Remove DiffMode::error_estimation by merging it with DiffMode::reverse

Unlike pullback requests, gradient requests track information about error estimation by having 2 separate modes: `DiffMode::reverse` and `DiffMode::error_estimation`. Even though `DiffMode::error_estimation` is immediately changed to `DiffMode::reverse` at the beginning of the reverse mode visitation. This PR also removes `DiffMode::error_estimation` completely, as this information is already tracked with `EnableErrorEstimation`.

Author: PetroZarytskyi

include/clad/Differentiator/DiffMode.h

@@ -13,8 +13,7 @@ enum class DiffMode {
   hessian,
   hessian_diagonal,
   jacobian,
-  reverse_mode_forward_pass,
-  error_estimation
+  reverse_mode_forward_pass
 };
 
 /// Convert enum value to string.
@@ -40,8 +39,6 @@ inline const char* DiffModeToString(DiffMode mode) {
     return "jacobian";
   case DiffMode::reverse_mode_forward_pass:
     return "reverse_forw";
-  case DiffMode::error_estimation:
-    return "error_estimation";
   default:
     return "unknown";
   }

include/clad/Differentiator/DiffPlanner.h

@@ -104,7 +104,6 @@ struct DiffRequest {
   bool ImmediateMode = false;
   /// A flag specifying whether this differentiation is to be used
   /// for error estimation.
-  /// FIXME: Should this be reflected in the DiffMode?
   bool EnableErrorEstimation = false;
   /// Puts the derived function and its code in the diff call
   void updateCall(clang::FunctionDecl* FD, clang::FunctionDecl* OverloadedFD,

lib/Differentiator/CladUtils.cpp

@@ -1187,8 +1187,7 @@ namespace clad {
         return resType;
       }
 
-      if (Mode == DiffMode::reverse || Mode == DiffMode::pullback ||
-          Mode == DiffMode::error_estimation) {
+      if (Mode == DiffMode::reverse || Mode == DiffMode::pullback) {
         QualType ValueType = GetNonConstValueType(Type);
         QualType nonRefValueType = ValueType.getNonReferenceType();
         return C.getPointerType(nonRefValueType);
@@ -1253,7 +1252,6 @@ namespace clad {
       QualType dRetTy = C.VoidTy;
       bool returnVoid = mode == DiffMode::reverse ||
                         mode == DiffMode::pullback ||
-                        mode == DiffMode::error_estimation ||
                         mode == DiffMode::vector_forward_mode;
       if (mode == DiffMode::reverse_mode_forward_pass) {
         if (isMemoryType(oRetTy) || isa<CXXConstructorDecl>(FD)) {

lib/Differentiator/DerivativeBuilder.cpp

@@ -552,10 +552,8 @@ static void registerDerivative(Decl* D, Sema& S, const DiffRequest& R) {
     } else if (request.Mode == DiffMode::vector_pushforward) {
       VectorPushForwardModeVisitor V(*this, request);
       result = V.Derive();
-    } else if (request.Mode == DiffMode::reverse) {
-      ReverseModeVisitor V(*this, request);
-      result = V.Derive();
-    } else if (request.Mode == DiffMode::pullback) {
+    } else if (request.Mode == DiffMode::reverse ||
+               request.Mode == DiffMode::pullback) {
       ErrorEstimationHandler handler;
       std::unique_ptr<FPErrorEstimationModel> model;
       ReverseModeVisitor V(*this, request);
@@ -575,14 +573,6 @@ static void registerDerivative(Decl* D, Sema& S, const DiffRequest& R) {
     } else if (request.Mode == DiffMode::jacobian) {
       JacobianModeVisitor J(*this, request);
       result = J.Derive();
-    } else if (request.Mode == DiffMode::error_estimation) {
-      ErrorEstimationHandler handler;
-      FPErrorEstimationModel model(*this, request);
-      handler.SetErrorEstimationModel(&model);
-      ReverseModeVisitor R(*this, request);
-      R.AddExternalSource(handler);
-      // Finally begin estimation.
-      result = R.Derive();
     } else if (const VarDecl* VD = request.Global) {
       // The request represents a global variable, construct the adjoint and
       // register it.

lib/Differentiator/DiffPlanner.cpp

@@ -812,7 +812,7 @@ static QualType GetDerivedFunctionType(const CallExpr* CE) {
     std::string Annotation = A->getAnnotation().str();
     if (Annotation == "E") {
       // Error estimation has no options yet.
-      request.Mode = DiffMode::error_estimation;
+      request.Mode = DiffMode::reverse;
       request.EnableErrorEstimation = true;
       return false;
     }
@@ -827,9 +827,7 @@ static QualType GetDerivedFunctionType(const CallExpr* CE) {
       request.Mode = DiffMode::reverse;
     else
       llvm_unreachable("unknown mode");
-    if (request.Mode == DiffMode::reverse ||
-        request.Mode == DiffMode::hessian ||
-        request.Mode == DiffMode::error_estimation)
+    if (request.Mode == DiffMode::reverse || request.Mode == DiffMode::hessian)
       request.EnableTBRAnalysis = ReqOpts.EnableTBRAnalysis;
     request.EnableVariedAnalysis = ReqOpts.EnableVariedAnalysis;
     request.EnableUsefulAnalysis = ReqOpts.EnableUsefulAnalysis;
@@ -1184,6 +1182,7 @@ static QualType GetDerivedFunctionType(const CallExpr* CE) {
       request.CallContext = E;
       bool canUsePushforwardInRevMode =
           m_TopMostReq->Mode == DiffMode::reverse &&
+          !request.EnableErrorEstimation &&
           utils::canUsePushforwardInRevMode(FD);
 
       std::string FDName = FD->getNameAsString();
@@ -1201,8 +1200,7 @@ static QualType GetDerivedFunctionType(const CallExpr* CE) {
                m_TopMostReq->Mode == DiffMode::hessian ||
                canUsePushforwardInRevMode)
         request.Mode = DiffMode::pushforward;
-      else if (m_TopMostReq->Mode == DiffMode::reverse ||
-               m_TopMostReq->Mode == DiffMode::error_estimation)
+      else if (m_TopMostReq->Mode == DiffMode::reverse)
         request.Mode = DiffMode::pullback;
       else if (m_TopMostReq->Mode == DiffMode::vector_forward_mode ||
                m_TopMostReq->Mode == DiffMode::jacobian ||

lib/Differentiator/ReverseModeVisitor.cpp

@@ -255,9 +255,6 @@ Expr* ReverseModeVisitor::getStdInitListSizeExpr(const Expr* E) {
 
     if (m_ExternalSource)
       m_ExternalSource->ActOnStartOfDerive();
-    if (m_DiffReq.Mode == DiffMode::error_estimation)
-      // NOLINTNEXTLINE(cppcoreguidelines-pro-type-const-cast)
-      const_cast<DiffRequest&>(m_DiffReq).Mode = DiffMode::reverse;
 
     QualType returnTy = m_DiffReq->getReturnType();
     // If reverse mode differentiates only part of the arguments it needs to