast.
   Value = Builder.CreateBitCast(Value, DerivedPtrTy);
diff --git a/test/CodeGenCXX/catch-undef-behavior.cpp b/test/CodeGenCXX/catch-undef-behavior.cpp
index e828753898..786c6da21a 100644
--- a/test/CodeGenCXX/catch-undef-behavior.cpp
+++ b/test/CodeGenCXX/catch-undef-behavior.cpp
@@ -371,7 +371,7 @@ class C : public A, public B // align=16
 void downcast_pointer(B *b) {
   (void) static_cast<C*>(b);
   // Alignment check from EmitTypeCheck(TCK_DowncastPointer, ...)
-  // CHECK: [[SUB:%[.a-z0-9]*]] = getelementptr i8, i8* {{.*}}, i64 -16
+  // CHECK: [[SUB:%[.a-z0-9]*]] = getelementptr inbounds i8, i8* {{.*}}, i64 -16
   // CHECK-NEXT: [[C:%.+]] = bitcast i8* [[SUB]] to %class.C*
   // null check goes here
   // CHECK: [[FROM_PHI:%.+]] = phi %class.C* [ [[C]], {{.*}} ], {{.*}}
@@ -388,7 +388,7 @@ void downcast_pointer(B *b) {
 void downcast_reference(B &b) {
   (void) static_cast<C&>(b);
   // Alignment check from EmitTypeCheck(TCK_DowncastReference, ...)
-  // CHECK:      [[SUB:%[.a-z0-9]*]] = getelementptr i8, i8* {{.*}}, i64 -16
+  // CHECK:      [[SUB:%[.a-z0-9]*]] = getelementptr inbounds i8, i8* {{.*}}, i64 -16
   // CHECK-NEXT: [[C:%.+]] = bitcast i8* [[SUB]] to %class.C*
   // Objectsize check goes here
   // CHECK:      [[C_INT:%.+]] = ptrtoint %class.C* [[C]] to i64
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
+}
+