PR30937: don't devirtualize if we find that the callee is a pure virtual

function. In that case, there is no requirement that the callee is actually
defined, and the code may in fact be valid and have defined behavior if the
virtual call is unreachable.

llvm-svn: 286534

Author: zygoloid

clang/lib/CodeGen/CGClass.cpp

@@ -2851,41 +2851,38 @@ CodeGenFunction::CanDevirtualizeMemberFunctionCall(const Expr *Base,
     return false;
 
   // If the member function is marked 'final', we know that it can't be
-  // overridden and can therefore devirtualize it.
+  // overridden and can therefore devirtualize it unless it's pure virtual.
   if (MD->hasAttr<FinalAttr>())
-    return true;
+    return !MD->isPure();
 
   // If the base expression (after skipping derived-to-base conversions) is a
   // class prvalue, then we can devirtualize.
   Base = Base->getBestDynamicClassTypeExpr();
   if (Base->isRValue() && Base->getType()->isRecordType())
     return true;
 
-  // If the most derived class is marked final, we know that no subclass can
-  // override this member function and so we can devirtualize it. For example:
-  //
-  // struct A { virtual void f(); }
-  // struct B final : A { };
-  //
-  // void f(B *b) {
-  //   b->f();
-  // }
-  //
-  if (const CXXRecordDecl *BestDynamicDecl = Base->getBestDynamicClassType()) {
-    if (BestDynamicDecl->hasAttr<FinalAttr>())
-      return true;
+  // If we don't even know what we would call, we can't devirtualize.
+  const CXXRecordDecl *BestDynamicDecl = Base->getBestDynamicClassType();
+  if (!BestDynamicDecl)
+    return false;
 
-    // There may be a method corresponding to MD in a derived class. If that
-    // method is marked final, we can devirtualize it.
-    const CXXMethodDecl *DevirtualizedMethod =
-        MD->getCorrespondingMethodInClass(BestDynamicDecl);
-    if (DevirtualizedMethod->hasAttr<FinalAttr>())
-      return true;
-  }
+  // There may be a method corresponding to MD in a derived class.
+  const CXXMethodDecl *DevirtualizedMethod =
+      MD->getCorrespondingMethodInClass(BestDynamicDecl);
+
+  // If that method is pure virtual, we can't devirtualize. If this code is
+  // reached, the result would be UB, not a direct call to the derived class
+  // function, and we can't assume the derived class function is defined.
+  if (DevirtualizedMethod->isPure())
+    return false;
+
+  // If that method is marked final, we can devirtualize it.
+  if (DevirtualizedMethod->hasAttr<FinalAttr>())
+    return true;
 
   // Similarly, if the class itself is marked 'final' it can't be overridden
   // and we can therefore devirtualize the member function call.
-  if (MD->getParent()->hasAttr<FinalAttr>())
+  if (BestDynamicDecl->hasAttr<FinalAttr>())
     return true;
 
   if (const DeclRefExpr *DRE = dyn_cast<DeclRefExpr>(Base)) {

clang/test/CodeGenCXX/devirtualize-virtual-function-calls.cpp

@@ -161,3 +161,37 @@ namespace test4 {
     p->fish.eat();
   }
 }
+
+// Do not devirtualize to pure virtual function calls.
+namespace test5 {
+  struct X {
+    virtual void f() = 0;
+  };
+  struct Y {};
+  // CHECK-LABEL: define {{.*}} @_ZN5test51f
+  void f(Y &y, X Y::*p) {
+    // CHECK-NOT: call {{.*}} @_ZN5test51X1fEv
+    // CHECK: call void %
+    (y.*p).f();
+  };
+
+  struct Z final {
+    virtual void f() = 0;
+  };
+  // CHECK-LABEL: define {{.*}} @_ZN5test51g
+  void g(Z &z) {
+    // CHECK-NOT: call {{.*}} @_ZN5test51Z1fEv
+    // CHECK: call void %
+    z.f();
+  }
+
+  struct Q {
+    virtual void f() final = 0;
+  };
+  // CHECK-LABEL: define {{.*}} @_ZN5test51h
+  void h(Q &q) {
+    // CHECK-NOT: call {{.*}} @_ZN5test51Q1fEv
+    // CHECK: call void %
+    q.f();
+  }
+}