From 4449e7f008cbb6da6dbe8a93fdf4cfd89851f7b1 Mon Sep 17 00:00:00 2001
From: Artem Dergachev <artem.dergachev@gmail.com>
Date: Tue, 27 Feb 2018 21:10:08 +0000
Subject: [PATCH] [analyzer] Fix trivial copy for empty objects.

The SVal for any empty C++ object is an UnknownVal. Because RegionStore does
not have binding extents, binding an empty object to an UnknownVal may
potentially overwrite existing bindings at the same offset.

Therefore, when performing a trivial copy of an empty object, don't try to
take the value of the object and bind it to the copy. Doing nothing is accurate
enough, and it doesn't screw any existing bindings.

Differential Revision: https://reviews.llvm.org/D43714

llvm-svn: 326247
---
 clang/lib/StaticAnalyzer/Core/ExprEngineCXX.cpp | 11 +++++++++++
 clang/test/Analysis/ctor.mm                     | 20 ++++++++++++++++++++
 2 files changed, 31 insertions(+)

diff --git a/clang/lib/StaticAnalyzer/Core/ExprEngineCXX.cpp b/clang/lib/StaticAnalyzer/Core/ExprEngineCXX.cpp
index 6502af7..f4aa56f 100644
--- a/clang/lib/StaticAnalyzer/Core/ExprEngineCXX.cpp
+++ b/clang/lib/StaticAnalyzer/Core/ExprEngineCXX.cpp
@@ -42,19 +42,30 @@ void ExprEngine::performTrivialCopy(NodeBuilder &Bldr, ExplodedNode *Pred,
                                     const CallEvent &Call) {
   SVal ThisVal;
   bool AlwaysReturnsLValue;
+  const CXXRecordDecl *ThisRD = nullptr;
   if (const CXXConstructorCall *Ctor = dyn_cast<CXXConstructorCall>(&Call)) {
     assert(Ctor->getDecl()->isTrivial());
     assert(Ctor->getDecl()->isCopyOrMoveConstructor());
     ThisVal = Ctor->getCXXThisVal();
+    ThisRD = Ctor->getDecl()->getParent();
     AlwaysReturnsLValue = false;
   } else {
     assert(cast<CXXMethodDecl>(Call.getDecl())->isTrivial());
     assert(cast<CXXMethodDecl>(Call.getDecl())->getOverloadedOperator() ==
            OO_Equal);
     ThisVal = cast<CXXInstanceCall>(Call).getCXXThisVal();
+    ThisRD = cast<CXXMethodDecl>(Call.getDecl())->getParent();
     AlwaysReturnsLValue = true;
   }
 
+  assert(ThisRD);
+  if (ThisRD->isEmpty()) {
+    // Do nothing for empty classes. Otherwise it'd retrieve an UnknownVal
+    // and bind it and RegionStore would think that the actual value
+    // in this region at this offset is unknown.
+    return;
+  }
+
   const LocationContext *LCtx = Pred->getLocationContext();
 
   ExplodedNodeSet Dst;
diff --git a/clang/test/Analysis/ctor.mm b/clang/test/Analysis/ctor.mm
index 5ef2bee..cfae8ed 100644
--- a/clang/test/Analysis/ctor.mm
+++ b/clang/test/Analysis/ctor.mm
@@ -729,3 +729,23 @@ namespace NoCrashOnEmptyBaseOptimization {
     S s;
   }
 }
+
+namespace EmptyBaseAssign {
+struct B1 {};
+struct B2 { int x; };
+struct D: public B1, public B2 {
+const D &operator=(const D &d) {
+  *((B2 *)this) = d;
+  *((B1 *)this) = d;
+  return *this;
+}
+};
+
+void test() {
+  D d1;
+  d1.x = 1;
+  D d2;
+  d2 = d1;
+  clang_analyzer_eval(d2.x == 1); // expected-warning{{TRUE}}
+}
+}
-- 
2.7.4