Fix gradient of fxns with const reference parameters

lib/Differentiator/ReverseModeVisitor.cpp

@@ -1455,6 +1455,13 @@ Expr* getArraySizeExpr(const ArrayType* AT, ASTContext& context,
           FD->getParamDecl(i - static_cast<unsigned long>(isCXXOperatorCall));
       StmtDiff argDiff{};
       bool passByRef = utils::IsReferenceOrPointerType(PVD->getType());
+      if (passByRef && isa<MaterializeTemporaryExpr>(arg)) {
+        // If the argument is a temporary variable, this means that param type
+        // is a reference to a const type and we are passing a temporary
+        // variable to it. In this case, we should not pass the derivative
+        // argument by reference.
+        passByRef = false;
+      }
       // We do not need to create result arg for arguments passed by reference
       // because the derivatives of arguments passed by reference are directly
       // modified by the derived callee function.
@@ -1498,7 +1505,8 @@ Expr* getArraySizeExpr(const ArrayType* AT, ASTContext& context,
         // same as the call expression as it is the type used to declare the
         // _gradX array
         Expr* dArg;
-        dArg = StoreAndRef(/*E=*/nullptr, arg->getType(), direction::reverse, "_r",
+        QualType argType = utils::GetValueType(arg->getType());
+        dArg = StoreAndRef(/*E=*/nullptr, argType, direction::reverse, "_r",
                            /*forceDeclCreation=*/true);
         ArgResultDecls.push_back(
             cast<VarDecl>(cast<DeclRefExpr>(dArg)->getDecl()));
@@ -1673,6 +1681,13 @@ Expr* getArraySizeExpr(const ArrayType* AT, ASTContext& context,
 
         auto PVD = FD->getParamDecl(idx);
         bool passByRef = utils::IsReferenceOrPointerType(PVD->getType());
+        if (passByRef && isa<MaterializeTemporaryExpr>(CE->getArg(idx))) {
+          // If the argument is a temporary variable, this means that param type
+          // is a reference to a const type and we are passing a temporary
+          // variable to it. In this case, we should not pass the derivative
+          // argument by reference.
+          passByRef = false;
+        }
         if (passByRef) {
           // If derivative type is constant array type instead of
           // `clad::array_ref` or `clad::array` type, then create an
@@ -1700,13 +1715,14 @@ Expr* getArraySizeExpr(const ArrayType* AT, ASTContext& context,
         } else {
           // Declare: diffArgType _grad;
           Expr* initVal = nullptr;
-          if (!PVD->getType()->isRecordType()) {
+          QualType gradVarType = utils::GetValueType(PVD->getType());
+          if (!gradVarType->isRecordType()) {
             // If the argument is not a class type, then initialize the grad
             // variable with 0.
             initVal =
-                ConstantFolder::synthesizeLiteral(PVD->getType(), m_Context, 0);
+                ConstantFolder::synthesizeLiteral(gradVarType, m_Context, 0);
           }
-          gradVarDecl = BuildVarDecl(PVD->getType(), gradVarII, initVal);
+          gradVarDecl = BuildVarDecl(gradVarType, gradVarII, initVal);
           // Pass the address of the declared variable
           gradVarExpr = BuildDeclRef(gradVarDecl);
           gradArgExpr =

test/Gradient/FunctionCalls.C

@@ -491,6 +491,48 @@ double fn8(double x, double y) {
 // CHECK-NEXT:     }
 // CHECK-NEXT: }
 
+double custom_max(const double& a, const double& b) {
+  return a > b ? a : b;
+}
+
+// CHECK: void custom_max_pullback(const double &a, const double &b, double _d_y, clad::array_ref<double> _d_a, clad::array_ref<double> _d_b) {
+// CHECK-NEXT:     bool _cond0;
+// CHECK-NEXT:     _cond0 = a > b;
+// CHECK-NEXT:     goto _label0;
+// CHECK-NEXT:   _label0:
+// CHECK-NEXT:     if (_cond0)
+// CHECK-NEXT:         * _d_a += _d_y;
+// CHECK-NEXT:     else
+// CHECK-NEXT:         * _d_b += _d_y;
+// CHECK-NEXT: }
+
+double fn9(double x, double y) {
+  return custom_max(x*y, y);
+}
+
+// CHECK: void fn9_grad(double x, double y, clad::array_ref<double> _d_x, clad::array_ref<double> _d_y) {
+// CHECK-NEXT:    double _t0;
+// CHECK-NEXT:    double _t1;
+// CHECK-NEXT:    double _t2;
+// CHECK-NEXT:    double _t3;
+// CHECK-NEXT:    _t1 = x;
+// CHECK-NEXT:    _t0 = y;
+// CHECK-NEXT:    _t2 = _t1 * _t0;
+// CHECK-NEXT:    _t3 = y;
+// CHECK-NEXT:    goto _label0;
+// CHECK-NEXT:  _label0:
+// CHECK-NEXT:    {
+// CHECK-NEXT:        double _grad0 = 0.;
+// CHECK-NEXT:        custom_max_pullback(_t2, _t3, 1, &_grad0, &* _d_y);
+// CHECK-NEXT:        double _r0 = _grad0;
+// CHECK-NEXT:        double _r1 = _r0 * _t0;
+// CHECK-NEXT:        * _d_x += _r1;
+// CHECK-NEXT:        double _r2 = _t1 * _r0;
+// CHECK-NEXT:        * _d_y += _r2;
+// CHECK-NEXT:        double _r3 = * _d_y;
+// CHECK-NEXT:    }
+// CHECK-NEXT: }
+
 template<typename T>
 void reset(T* arr, int n) {
   for (int i=0; i<n; ++i)
@@ -544,6 +586,7 @@ int main() {
   INIT(fn6);
   INIT(fn7);
   INIT(fn8);
+  INIT(fn9);
 
   TEST1_float(fn1, 11);         // CHECK-EXEC: {3.00}
   TEST2(fn2, 3, 5);             // CHECK-EXEC: {1.00, 3.00}
@@ -554,4 +597,5 @@ int main() {
   TEST2(fn6, 3, 5);             // CHECK-EXEC: {5.00, 3.00}
   TEST2(fn7, 3, 5);             // CHECK-EXEC: {10.00, 71.00}
   TEST2(fn8, 3, 5);             // CHECK-EXEC: {7.62, 4.57}
+  TEST2(fn9, 3, 5);             // CHECK-EXEC: {5.00, 3.00}
 }