Add support for Basic Forward Mode Differentiation with Enzyme

This commit adds support for differentiation of a function of type:
```cpp
double func(double x, double y){
	return x*y;
}
```

include/clad/Differentiator/ForwardModeVisitor.h

@@ -13,6 +13,9 @@
 #include "clang/AST/StmtVisitor.h"
 #include "clang/Sema/Sema.h"
 
+#include "clad/Differentiator/DiffPlanner.h"
+
+
 #include <array>
 #include <stack>
 #include <unordered_map>
@@ -28,6 +31,14 @@ namespace clad {
     unsigned m_IndependentVarIndex = ~0;
     unsigned m_DerivativeOrder = ~0;
     unsigned m_ArgIndex = ~0;
+    DiffInputVarsInfo m_DVI;
+    bool use_enzyme = false;
+
+    // Function to Differentiate with Clad as Backend
+    void DifferentiateWithClad();
+
+    // Function to Differentiate with Enzyme as Backend
+    void DifferentiateWithEnzyme();
 
   public:
     ForwardModeVisitor(DerivativeBuilder& builder);

lib/Differentiator/ForwardModeVisitor.cpp

@@ -9,7 +9,6 @@
 #include "ConstantFolder.h"
 
 #include "clad/Differentiator/CladUtils.h"
-#include "clad/Differentiator/DiffPlanner.h"
 #include "clad/Differentiator/ErrorEstimator.h"
 #include "clad/Differentiator/StmtClone.h"
 
@@ -214,8 +213,7 @@ namespace clad {
     m_DerivativeInFlight = true;
 
     DiffInputVarsInfo DVI = request.DVI;
-
-    DVI = request.DVI;
+    m_DVI=DVI;
 
     // FIXME: Shouldn't we give error here that no arg is specified?
     if (DVI.empty())
@@ -239,6 +237,10 @@ namespace clad {
     // FIXME: implement gradient-vector products to fix the issue.
     assert((DVI.size() == 1) &&
            "nested forward mode differentiation for several args is broken");
+
+        // Check if DiffRequest asks for use of enzyme as backend
+    if (request.use_enzyme)
+      use_enzyme = true;
 
     // FIXME: Differentiation variable cannot always be represented just by
     // `ValueDecl*` variable. For example -- `u.mem1.mem2,`, `arr[7]` etc.
@@ -307,6 +309,9 @@ namespace clad {
     for (auto field : diffVarInfo.fields)
       argInfo += "_" + field;
 
+    if(use_enzyme)
+      derivativeSuffix+="_enzyme";
+
     IdentifierInfo* II =
         &m_Context.Idents.get(request.BaseFunctionName + "_d" + s + "arg" +
                               argInfo + derivativeSuffix);
@@ -371,6 +376,33 @@ namespace clad {
     beginScope(Scope::FnScope | Scope::DeclScope);
     m_DerivativeFnScope = getCurrentScope();
     beginBlock();
+
+    if(!use_enzyme)
+      DifferentiateWithClad();
+    else 
+      DifferentiateWithEnzyme();
+
+
+    Stmt* derivativeBody = endBlock();
+    derivedFD->setBody(derivativeBody);
+
+    endScope(); // Function body scope
+    m_Sema.PopFunctionScopeInfo();
+    m_Sema.PopDeclContext();
+    endScope(); // Function decl scope
+
+    m_DerivativeInFlight = false;
+
+    return DerivativeAndOverload{result.first,
+                                 /*OverloadFunctionDecl=*/nullptr};
+  }
+
+  void ForwardModeVisitor::DifferentiateWithClad(){
+    const clang::FunctionDecl* FD = m_Function;
+    llvm::ArrayRef<ParmVarDecl*> params = m_Derivative->parameters();
+    DiffInputVarInfo diffVarInfo = m_DVI.back(); 
+
+
     // For each function parameter variable, store its derivative value.
     for (auto param : params) {
       // We cannot create derivatives of reference type since seed value is
@@ -507,18 +539,76 @@ namespace clad {
         addToCurrentBlock(S);
     else
       addToCurrentBlock(BodyDiff);
-    Stmt* derivativeBody = endBlock();
-    derivedFD->setBody(derivativeBody);
-
-    endScope(); // Function body scope
-    m_Sema.PopFunctionScopeInfo();
-    m_Sema.PopDeclContext();
-    endScope(); // Function decl scope
+  }
 
-    m_DerivativeInFlight = false;
+  void ForwardModeVisitor::DifferentiateWithEnzyme(){
+    llvm::ArrayRef<ParmVarDecl*> paramsRef = m_Derivative->parameters();
+    unsigned numParams = m_Function->getNumParams();
+    auto originalFnType = dyn_cast<FunctionProtoType>(m_Function->getType());
+    auto returnType = m_Derivative->getReturnType();
+
+
+    // Prepare Arguments and Parameters to enzyme_autodiff
+    llvm::SmallVector<Expr*, 16> enzymeArgs;
+    llvm::SmallVector<ParmVarDecl*, 16> enzymeParams;
+
+    // First add the function itself as a parameter/argument
+    enzymeArgs.push_back(BuildDeclRef(const_cast<FunctionDecl*>(m_Function)));
+    DeclContext* fdDeclContext =
+        const_cast<DeclContext*>(m_Function->getDeclContext());
+    enzymeParams.push_back(m_Sema.BuildParmVarDeclForTypedef(
+        fdDeclContext, noLoc, m_Function->getType()));
+
+    // Add rest of the parameters/arguments
+    for (int i = 0; i < numParams; i++) {
+      auto paramType = paramsRef[i]->getType().getNonReferenceType();
+
+      // First Add the original parameter
+      enzymeArgs.push_back(BuildDeclRef(paramsRef[i]));
+      enzymeParams.push_back(m_Sema.BuildParmVarDeclForTypedef(
+          fdDeclContext, noLoc, paramType));
+
+      float fDx=0;
+      if(paramsRef[i]==m_IndependentVar)
+        fDx=1;
+      Expr* dx = dyn_cast<Expr>(
+            FloatingLiteral::Create(m_Context, llvm::APFloat(fDx),
+                                   false,paramType, noLoc));
+
+      // Then add the dx argument
+      enzymeArgs.push_back(dx);
+      enzymeParams.push_back(m_Sema.BuildParmVarDeclForTypedef(
+          fdDeclContext, noLoc, paramType));
+    }
 
-    return DerivativeAndOverload{result.first,
-                                 /*OverloadFunctionDecl=*/nullptr};
+    llvm::SmallVector<QualType, 16> enzymeParamsType;
+    for (auto i : enzymeParams)
+      enzymeParamsType.push_back(i->getType());
+
+    // Prepare Function call
+    std::string enzymeCallName =
+        "__enzyme_fwddiff_" + m_Function->getNameAsString()+"_"+m_IndependentVar->getNameAsString();
+    IdentifierInfo* IIEnzyme = &m_Context.Idents.get(enzymeCallName);
+    DeclarationName nameEnzyme(IIEnzyme);
+    QualType enzymeFunctionType =
+        m_Sema.BuildFunctionType(returnType, enzymeParamsType, noLoc, nameEnzyme,
+                                 originalFnType->getExtProtoInfo());
+    FunctionDecl* enzymeCallFD = FunctionDecl::Create(
+        m_Context, fdDeclContext, noLoc, noLoc, nameEnzyme, enzymeFunctionType,
+        m_Function->getTypeSourceInfo(), SC_Extern);
+    enzymeCallFD->setParams(enzymeParams);
+    Expr* enzymeCall = BuildCallExprToFunction(enzymeCallFD, enzymeArgs);
+
+    auto diffDecl = BuildVarDecl(returnType, "diff", enzymeCall, true);
+    addToCurrentBlock(BuildDeclStmt(diffDecl));
+
+    // auto type = m_IndependentVar->getType().getNonReferenceType();
+    // Expr* retExpr = dyn_cast<Expr>(
+    //         FloatingLiteral::Create(m_Context, llvm::APFloat(5.01),
+    //                                true,type, noLoc));
+    Stmt* returnStmt = m_Sema.BuildReturnStmt(noLoc,BuildDeclRef(diffDecl)).get();
+
+    addToCurrentBlock(returnStmt);    
   }
 
   StmtDiff ForwardModeVisitor::VisitStmt(const Stmt* S) {

test/Enzyme/ForwardMode.C

@@ -1,17 +1,22 @@
 // RUN: %cladclang %s -lstdc++ -I%S/../../include -oEnzyme.out 2>&1 | FileCheck %s
-// RUN: ./Enzyme.out
+// RUN: ./Enzyme.out | FileCheck -check-prefix=CHECK-EXEC %s
 // CHECK-NOT: {{.*error|warning|note:.*}}
 // REQUIRES: Enzyme
-// XFAIL:*
 // Forward mode is not implemented yet
 
 #include "clad/Differentiator/Differentiator.h"
 
-double f(double x, double y) { return x * y; }
+double f(double x, double y) {
+      return x * y; 
+}
 
-// CHECK:     double f_darg0_enzyme(double x, double y) {
-// CHECK-NEXT:}
+// CHECK: double f_darg0_enzyme(double x, double y) {
+// CHECK-NEXT:     double diff = __enzyme_fwddiff_f_x(f, x, 1., y, 0.);
+// CHECK-NEXT:     return diff;
+// CHECK-NEXT: }
 
 int main(){
      auto f_dx = clad::differentiate<clad::opts::use_enzyme>(f, "x");
+     double ans = f_dx.execute(3,4);
+     printf("Ans = %.2f\n",ans); // CHECK-EXEC: Ans = 4.00
 }