Add support for simple lambda functions in forward mode

This commit provides support for the simplest lambda functions,
that is, those with no captures in forward mode. The original
lambda function is copied into the derivative, but the
corresponding lambda class gets extended to also have a pushforward
method for the call operator overload.

lib/Differentiator/BaseForwardModeVisitor.cpp

@@ -1030,6 +1030,10 @@ StmtDiff BaseForwardModeVisitor::VisitCallExpr(const CallExpr* CE) {
          "Differentiation of only direct calls is supported. Ignored");
     return StmtDiff(Clone(CE));
   }
+  // Calls to lambda functions are processed differently
+  bool isLambda =
+      (FD->getDeclContext()->isRecord() &&
+       FD->getDeclContext()->getOuterLexicalRecordContext()->isLambda());
 
   SourceLocation validLoc{CE->getBeginLoc()};
 
@@ -1064,18 +1068,32 @@ StmtDiff BaseForwardModeVisitor::VisitCallExpr(const CallExpr* CE) {
   // a direct or indirect (operator overload) call to member function.
   StmtDiff baseDiff;
   // Add derivative of the implicit `this` pointer to the `diffArgs`.
-  if (auto MD = dyn_cast<CXXMethodDecl>(FD)) {
-    if (MD->isInstance()) {
-      const Expr* baseOriginalE = nullptr;
-      if (auto MCE = dyn_cast<CXXMemberCallExpr>(CE))
-        baseOriginalE = MCE->getImplicitObjectArgument();
-      else if (auto OCE = dyn_cast<CXXOperatorCallExpr>(CE))
-        baseOriginalE = OCE->getArg(0);
-      baseDiff = Visit(baseOriginalE);
-      Expr* baseDerivative = baseDiff.getExpr_dx();
-      if (!baseDerivative->getType()->isPointerType())
-        baseDerivative = BuildOp(UnaryOperatorKind::UO_AddrOf, baseDerivative);
-      diffArgs.push_back(baseDerivative);
+  if (const auto* MD = dyn_cast<CXXMethodDecl>(FD)) {
+    if (isLambda) {
+      if (const auto* OCE = dyn_cast<CXXOperatorCallExpr>(CE)) {
+        QualType ptrType = m_Context.getPointerType(m_Context.getRecordType(
+            FD->getDeclContext()->getOuterLexicalRecordContext()));
+        // For now, only lambdas with no captures are supported, so we just pass
+        // a nullptr instead of the diff object.
+        baseDiff =
+            StmtDiff(Clone(OCE->getArg(0)),
+                     new (m_Context) CXXNullPtrLiteralExpr(ptrType, validLoc));
+        diffArgs.push_back(baseDiff.getExpr_dx());
+      }
+    } else { // isLambda == false
+      if (MD->isInstance()) {
+        const Expr* baseOriginalE = nullptr;
+        if (const auto* MCE = dyn_cast<CXXMemberCallExpr>(CE))
+          baseOriginalE = MCE->getImplicitObjectArgument();
+        else if (const auto* OCE = dyn_cast<CXXOperatorCallExpr>(CE))
+          baseOriginalE = OCE->getArg(0);
+        baseDiff = Visit(baseOriginalE);
+        Expr* baseDerivative = baseDiff.getExpr_dx();
+        if (!baseDerivative->getType()->isPointerType())
+          baseDerivative =
+              BuildOp(UnaryOperatorKind::UO_AddrOf, baseDerivative);
+        diffArgs.push_back(baseDerivative);
+      }
     }
   }
 
@@ -1131,34 +1149,36 @@ StmtDiff BaseForwardModeVisitor::VisitCallExpr(const CallExpr* CE) {
 
   auto customDerivativeArgs = pushforwardFnArgs;
 
-  if (baseDiff.getExpr()) {
-    Expr* baseE = baseDiff.getExpr();
+  if (Expr* baseE = baseDiff.getExpr()) {
     if (!baseE->getType()->isPointerType())
       baseE = BuildOp(UnaryOperatorKind::UO_AddrOf, baseE);
     customDerivativeArgs.insert(customDerivativeArgs.begin(), baseE);
   }
 
   // Try to find a user-defined overloaded derivative.
+  Expr* callDiff = nullptr;
   std::string customPushforward =
       clad::utils::ComputeEffectiveFnName(FD) + GetPushForwardFunctionSuffix();
-  Expr* callDiff = m_Builder.BuildCallToCustomDerivativeOrNumericalDiff(
+  callDiff = m_Builder.BuildCallToCustomDerivativeOrNumericalDiff(
       customPushforward, customDerivativeArgs, getCurrentScope(),
       const_cast<DeclContext*>(FD->getDeclContext()));
 
-  // Check if it is a recursive call.
-  if (!callDiff && (FD == m_DiffReq.Function) &&
-      m_Mode == GetPushForwardMode()) {
-    // The differentiated function is called recursively.
-    Expr* derivativeRef =
-        m_Sema
-            .BuildDeclarationNameExpr(CXXScopeSpec(),
-                                      m_Derivative->getNameInfo(), m_Derivative)
-            .get();
-    callDiff =
-        m_Sema
-            .ActOnCallExpr(m_Sema.getScopeForContext(m_Sema.CurContext),
-                           derivativeRef, validLoc, pushforwardFnArgs, validLoc)
-            .get();
+  if (!isLambda) {
+    // Check if it is a recursive call.
+    if (!callDiff && (FD == m_DiffReq.Function) &&
+        m_Mode == GetPushForwardMode()) {
+      // The differentiated function is called recursively.
+      Expr* derivativeRef =
+          m_Sema
+              .BuildDeclarationNameExpr(
+                  CXXScopeSpec(), m_Derivative->getNameInfo(), m_Derivative)
+              .get();
+      callDiff = m_Sema
+                     .ActOnCallExpr(
+                         m_Sema.getScopeForContext(m_Sema.CurContext),
+                         derivativeRef, validLoc, pushforwardFnArgs, validLoc)
+                     .get();
+    }
   }
 
   // If all arguments are constant literals, then this does not contribute to
@@ -1499,7 +1519,12 @@ StmtDiff BaseForwardModeVisitor::VisitDeclStmt(const DeclStmt* DS) {
     if (QT->isPointerType())
       QT = QT->getPointeeType();
     auto* typeDecl = QT->getAsCXXRecordDecl();
-    if (typeDecl && clad::utils::hasNonDifferentiableAttribute(typeDecl)) {
+    // For lambda functions, 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.
+    if (typeDecl && (clad::utils::hasNonDifferentiableAttribute(typeDecl) ||
+                     typeDecl->isLambda())) {
       for (auto* D : DS->decls()) {
         if (auto* VD = dyn_cast<VarDecl>(D))
           decls.push_back(VD);

test/ForwardMode/Lambdas.C

@@ -0,0 +1,29 @@
+// RUN: %cladclang %s -I%S/../../include -oLambdas.out 2>&1 | %filecheck %s
+// RUN: ./Lambdas.out | %filecheck_exec %s
+// CHECK-NOT: {{.*error|warning|note:.*}}
+
+#include "clad/Differentiator/Differentiator.h"
+
+double fn0(double x) {
+    auto _f = [](double _x) {
+        return _x*_x;
+    };
+    return _f(x) + 1;
+}
+
+double fn1(double x, double y) {
+    auto _f = [](double _x, double _y) {
+        return _x + _y;
+    };
+    return _f(x*x, x+2) + y;
+}
+
+int main() {
+    auto fn0_dx = clad::differentiate(fn0, 0);
+    printf("Result is = %.2f\n", fn0_dx.execute(7)); // CHECK-EXEC: Result is = 14.00
+    printf("Result is = %.2f\n", fn0_dx.execute(-1)); // CHECK-EXEC: Result is = -2.00
+
+    auto fn1_dx = clad::differentiate(fn1, 0);
+    printf("Result is = %.2f\n", fn1_dx.execute(7, 1)); // CHECK-EXEC: Result is = 15.00
+    printf("Result is = %.2f\n", fn1_dx.execute(-1, 1)); // CHECK-EXEC: Result is = -1.00
+}