Teach SimpleSValBuilder that (in the absence of more information) stack memory doesn't alias symbolic memory. This is a heuristic/hack, but works well in practice. Fixes <rdar://problem/10978247>.

llvm-svn: 152065

clang/lib/StaticAnalyzer/Core/SimpleSValBuilder.cpp

@@ -714,6 +714,24 @@ SVal SimpleSValBuilder::evalBinOpLL(ProgramStateRef state,
 
     // The two regions are from the same base region. See if they're both a
     // type of region we know how to compare.
+    const MemSpaceRegion *LeftMS = LeftBase->getMemorySpace();
+    const MemSpaceRegion *RightMS = RightBase->getMemorySpace();
+
+    // Heuristic: assume that no symbolic region (whose memory space is
+    // unknown) is on the stack.
+    // FIXME: we should be able to be more precise once we can do better
+    // aliasing constraints for symbolic regions, but this is a reasonable,
+    // albeit unsound, assumption that holds most of the time.
+    if (isa<StackSpaceRegion>(LeftMS) ^ isa<StackSpaceRegion>(RightMS)) {
+      switch (op) {
+        default:
+          break;
+        case BO_EQ:
+          return makeTruthVal(false, resultTy);
+        case BO_NE:
+          return makeTruthVal(true, resultTy);
+      }
+    }
 
     // FIXME: If/when there is a getAsRawOffset() for FieldRegions, this
     // ElementRegion path and the FieldRegion path below should be unified.

clang/test/Analysis/malloc.c

@@ -728,6 +728,38 @@ int my_main_warn(FILE *f) {
     return 0;// expected-warning {{leak}}
 }
 
+// <rdar://problem/10978247>.
+// some people use stack allocated memory as an optimization to avoid
+// a heap allocation for small work sizes.  This tests the analyzer's
+// understanding that the malloc'ed memory is not the same as stackBuffer.
+void radar10978247(int myValueSize) {
+  char stackBuffer[128];
+  char *buffer;
+
+  if (myValueSize <= sizeof(stackBuffer))
+    buffer = stackBuffer;
+  else 
+    buffer = malloc(myValueSize);
+
+  // do stuff with the buffer
+  if (buffer != stackBuffer)
+    free(buffer);
+}
+
+void radar10978247_positive(int myValueSize) {
+  char stackBuffer[128];
+  char *buffer;
+
+  if (myValueSize <= sizeof(stackBuffer))
+    buffer = stackBuffer;
+  else 
+    buffer = malloc(myValueSize);
+
+  // do stuff with the buffer
+  if (buffer == stackBuffer) // expected-warning {{leak}}
+    return;
+}
+
 // ----------------------------------------------------------------------------
 // Below are the known false positives.
 

clang/test/Analysis/ptr-arith.c

@@ -269,7 +269,7 @@ void symbolic_region(int *p) {
   int a;
 
   if (&a == p)
-    WARN; // expected-warning{{}}
+    WARN; // no-warning
   if (&a != p)
     WARN; // expected-warning{{}}
   if (&a > p)