Add support for non-differentiable attribute in reverse mode

fixes #717

lib/Differentiator/ReverseModeVisitor.cpp

@@ -1432,6 +1432,26 @@ Expr* getArraySizeExpr(const ArrayType* AT, ASTContext& context,
       return StmtDiff(Clone(CE));
     }
 
+    // If the function is non_differentiable, return zero derivative.
+    if (clad::utils::hasNonDifferentiableAttribute(CE)) {
+      // Calling the function without computing derivatives
+      llvm::SmallVector<Expr*, 4> ClonedArgs;
+      for (unsigned i = 0, e = CE->getNumArgs(); i < e; ++i)
+        ClonedArgs.push_back(Clone(CE->getArg(i)));
+
+      SourceLocation validLoc = clad::utils::GetValidSLoc(m_Sema);
+      Expr* Call = m_Sema
+                       .ActOnCallExpr(getCurrentScope(), Clone(CE->getCallee()),
+                                      validLoc, ClonedArgs, validLoc)
+                       .get();
+      // Creating a zero derivative
+      auto* zero = ConstantFolder::synthesizeLiteral(m_Context.IntTy, m_Context,
+                                                     /*val=*/0);
+
+      // Returning the function call and zero derivative
+      return StmtDiff(Call, zero);
+    }
+
     auto NArgs = FD->getNumParams();
     // If the function has no args and is not a member function call then we
     // assume that it is not related to independent variables and does not
@@ -2073,6 +2093,13 @@ Expr* getArraySizeExpr(const ArrayType* AT, ASTContext& context,
     } else if (opCode == UnaryOperatorKind::UO_Deref) {
       diff = Visit(E);
       Expr* cloneE = BuildOp(UnaryOperatorKind::UO_Deref, diff.getExpr());
+
+      // If we have a pointer to a member expression, which is
+      // non-differentiable, we just return a clone of the original expression.
+      if (auto* ME = dyn_cast<MemberExpr>(diff.getExpr()))
+        if (clad::utils::hasNonDifferentiableAttribute(ME->getMemberDecl()))
+          return {cloneE};
+
       Expr* diff_dx = diff.getExpr_dx();
       bool specialDThisCase = false;
       Expr* derivedE = nullptr;
@@ -2662,9 +2689,14 @@ Expr* getArraySizeExpr(const ArrayType* AT, ASTContext& context,
     // If `VD` is a reference to a non-local variable then also there's no
     // need to call `Visit` since non-local variables are not differentiated.
     if (!isDerivativeOfRefType && (!isPointerType || isInitializedByNewExpr)) {
-      Expr* derivedE = BuildDeclRef(VDDerived);
-      if (isInitializedByNewExpr)
-        derivedE = BuildOp(UnaryOperatorKind::UO_Deref, derivedE);
+      Expr* derivedE = nullptr;
+
+      if (!clad::utils::hasNonDifferentiableAttribute(VD)) {
+        derivedE = BuildDeclRef(VDDerived);
+        if (isInitializedByNewExpr)
+          derivedE = BuildOp(UnaryOperatorKind::UO_Deref, derivedE);
+      }
+
       if (VD->getInit()) {
         if (isa<CXXConstructExpr>(VD->getInit()))
           initDiff = Visit(VD->getInit());
@@ -2696,6 +2728,7 @@ Expr* getArraySizeExpr(const ArrayType* AT, ASTContext& context,
         addToCurrentBlock(assignToZero, direction::reverse);
       }
     }
+
     VarDecl* VDClone = nullptr;
     Expr* derivedVDE = nullptr;
     if (VDDerived)
@@ -2822,19 +2855,21 @@ Expr* getArraySizeExpr(const ArrayType* AT, ASTContext& context,
     if (declsBegin != DS->decls().end() && isa<VarDecl>(*declsBegin)) {
       auto* VD = dyn_cast<VarDecl>(*declsBegin);
       QualType QT = VD->getType();
-      if (!QT->isPointerType()) {
-        auto* typeDecl = QT->getAsCXXRecordDecl();
-        // We should also simply copy the original lambda. The differentiation
-        // of lambdas is happening in the `VisitCallExpr`. For now, only the
-        // declarations with lambda expressions without captures are supported.
-        isLambda = typeDecl && typeDecl->isLambda();
-        if (isLambda) {
-          for (auto* D : DS->decls())
-            if (auto* VD = dyn_cast<VarDecl>(D))
-              decls.push_back(VD);
-          Stmt* DSClone = BuildDeclStmt(decls);
-          return StmtDiff(DSClone, nullptr);
-        }
+      if (QT->isPointerType())
+        QT = QT->getPointeeType();
+
+      auto* typeDecl = QT->getAsCXXRecordDecl();
+      // We should also simply copy the original lambda. The differentiation
+      // of lambdas is happening in the `VisitCallExpr`. For now, only the
+      // declarations with lambda expressions without captures are supported.
+      isLambda = typeDecl && typeDecl->isLambda();
+      if (isLambda ||
+          (typeDecl && clad::utils::hasNonDifferentiableAttribute(typeDecl))) {
+        for (auto* D : DS->decls())
+          if (auto* VD = dyn_cast<VarDecl>(D))
+            decls.push_back(VD);
+        Stmt* DSClone = BuildDeclStmt(decls);
+        return StmtDiff(DSClone, nullptr);
       }
     }
 
@@ -2846,6 +2881,7 @@ Expr* getArraySizeExpr(const ArrayType* AT, ASTContext& context,
     for (auto* D : DS->decls()) {
       if (auto* VD = dyn_cast<VarDecl>(D)) {
         DeclDiff<VarDecl> VDDiff;
+
         if (!isLambda)
           VDDiff = DifferentiateVarDecl(VD);
 
@@ -3021,6 +3057,11 @@ Expr* getArraySizeExpr(const ArrayType* AT, ASTContext& context,
            "CXXMethodDecl nodes not supported yet!");
     MemberExpr* clonedME = utils::BuildMemberExpr(
         m_Sema, getCurrentScope(), baseDiff.getExpr(), field->getName());
+    if (clad::utils::hasNonDifferentiableAttribute(ME)) {
+      auto* zero = ConstantFolder::synthesizeLiteral(m_Context.IntTy, m_Context,
+                                                     /*val=*/0);
+      return {clonedME, zero};
+    }
     if (!baseDiff.getExpr_dx())
       return {clonedME, nullptr};
     MemberExpr* derivedME = utils::BuildMemberExpr(