From f975ae4867d1fdfaba11a3ec7e479da8fbfd82d8 Mon Sep 17 00:00:00 2001
From: Zequan Wu <zequanwu@google.com>
Date: Mon, 14 Sep 2020 10:57:23 -0700
Subject: [PATCH] [CodeGen][typeid] Emit typeinfo directly if type is known at
 compile-time

Differential Revision: https://reviews.llvm.org/D87425
---
 clang/include/clang/AST/ExprCXX.h              |  4 ++++
 clang/lib/AST/ExprCXX.cpp                      | 12 ++++++++++++
 clang/lib/CodeGen/CGExprCXX.cpp                |  3 ++-
 clang/test/CodeGenCXX/microsoft-abi-typeid.cpp |  8 +++++---
 4 files changed, 23 insertions(+), 4 deletions(-)

diff --git a/clang/include/clang/AST/ExprCXX.h b/clang/include/clang/AST/ExprCXX.h
index 0ba5e41..9658f37 100644
--- a/clang/include/clang/AST/ExprCXX.h
+++ b/clang/include/clang/AST/ExprCXX.h
@@ -858,6 +858,10 @@ public:
   /// evaluated, per C++11 [expr.typeid]p3.
   bool isPotentiallyEvaluated() const;
 
+  /// Best-effort check if the expression operand refers to a most derived
+  /// object. This is not a strong guarantee.
+  bool isMostDerived(ASTContext &Context) const;
+
   bool isTypeOperand() const { return Operand.is<TypeSourceInfo *>(); }
 
   /// Retrieves the type operand of this typeid() expression after
diff --git a/clang/lib/AST/ExprCXX.cpp b/clang/lib/AST/ExprCXX.cpp
index 3f3f230..1fd2b8e 100644
--- a/clang/lib/AST/ExprCXX.cpp
+++ b/clang/lib/AST/ExprCXX.cpp
@@ -146,6 +146,18 @@ bool CXXTypeidExpr::isPotentiallyEvaluated() const {
   return false;
 }
 
+bool CXXTypeidExpr::isMostDerived(ASTContext &Context) const {
+  assert(!isTypeOperand() && "Cannot call isMostDerived for typeid(type)");
+  const Expr *E = getExprOperand()->IgnoreParenNoopCasts(Context);
+  if (const auto *DRE = dyn_cast<DeclRefExpr>(E)) {
+    QualType Ty = DRE->getDecl()->getType();
+    if (!Ty->isPointerType() && !Ty->isReferenceType())
+      return true;
+  }
+
+  return false;
+}
+
 QualType CXXTypeidExpr::getTypeOperand(ASTContext &Context) const {
   assert(isTypeOperand() && "Cannot call getTypeOperand for typeid(expr)");
   Qualifiers Quals;
diff --git a/clang/lib/CodeGen/CGExprCXX.cpp b/clang/lib/CodeGen/CGExprCXX.cpp
index 50b6079..e33730b 100644
--- a/clang/lib/CodeGen/CGExprCXX.cpp
+++ b/clang/lib/CodeGen/CGExprCXX.cpp
@@ -2199,7 +2199,8 @@ llvm::Value *CodeGenFunction::EmitCXXTypeidExpr(const CXXTypeidExpr *E) {
   //   polymorphic class type, the result refers to a std::type_info object
   //   representing the type of the most derived object (that is, the dynamic
   //   type) to which the glvalue refers.
-  if (E->isPotentiallyEvaluated())
+  // If the operand is already most derived object, no need to look up vtable.
+  if (E->isPotentiallyEvaluated() && !E->isMostDerived(getContext()))
     return EmitTypeidFromVTable(*this, E->getExprOperand(),
                                 StdTypeInfoPtrTy);
 
diff --git a/clang/test/CodeGenCXX/microsoft-abi-typeid.cpp b/clang/test/CodeGenCXX/microsoft-abi-typeid.cpp
index f3bd7e6..8598396 100644
--- a/clang/test/CodeGenCXX/microsoft-abi-typeid.cpp
+++ b/clang/test/CodeGenCXX/microsoft-abi-typeid.cpp
@@ -46,9 +46,11 @@ const std::type_info* test4_typeid() { return &typeid(b); }
 
 const std::type_info* test5_typeid() { return &typeid(v); }
 // CHECK: define dso_local %struct.type_info* @"?test5_typeid@@YAPBUtype_info@@XZ"()
-// CHECK:        [[RT:%.*]] = call i8* @__RTtypeid(i8* bitcast (%struct.V* @"?v@@3UV@@A" to i8*))
-// CHECK-NEXT:   [[RET:%.*]] = bitcast i8* [[RT]] to %struct.type_info*
-// CHECK-NEXT:   ret %struct.type_info* [[RET]]
+// CHECK:   ret %struct.type_info* bitcast (%rtti.TypeDescriptor7* @"??_R0?AUV@@@8" to %struct.type_info*)
+
+const std::type_info *test6_typeid() { return &typeid((V &)v); }
+// CHECK: define dso_local %struct.type_info* @"?test6_typeid@@YAPBUtype_info@@XZ"()
+// CHECK:   ret %struct.type_info* bitcast (%rtti.TypeDescriptor7* @"??_R0?AUV@@@8" to %struct.type_info*)
 
 namespace PR26329 {
 struct Polymorphic {
-- 
2.7.4