[analyzer] Teach FindLastStoreBRVisitor to understand stores of the same value.

Consider this case:

  int *p = 0;
  p = getPointerThatMayBeNull();
  *p = 1;

If we inline 'getPointerThatMayBeNull', we might know that the value of 'p'
is NULL, and thus emit a null pointer dereference report. However, we
usually want to suppress such warnings as error paths, and we do so by using
FindLastStoreBRVisitor to see where the NULL came from. In this case, though,
because 'p' was NULL both before and after the assignment, the visitor
would decide that the "last store" was the initialization, not the
re-assignment.

This commit changes FindLastStoreBRVisitor to consider all PostStore nodes
that assign to this region. This still won't catches changes made directly
by checkers if they re-assign the same value, but it does handle the common
case in user-written code and will trigger ReturnVisitor's suppression
machinery as expected.

<rdar://problem/13299738>

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@176201 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 11 insertions, 4 deletions

diff --git a/lib/StaticAnalyzer/Core/BugReporterVisitors.cpp b/lib/StaticAnalyzer/Core/BugReporterVisitors.cpp
index 8dbfc8b61d..d712c18b78 100644
--- a/lib/StaticAnalyzer/Core/BugReporterVisitors.cpp
+++ b/lib/StaticAnalyzer/Core/BugReporterVisitors.cpp
@@ -378,13 +378,20 @@ PathDiagnosticPiece *FindLastStoreBRVisitor::VisitNode(const ExplodedNode *Succ,
     }
   }
 
-  // Otherwise, check that Succ has this binding and Pred does not, i.e. this is
-  // where the binding first occurred.
+  // Otherwise, see if this is the store site:
+  // (1) Succ has this binding and Pred does not, i.e. this is
+  //     where the binding first occurred.
+  // (2) Succ has this binding and is a PostStore node for this region, i.e.
+  //     the same binding was re-assigned here.
   if (!StoreSite) {
     if (Succ->getState()->getSVal(R) != V)
       return NULL;
-    if (Pred->getState()->getSVal(R) == V)
-      return NULL;
+
+    if (Pred->getState()->getSVal(R) == V) {
+      Optional<PostStore> PS = Succ->getLocationAs<PostStore>();
+      if (!PS || PS->getLocationValue() != R)
+        return NULL;
+    }
 
     StoreSite = Succ;