r210377 - [MS-ABI] Implement dynamic_cast
Warren Hunt
whunt at google.com
Fri Jun 6 15:12:37 PDT 2014
Author: whunt
Date: Fri Jun 6 17:12:37 2014
New Revision: 210377
URL: http://llvm.org/viewvc/llvm-project?rev=210377&view=rev
Log:
[MS-ABI] Implement dynamic_cast
This patch implements call lower from dynamic_cast to __RTDynamicCast
and __RTCastToVoid. Test cases are included. A feature of note is that
helper function getPolymorphicOffset is placed in such a way that it can
be used by EmitTypeid (to be implemented in a later patch) without being
moved. Details are included as comments directly in the code.
Added:
cfe/trunk/test/CodeGenCXX/microsoft-abi-dynamic-cast.cpp
Modified:
cfe/trunk/lib/CodeGen/CGExprCXX.cpp
Modified: cfe/trunk/lib/CodeGen/CGExprCXX.cpp
URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/CodeGen/CGExprCXX.cpp?rev=210377&r1=210376&r2=210377&view=diff
==============================================================================
--- cfe/trunk/lib/CodeGen/CGExprCXX.cpp (original)
+++ cfe/trunk/lib/CodeGen/CGExprCXX.cpp Fri Jun 6 17:12:37 2014
@@ -1628,6 +1628,20 @@ static void EmitBadTypeidCall(CodeGenFun
CGF.Builder.CreateUnreachable();
}
+/// \brief Gets the offset to the virtual base that contains the vfptr for
+/// MS-ABI polymorphic types.
+static llvm::Value *getPolymorphicOffset(CodeGenFunction &CGF,
+ const CXXRecordDecl *RD,
+ llvm::Value *Value) {
+ const ASTContext &Context = RD->getASTContext();
+ for (const CXXBaseSpecifier &Base : RD->vbases())
+ if (Context.getASTRecordLayout(Base.getType()->getAsCXXRecordDecl())
+ .hasExtendableVFPtr())
+ return CGF.CGM.getCXXABI().GetVirtualBaseClassOffset(
+ CGF, Value, RD, Base.getType()->getAsCXXRecordDecl());
+ llvm_unreachable("One of our vbases should be polymorphic.");
+}
+
static llvm::Value *EmitTypeidFromVTable(CodeGenFunction &CGF,
const Expr *E,
llvm::Type *StdTypeInfoPtrTy) {
@@ -1686,7 +1700,7 @@ llvm::Value *CodeGenFunction::EmitCXXTyp
StdTypeInfoPtrTy);
}
-static llvm::Constant *getDynamicCastFn(CodeGenFunction &CGF) {
+static llvm::Constant *getItaniumDynamicCastFn(CodeGenFunction &CGF) {
// void *__dynamic_cast(const void *sub,
// const abi::__class_type_info *src,
// const abi::__class_type_info *dst,
@@ -1774,7 +1788,7 @@ static CharUnits computeOffsetHint(ASTCo
}
static llvm::Value *
-EmitDynamicCastCall(CodeGenFunction &CGF, llvm::Value *Value,
+EmitItaniumDynamicCastCall(CodeGenFunction &CGF, llvm::Value *Value,
QualType SrcTy, QualType DestTy,
llvm::BasicBlock *CastEnd) {
llvm::Type *PtrDiffLTy =
@@ -1834,7 +1848,7 @@ EmitDynamicCastCall(CodeGenFunction &CGF
Value = CGF.EmitCastToVoidPtr(Value);
llvm::Value *args[] = { Value, SrcRTTI, DestRTTI, OffsetHint };
- Value = CGF.EmitNounwindRuntimeCall(getDynamicCastFn(CGF), args);
+ Value = CGF.EmitNounwindRuntimeCall(getItaniumDynamicCastFn(CGF), args);
Value = CGF.Builder.CreateBitCast(Value, DestLTy);
/// C++ [expr.dynamic.cast]p9:
@@ -1867,8 +1881,125 @@ static llvm::Value *EmitDynamicCastToNul
return llvm::UndefValue::get(DestLTy);
}
+namespace {
+struct MSDynamicCastBuilder {
+ MSDynamicCastBuilder(CodeGenFunction &CGF, const CXXDynamicCastExpr *DCE);
+ llvm::Value *emitDynamicCastCall(llvm::Value *Value);
+ llvm::Value *emitDynamicCast(llvm::Value *Value);
+
+ CodeGenFunction &CGF;
+ CGBuilderTy &Builder;
+ llvm::PointerType *Int8PtrTy;
+ QualType SrcTy, DstTy;
+ const CXXRecordDecl *SrcDecl;
+ bool IsPtrCast, IsCastToVoid, IsCastOfNull;
+};
+} // namespace
+
+MSDynamicCastBuilder::MSDynamicCastBuilder(CodeGenFunction &CGF,
+ const CXXDynamicCastExpr *DCE)
+ : CGF(CGF), Builder(CGF.Builder), Int8PtrTy(CGF.Int8PtrTy),
+ SrcDecl(nullptr) {
+ DstTy = DCE->getTypeAsWritten();
+ IsPtrCast = DstTy->isPointerType();
+ // Get the PointeeTypes. After this point the original types are not used.
+ DstTy = IsPtrCast ? DstTy->castAs<PointerType>()->getPointeeType()
+ : DstTy->castAs<ReferenceType>()->getPointeeType();
+ IsCastToVoid = DstTy->isVoidType();
+ IsCastOfNull = DCE->isAlwaysNull();
+ if (IsCastOfNull)
+ return;
+ SrcTy = DCE->getSubExpr()->getType();
+ SrcTy = IsPtrCast ? SrcTy->castAs<PointerType>()->getPointeeType() : SrcTy;
+ SrcDecl = SrcTy->getAsCXXRecordDecl();
+ // If we don't need a base adjustment, we don't need a SrcDecl so clear it
+ // here. Later we use the existance of the SrcDecl to determine the need for
+ // a base adjustment.
+ if (CGF.getContext().getASTRecordLayout(SrcDecl).hasExtendableVFPtr())
+ SrcDecl = nullptr;
+}
+
+llvm::Value *MSDynamicCastBuilder::emitDynamicCastCall(llvm::Value *Value) {
+ llvm::IntegerType *Int32Ty = CGF.Int32Ty;
+ llvm::Value *Offset = llvm::ConstantInt::get(Int32Ty, 0);
+ Value = Builder.CreateBitCast(Value, Int8PtrTy);
+ // If we need to perform a base adjustment, do it here.
+ if (SrcDecl) {
+ Offset = getPolymorphicOffset(CGF, SrcDecl, Value);
+ Value = Builder.CreateInBoundsGEP(Value, Offset);
+ Offset = Builder.CreateTrunc(Offset, Int32Ty);
+ }
+ if (IsCastToVoid) {
+ // PVOID __RTCastToVoid(
+ // PVOID inptr)
+ llvm::Type *ArgTypes[] = {Int8PtrTy};
+ llvm::Constant *Function = CGF.CGM.CreateRuntimeFunction(
+ llvm::FunctionType::get(Int8PtrTy, ArgTypes, false), "__RTCastToVoid");
+ llvm::Value *Args[] = {Value};
+ return CGF.EmitRuntimeCall(Function, Args);
+ }
+ // PVOID __RTDynamicCast(
+ // PVOID inptr,
+ // LONG VfDelta,
+ // PVOID SrcType,
+ // PVOID TargetType,
+ // BOOL isReference)
+ llvm::Type *ArgTypes[] = {Int8PtrTy, Int32Ty, Int8PtrTy, Int8PtrTy, Int32Ty};
+ llvm::Constant *Function = CGF.CGM.CreateRuntimeFunction(
+ llvm::FunctionType::get(Int8PtrTy, ArgTypes, false), "__RTDynamicCast");
+ llvm::Value *Args[] = {
+ Value, Offset,
+ CGF.CGM.GetAddrOfRTTIDescriptor(SrcTy.getUnqualifiedType()),
+ CGF.CGM.GetAddrOfRTTIDescriptor(DstTy.getUnqualifiedType()),
+ llvm::ConstantInt::get(Int32Ty, IsPtrCast ? 0 : 1)};
+ return CGF.EmitRuntimeCall(Function, Args);
+}
+
+llvm::Value *MSDynamicCastBuilder::emitDynamicCast(llvm::Value *Value) {
+ // Note about undefined behavior: If the dynamic cast is casting to a
+ // reference type and the input is null, we hit a grey area in the standard.
+ // Here we're interpreting the behavior as undefined. The effects are the
+ // following: If the compiler determines that the argument is statically null
+ // or if the argument is dynamically null but does not require base
+ // adjustment, __RTDynamicCast will be called with a null argument and the
+ // isreference bit set. In this case __RTDynamicCast will throw
+ // std::bad_cast. If the argument is dynamically null and a base adjustment is
+ // required the resulting code will produce an out of bounds memory reference
+ // when trying to read VBTblPtr. In Itanium mode clang also emits a vtable
+ // load that fails at run time.
+ llvm::PointerType *DstLTy = CGF.ConvertType(DstTy)->getPointerTo();
+ if (IsCastOfNull && IsPtrCast)
+ return Builder.CreateBitCast(Value, DstLTy);
+ if (IsCastOfNull || !IsPtrCast || !SrcDecl)
+ return Builder.CreateBitCast(emitDynamicCastCall(Value), DstLTy);
+ // !IsCastOfNull && IsPtrCast && SrcDecl
+ // In this case we have a pointer that requires a base adjustment. An
+ // adjustment is only required if the pointer is actually valid so here we
+ // perform a null check before doing the base adjustment and calling
+ // __RTDynamicCast. In the case that the argument is null we simply return
+ // null without calling __RTDynamicCast.
+ llvm::BasicBlock *EntryBlock = Builder.GetInsertBlock();
+ llvm::BasicBlock *CallBlock = CGF.createBasicBlock("dynamic_cast.valid");
+ llvm::BasicBlock *ExitBlock = CGF.createBasicBlock("dynamic_cast.call");
+ Builder.CreateCondBr(Builder.CreateIsNull(Value), ExitBlock, CallBlock);
+ // Emit the call block and code for it.
+ CGF.EmitBlock(CallBlock);
+ Value = emitDynamicCastCall(Value);
+ // Emit the call block and the phi nodes for it.
+ CGF.EmitBlock(ExitBlock);
+ llvm::PHINode *ValuePHI = Builder.CreatePHI(Int8PtrTy, 2);
+ ValuePHI->addIncoming(Value, CallBlock);
+ ValuePHI->addIncoming(llvm::Constant::getNullValue(Int8PtrTy), EntryBlock);
+ return Builder.CreateBitCast(ValuePHI, DstLTy);
+}
+
llvm::Value *CodeGenFunction::EmitDynamicCast(llvm::Value *Value,
const CXXDynamicCastExpr *DCE) {
+ if (getTarget().getCXXABI().isMicrosoft()) {
+ MSDynamicCastBuilder Builder(*this, DCE);
+ return Builder.emitDynamicCast(Value);
+ }
+
QualType DestTy = DCE->getTypeAsWritten();
if (DCE->isAlwaysNull())
@@ -1894,7 +2025,7 @@ llvm::Value *CodeGenFunction::EmitDynami
EmitBlock(CastNotNull);
}
- Value = EmitDynamicCastCall(*this, Value, SrcTy, DestTy, CastEnd);
+ Value = EmitItaniumDynamicCastCall(*this, Value, SrcTy, DestTy, CastEnd);
if (ShouldNullCheckSrcValue) {
EmitBranch(CastEnd);
Added: cfe/trunk/test/CodeGenCXX/microsoft-abi-dynamic-cast.cpp
URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/test/CodeGenCXX/microsoft-abi-dynamic-cast.cpp?rev=210377&view=auto
==============================================================================
--- cfe/trunk/test/CodeGenCXX/microsoft-abi-dynamic-cast.cpp (added)
+++ cfe/trunk/test/CodeGenCXX/microsoft-abi-dynamic-cast.cpp Fri Jun 6 17:12:37 2014
@@ -0,0 +1,157 @@
+// RUN: %clang_cc1 -emit-llvm -O2 -optzns -o - -triple=i386-pc-win32 2>/dev/null %s | FileCheck %s
+
+struct S { char a; };
+struct V { virtual void f(){} };
+struct A : virtual V {};
+struct B : S, virtual V {};
+struct T {};
+
+T* test0() { return dynamic_cast<T*>((B*)0); }
+// CHECK: define noalias %struct.T* @"\01?test0@@YAPAUT@@XZ"() #0 {
+// CHECK-NEXT: entry:
+// CHECK-NEXT: ret %struct.T* null
+// CHECK-NEXT: }
+
+T* test1(V* x) { return &dynamic_cast<T&>(*x); }
+// CHECK: define %struct.T* @"\01?test1@@YAPAUT@@PAUV@@@Z"(%struct.V* %x) #1 {
+// CHECK-NEXT: entry:
+// CHECK-NEXT: %0 = bitcast %struct.V* %x to i8*
+// CHECK-NEXT: %1 = tail call i8* @__RTDynamicCast(i8* %0, i32 0, i8* bitcast (%"MSRTTITypeDescriptor\07"* @"\01??_R0?AUV@@@8" to i8*), i8* bitcast (%"MSRTTITypeDescriptor\07"* @"\01??_R0?AUT@@@8" to i8*), i32 1) #2
+// CHECK-NEXT: %2 = bitcast i8* %1 to %struct.T*
+// CHECK-NEXT: ret %struct.T* %2
+// CHECK-NEXT: }
+
+T* test2(A* x) { return &dynamic_cast<T&>(*x); }
+// CHECK: define %struct.T* @"\01?test2@@YAPAUT@@PAUA@@@Z"(%struct.A* %x) #1 {
+// CHECK-NEXT: entry:
+// CHECK-NEXT: %0 = bitcast %struct.A* %x to i8*
+// CHECK-NEXT: %1 = bitcast %struct.A* %x to i8**
+// CHECK-NEXT: %vbtable = load i8** %1, align 4
+// CHECK-NEXT: %2 = getelementptr inbounds i8* %vbtable, i32 4
+// CHECK-NEXT: %3 = bitcast i8* %2 to i32*
+// CHECK-NEXT: %vbase_offs = load i32* %3, align 4
+// CHECK-NEXT: %4 = getelementptr inbounds i8* %0, i32 %vbase_offs
+// CHECK-NEXT: %5 = tail call i8* @__RTDynamicCast(i8* %4, i32 %vbase_offs, i8* bitcast (%"MSRTTITypeDescriptor\07"* @"\01??_R0?AUA@@@8" to i8*), i8* bitcast (%"MSRTTITypeDescriptor\07"* @"\01??_R0?AUT@@@8" to i8*), i32 1) #2
+// CHECK-NEXT: %6 = bitcast i8* %5 to %struct.T*
+// CHECK-NEXT: ret %struct.T* %6
+// CHECK-NEXT: }
+
+T* test3(B* x) { return &dynamic_cast<T&>(*x); }
+// CHECK: define %struct.T* @"\01?test3@@YAPAUT@@PAUB@@@Z"(%struct.B* %x) #1 {
+// CHECK-NEXT: entry:
+// CHECK-NEXT: %0 = getelementptr inbounds %struct.B* %x, i32 0, i32 0, i32 0
+// CHECK-NEXT: %vbptr = getelementptr inbounds i8* %0, i32 4
+// CHECK-NEXT: %1 = bitcast i8* %vbptr to i8**
+// CHECK-NEXT: %vbtable = load i8** %1, align 4
+// CHECK-NEXT: %2 = getelementptr inbounds i8* %vbtable, i32 4
+// CHECK-NEXT: %3 = bitcast i8* %2 to i32*
+// CHECK-NEXT: %vbase_offs = load i32* %3, align 4
+// CHECK-NEXT: %4 = add nsw i32 %vbase_offs, 4
+// CHECK-NEXT: %5 = getelementptr inbounds i8* %0, i32 %4
+// CHECK-NEXT: %6 = tail call i8* @__RTDynamicCast(i8* %5, i32 %4, i8* bitcast (%"MSRTTITypeDescriptor\07"* @"\01??_R0?AUB@@@8" to i8*), i8* bitcast (%"MSRTTITypeDescriptor\07"* @"\01??_R0?AUT@@@8" to i8*), i32 1) #2
+// CHECK-NEXT: %7 = bitcast i8* %6 to %struct.T*
+// CHECK-NEXT: ret %struct.T* %7
+// CHECK-NEXT: }
+
+T* test4(V* x) { return dynamic_cast<T*>(x); }
+// CHECK: define %struct.T* @"\01?test4@@YAPAUT@@PAUV@@@Z"(%struct.V* %x) #1 {
+// CHECK-NEXT: entry:
+// CHECK-NEXT: %0 = bitcast %struct.V* %x to i8*
+// CHECK-NEXT: %1 = tail call i8* @__RTDynamicCast(i8* %0, i32 0, i8* bitcast (%"MSRTTITypeDescriptor\07"* @"\01??_R0?AUV@@@8" to i8*), i8* bitcast (%"MSRTTITypeDescriptor\07"* @"\01??_R0?AUT@@@8" to i8*), i32 0) #2
+// CHECK-NEXT: %2 = bitcast i8* %1 to %struct.T*
+// CHECK-NEXT: ret %struct.T* %2
+// CHECK-NEXT: }
+
+T* test5(A* x) { return dynamic_cast<T*>(x); }
+// CHECK: define %struct.T* @"\01?test5@@YAPAUT@@PAUA@@@Z"(%struct.A* %x) #1 {
+// CHECK-NEXT: entry:
+// CHECK-NEXT: %0 = icmp eq %struct.A* %x, null
+// CHECK-NEXT: br i1 %0, label %dynamic_cast.call, label %dynamic_cast.valid
+// CHECK: dynamic_cast.valid: ; preds = %entry
+// CHECK-NEXT: %1 = bitcast %struct.A* %x to i8*
+// CHECK-NEXT: %2 = bitcast %struct.A* %x to i8**
+// CHECK-NEXT: %vbtable = load i8** %2, align 4
+// CHECK-NEXT: %3 = getelementptr inbounds i8* %vbtable, i32 4
+// CHECK-NEXT: %4 = bitcast i8* %3 to i32*
+// CHECK-NEXT: %vbase_offs = load i32* %4, align 4
+// CHECK-NEXT: %5 = getelementptr inbounds i8* %1, i32 %vbase_offs
+// CHECK-NEXT: %6 = tail call i8* @__RTDynamicCast(i8* %5, i32 %vbase_offs, i8* bitcast (%"MSRTTITypeDescriptor\07"* @"\01??_R0?AUA@@@8" to i8*), i8* bitcast (%"MSRTTITypeDescriptor\07"* @"\01??_R0?AUT@@@8" to i8*), i32 0) #2
+// CHECK-NEXT: %phitmp = bitcast i8* %6 to %struct.T*
+// CHECK-NEXT: br label %dynamic_cast.call
+// CHECK: dynamic_cast.call: ; preds = %dynamic_cast.valid, %entry
+// CHECK-NEXT: %7 = phi %struct.T* [ %phitmp, %dynamic_cast.valid ], [ null, %entry ]
+// CHECK-NEXT: ret %struct.T* %7
+// CHECK-NEXT: }
+
+T* test6(B* x) { return dynamic_cast<T*>(x); }
+// CHECK: define %struct.T* @"\01?test6@@YAPAUT@@PAUB@@@Z"(%struct.B* %x) #1 {
+// CHECK-NEXT: entry:
+// CHECK-NEXT: %0 = icmp eq %struct.B* %x, null
+// CHECK-NEXT: br i1 %0, label %dynamic_cast.call, label %dynamic_cast.valid
+// CHECK: dynamic_cast.valid: ; preds = %entry
+// CHECK-NEXT: %1 = getelementptr inbounds %struct.B* %x, i32 0, i32 0, i32 0
+// CHECK-NEXT: %vbptr = getelementptr inbounds i8* %1, i32 4
+// CHECK-NEXT: %2 = bitcast i8* %vbptr to i8**
+// CHECK-NEXT: %vbtable = load i8** %2, align 4
+// CHECK-NEXT: %3 = getelementptr inbounds i8* %vbtable, i32 4
+// CHECK-NEXT: %4 = bitcast i8* %3 to i32*
+// CHECK-NEXT: %vbase_offs = load i32* %4, align 4
+// CHECK-NEXT: %5 = add nsw i32 %vbase_offs, 4
+// CHECK-NEXT: %6 = getelementptr inbounds i8* %1, i32 %5
+// CHECK-NEXT: %7 = tail call i8* @__RTDynamicCast(i8* %6, i32 %5, i8* bitcast (%"MSRTTITypeDescriptor\07"* @"\01??_R0?AUB@@@8" to i8*), i8* bitcast (%"MSRTTITypeDescriptor\07"* @"\01??_R0?AUT@@@8" to i8*), i32 0) #2
+// CHECK-NEXT: %phitmp = bitcast i8* %7 to %struct.T*
+// CHECK-NEXT: br label %dynamic_cast.call
+// CHECK: dynamic_cast.call: ; preds = %dynamic_cast.valid, %entry
+// CHECK-NEXT: %8 = phi %struct.T* [ %phitmp, %dynamic_cast.valid ], [ null, %entry ]
+// CHECK-NEXT: ret %struct.T* %8
+// CHECK-NEXT: }
+
+void* test7(V* x) { return dynamic_cast<void*>(x); }
+// CHECK: define i8* @"\01?test7@@YAPAXPAUV@@@Z"(%struct.V* %x) #1 {
+// CHECK-NEXT: entry:
+// CHECK-NEXT: %0 = bitcast %struct.V* %x to i8*
+// CHECK-NEXT: %1 = tail call i8* @__RTCastToVoid(i8* %0) #2
+// CHECK-NEXT: ret i8* %1
+// CHECK-NEXT: }
+
+void* test8(A* x) { return dynamic_cast<void*>(x); }
+// CHECK: define i8* @"\01?test8@@YAPAXPAUA@@@Z"(%struct.A* %x) #1 {
+// CHECK-NEXT: entry:
+// CHECK-NEXT: %0 = icmp eq %struct.A* %x, null
+// CHECK-NEXT: br i1 %0, label %dynamic_cast.call, label %dynamic_cast.valid
+// CHECK: dynamic_cast.valid: ; preds = %entry
+// CHECK-NEXT: %1 = bitcast %struct.A* %x to i8*
+// CHECK-NEXT: %2 = bitcast %struct.A* %x to i8**
+// CHECK-NEXT: %vbtable = load i8** %2, align 4
+// CHECK-NEXT: %3 = getelementptr inbounds i8* %vbtable, i32 4
+// CHECK-NEXT: %4 = bitcast i8* %3 to i32*
+// CHECK-NEXT: %vbase_offs = load i32* %4, align 4
+// CHECK-NEXT: %5 = getelementptr inbounds i8* %1, i32 %vbase_offs
+// CHECK-NEXT: %6 = tail call i8* @__RTCastToVoid(i8* %5) #2
+// CHECK-NEXT: br label %dynamic_cast.call
+// CHECK: dynamic_cast.call: ; preds = %dynamic_cast.valid, %entry
+// CHECK-NEXT: %7 = phi i8* [ %6, %dynamic_cast.valid ], [ null, %entry ]
+// CHECK-NEXT: ret i8* %7
+// CHECK-NEXT: }
+
+void* test9(B* x) { return dynamic_cast<void*>(x); }
+// CHECK: define i8* @"\01?test9@@YAPAXPAUB@@@Z"(%struct.B* %x) #1 {
+// CHECK-NEXT: entry:
+// CHECK-NEXT: %0 = icmp eq %struct.B* %x, null
+// CHECK-NEXT: br i1 %0, label %dynamic_cast.call, label %dynamic_cast.valid
+// CHECK: dynamic_cast.valid: ; preds = %entry
+// CHECK-NEXT: %1 = getelementptr inbounds %struct.B* %x, i32 0, i32 0, i32 0
+// CHECK-NEXT: %vbptr = getelementptr inbounds i8* %1, i32 4
+// CHECK-NEXT: %2 = bitcast i8* %vbptr to i8**
+// CHECK-NEXT: %vbtable = load i8** %2, align 4
+// CHECK-NEXT: %3 = getelementptr inbounds i8* %vbtable, i32 4
+// CHECK-NEXT: %4 = bitcast i8* %3 to i32*
+// CHECK-NEXT: %vbase_offs = load i32* %4, align 4
+// CHECK-NEXT: %5 = add nsw i32 %vbase_offs, 4
+// CHECK-NEXT: %6 = getelementptr inbounds i8* %1, i32 %5
+// CHECK-NEXT: %7 = tail call i8* @__RTCastToVoid(i8* %6) #2
+// CHECK-NEXT: br label %dynamic_cast.call
+// CHECK: dynamic_cast.call: ; preds = %dynamic_cast.valid, %entry
+// CHECK-NEXT: %8 = phi i8* [ %7, %dynamic_cast.valid ], [ null, %entry ]
+// CHECK-NEXT: ret i8* %8
+// CHECK-NEXT: }
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