[clang][Interp] Handle DeclRefExpr of reference types

References are implemented through pointers, so we need a second deref
when encountering a DeclRefExpr of a reference type.

Differential Revision: https://reviews.llvm.org/D132997

clang/lib/AST/Interp/ByteCodeExprGen.cpp

@@ -814,15 +814,38 @@ bool ByteCodeExprGen<Emitter>::VisitUnaryOperator(const UnaryOperator *E) {
 template <class Emitter>
 bool ByteCodeExprGen<Emitter>::VisitDeclRefExpr(const DeclRefExpr *E) {
   const auto *Decl = E->getDecl();
+  bool IsReference = Decl->getType()->isReferenceType();
+  bool FoundDecl = false;
 
   if (auto It = Locals.find(Decl); It != Locals.end()) {
     const unsigned Offset = It->second.Offset;
-    return this->emitGetPtrLocal(Offset, E);
+    if (!this->emitGetPtrLocal(Offset, E))
+      return false;
+
+    FoundDecl = true;
   } else if (auto GlobalIndex = P.getGlobal(Decl)) {
-    return this->emitGetPtrGlobal(*GlobalIndex, E);
+    if (!this->emitGetPtrGlobal(*GlobalIndex, E))
+      return false;
+
+    FoundDecl = true;
   } else if (const auto *PVD = dyn_cast<ParmVarDecl>(Decl)) {
-    if (auto It = this->Params.find(PVD); It != this->Params.end())
-      return this->emitGetPtrParam(It->second, E);
+    if (auto It = this->Params.find(PVD); It != this->Params.end()) {
+      if (!this->emitGetPtrParam(It->second, E))
+        return false;
+
+      FoundDecl = true;
+    }
+  }
+
+  // References are implemented using pointers, so when we get here,
+  // we have a pointer to a pointer, which we need to de-reference once.
+  if (FoundDecl) {
+    if (IsReference) {
+      if (!this->emitLoadPopPtr(E))
+        return false;
+    }
+
+    return true;
   }
 
   return false;

clang/test/AST/Interp/arrays.cpp

@@ -45,6 +45,13 @@ constexpr T getElementOf(T* array, int i) {
 static_assert(getElementOf(foo[0], 1) == &m, "");
 
 
+template <typename T, int N>
+constexpr T& getElementOfArray(T (&array)[N], int I) {
+  return array[I];
+}
+static_assert(getElementOfArray(foo[2], 3) == &m, "");
+
+
 constexpr int data[] = {5, 4, 3, 2, 1};
 static_assert(data[0] == 4, ""); // expected-error{{failed}} \
                                  // expected-note{{5 == 4}} \

clang/test/AST/Interp/references.cpp

@@ -0,0 +1,91 @@
+// RUN: %clang_cc1 -fexperimental-new-constant-interpreter -verify %s
+// RUN: %clang_cc1 -verify=ref %s
+
+
+// ref-no-diagnostics
+
+constexpr int a = 10;
+constexpr const int &b = a;
+static_assert(a == b, "");
+
+constexpr int assignToReference() {
+  int a = 20;
+  int &b = a;
+
+  b = 100;
+  return a;
+}
+static_assert(assignToReference() == 100, "");
+
+
+constexpr void setValue(int &dest, int val) {
+  dest = val;
+}
+
+constexpr int checkSetValue() {
+  int l = 100;
+  setValue(l, 200);
+  return l;
+}
+static_assert(checkSetValue() == 200, "");
+
+constexpr int readLocalRef() {
+  int a = 20;
+  int &b = a;
+  return b;
+}
+static_assert(readLocalRef() == 20, "");
+
+constexpr int incRef() {
+  int a = 0;
+  int &b = a;
+
+  b = b + 1;
+
+  return a;
+}
+static_assert(incRef() == 1, "");
+
+
+template<const int &V>
+constexpr void Plus3(int &A) {
+  A = V + 3;
+}
+constexpr int foo = 4;
+
+constexpr int callTemplate() {
+  int a = 3;
+  Plus3<foo>(a);
+  return a;
+}
+static_assert(callTemplate() == 7, "");
+
+
+constexpr int& getValue(int *array, int index) {
+  return array[index];
+}
+constexpr int testGetValue() {
+  int values[] = {1, 2, 3, 4};
+  getValue(values, 2) = 30;
+  return values[2];
+}
+static_assert(testGetValue() == 30, "");
+
+// FIXME: ExprWithCleanups + MaterializeTemporaryExpr not implemented
+constexpr const int &MCE = 1; // expected-error{{must be initialized by a constant expression}}
+
+
+struct S {
+  int i, j;
+};
+
+constexpr int RefToMemberExpr() {
+  S s{1, 2};
+
+  int &j = s.i;
+  j += 10;
+
+  return j;
+}
+// FIXME: Should be accepted.
+static_assert(RefToMemberExpr() == 11, ""); // expected-error{{not an integral constant expression}}