[analyzer] canonicalize special case of structure/pointer deref

This simple change addresses a special case of structure/pointer
aliasing that produced different symbolvals, leading to false positives
during analysis.

The reproducer is as simple as this.

```lang=C++
struct s {
  int v;
};

void foo(struct s *ps) {
  struct s ss = *ps;
  clang_analyzer_dump(ss.v); // reg_$1<int Element{SymRegion{reg_$0<struct s *ps>},0 S64b,struct s}.v>
  clang_analyzer_dump(ps->v); //reg_$3<int SymRegion{reg_$0<struct s *ps>}.v>
  clang_analyzer_eval(ss.v == ps->v); // UNKNOWN
}
```

Acks: Many thanks to @steakhal and @martong for the group debug session.

Reviewed By: steakhal, martong

Differential Revision: https://reviews.llvm.org/D110625

Author: vabridgers

clang/lib/StaticAnalyzer/Core/Store.cpp

@@ -442,6 +442,19 @@ SVal StoreManager::getLValueIvar(const ObjCIvarDecl *decl, SVal base) {
 
 SVal StoreManager::getLValueElement(QualType elementType, NonLoc Offset,
                                     SVal Base) {
+
+  // Special case, if index is 0, return the same type as if
+  // this was not an array dereference.
+  if (Offset.isZeroConstant()) {
+    QualType BT = Base.getType(this->Ctx);
+    if (!BT.isNull() && !elementType.isNull()) {
+      QualType PointeeTy = BT->getPointeeType();
+      if (!PointeeTy.isNull() &&
+          PointeeTy.getCanonicalType() == elementType.getCanonicalType())
+        return Base;
+    }
+  }
+
   // If the base is an unknown or undefined value, just return it back.
   // FIXME: For absolute pointer addresses, we just return that value back as
   //  well, although in reality we should return the offset added to that

clang/test/Analysis/ptr-arith.c

@@ -2,6 +2,7 @@
 // RUN: %clang_analyze_cc1 -analyzer-checker=alpha.core.FixedAddr,alpha.core.PointerArithm,alpha.core.PointerSub,debug.ExprInspection -analyzer-store=region -Wno-pointer-to-int-cast -verify -triple i686-apple-darwin9 -Wno-tautological-pointer-compare -analyzer-config eagerly-assume=false %s
 
 void clang_analyzer_eval(int);
+void clang_analyzer_dump(int);
 
 void f1() {
   int a[10];
@@ -330,3 +331,59 @@ float test_nowarning_on_vector_deref() {
   simd_float2 x = {0, 1};
   return x[1]; // no-warning
 }
+
+struct s {
+  int v;
+};
+
+// These three expressions should produce the same sym vals.
+void struct_pointer_canon(struct s *ps) {
+  struct s ss = *ps;
+  clang_analyzer_dump((*ps).v);
+  // expected-warning-re@-1{{reg_${{[[:digit:]]+}}<int SymRegion{reg_${{[[:digit:]]+}}<struct s * ps>}.v>}}
+  clang_analyzer_dump(ps[0].v);
+  // expected-warning-re@-1{{reg_${{[[:digit:]]+}}<int SymRegion{reg_${{[[:digit:]]+}}<struct s * ps>}.v>}}
+  clang_analyzer_dump(ps->v);
+  // expected-warning-re@-1{{reg_${{[[:digit:]]+}}<int SymRegion{reg_${{[[:digit:]]+}}<struct s * ps>}.v>}}
+  clang_analyzer_eval((*ps).v == ps[0].v); // expected-warning{{TRUE}}
+  clang_analyzer_eval((*ps).v == ps->v);   // expected-warning{{TRUE}}
+  clang_analyzer_eval(ps[0].v == ps->v);   // expected-warning{{TRUE}}
+}
+
+void struct_pointer_canon_bidim(struct s **ps) {
+  struct s ss = **ps;
+  clang_analyzer_eval(&(ps[0][0].v) == &((*ps)->v)); // expected-warning{{TRUE}}
+}
+
+typedef struct s T1;
+typedef struct s T2;
+void struct_pointer_canon_typedef(T1 *ps) {
+  T2 ss = *ps;
+  clang_analyzer_dump((*ps).v);
+  // expected-warning-re@-1{{reg_${{[[:digit:]]+}}<int SymRegion{reg_${{[[:digit:]]+}}<T1 * ps>}.v>}}
+  clang_analyzer_dump(ps[0].v);
+  // expected-warning-re@-1{{reg_${{[[:digit:]]+}}<int SymRegion{reg_${{[[:digit:]]+}}<T1 * ps>}.v>}}
+  clang_analyzer_dump(ps->v);
+  // expected-warning-re@-1{{reg_${{[[:digit:]]+}}<int SymRegion{reg_${{[[:digit:]]+}}<T1 * ps>}.v>}}
+  clang_analyzer_eval((*ps).v == ps[0].v); // expected-warning{{TRUE}}
+  clang_analyzer_eval((*ps).v == ps->v);   // expected-warning{{TRUE}}
+  clang_analyzer_eval(ps[0].v == ps->v);   // expected-warning{{TRUE}}
+}
+
+void struct_pointer_canon_bidim_typedef(T1 **ps) {
+  T2 ss = **ps;
+  clang_analyzer_eval(&(ps[0][0].v) == &((*ps)->v)); // expected-warning{{TRUE}}
+}
+
+void struct_pointer_canon_const(const struct s *ps) {
+  struct s ss = *ps;
+  clang_analyzer_dump((*ps).v);
+  // expected-warning-re@-1{{reg_${{[[:digit:]]+}}<int SymRegion{reg_${{[[:digit:]]+}}<const struct s * ps>}.v>}}
+  clang_analyzer_dump(ps[0].v);
+  // expected-warning-re@-1{{reg_${{[[:digit:]]+}}<int SymRegion{reg_${{[[:digit:]]+}}<const struct s * ps>}.v>}}
+  clang_analyzer_dump(ps->v);
+  // expected-warning-re@-1{{reg_${{[[:digit:]]+}}<int SymRegion{reg_${{[[:digit:]]+}}<const struct s * ps>}.v>}}
+  clang_analyzer_eval((*ps).v == ps[0].v); // expected-warning{{TRUE}}
+  clang_analyzer_eval((*ps).v == ps->v);   // expected-warning{{TRUE}}
+  clang_analyzer_eval(ps[0].v == ps->v);   // expected-warning{{TRUE}}
+}