I'd like to modify my compiler's code generator to use visitor pattern since the current approach must use multiple conditional statement to check the real type of a child before generating the corresponding code. However, I have problems to get children attributes after they're visited. For instance, in binary expression I use this:

LHSCode := GenerateExpressionCode(LHSNode);
RHSCode := GenerateExpressionCode(RHSNode);
CreateBinaryExpression(Self,LHS,RHS);

In visitor pattern the visit method is usually void, so I can't get the expression code from LHS and RHS. Keeping shared global variables isn't an option since expression code generation is recursive thus could erase previous values kept in the variables.

I'll just show the binary expression as this is the most complicated part (for now):

function TLLVMCodeGenerator.GenerateExpressionCode(
  Expr: TASTExpression): TLLVMValue;
var
  BinExpr: TASTBinaryExpression;
  UnExpr: TASTUnaryExpression;
  LHSCode, RHSCode, ExprCode: TLLVMValue;
  VarExpr: TASTVariableExpression;
begin
  if Expr is TASTBinaryExpression then begin
    BinExpr := Expr as TASTBinaryExpression;
    LHSCode := GenerateExpressionCode(BinExpr.LHS);
    RHSCode := GenerateExpressionCode(BinExpr.RHS);
    case BinExpr.Op of
      '<': Result := FBuilder.CreateICmp(ccSLT, LHSCode, RHSCode);
      '<=': Result := FBuilder.CreateICmp(ccSLE, LHSCode, RHSCode);
      '>': Result := FBuilder.CreateICmp(ccSGT, LHSCode, RHSCode);
      '>=': Result := FBuilder.CreateICmp(ccSGE, LHSCode, RHSCode);
      '==': Result := FBuilder.CreateICmp(ccEQ, LHSCode, RHSCode);
      '<>': Result := FBuilder.CreateICmp(ccNE, LHSCode, RHSCode);
      '/\': Result := FBuilder.CreateAnd(LHSCode, RHSCode);
      '\/': Result := FBuilder.CreateOr(LHSCode, RHSCode);
      '+': Result := FBuilder.CreateAdd(LHSCode, RHSCode);
      '-': Result := FBuilder.CreateSub(LHSCode, RHSCode);
      '*': Result := FBuilder.CreateMul(LHSCode, RHSCode);
      '/': Result := FBuilder.CreateSDiv(LHSCode, RHSCode);
    end;
  end else if Expr is TASTPrimaryExpression then
    if Expr is TASTBooleanConstant then
      with Expr as TASTBooleanConstant do
        Result := FBuilder.CreateConstant(Ord(Value), ltI1)
    else if Expr is TASTIntegerConstant then
      with Expr as TASTIntegerConstant do
        Result := FBuilder.CreateConstant(Value, ltI32)
    else if Expr is TASTUnaryExpression then begin
      UnExpr := Expr as TASTUnaryExpression;
      ExprCode := GenerateExpressionCode(UnExpr.Expr);
      case UnExpr.Op of
        '~': Result := FBuilder.CreateXor(
            FBuilder.CreateConstant(1, ltI1), ExprCode);
        '-': Result := FBuilder.CreateSub(
            FBuilder.CreateConstant(0, ltI32), ExprCode);
      end;
    end else if Expr is TASTVariableExpression then begin
      VarExpr := Expr as TASTVariableExpression;
      with VarExpr.VarDecl do
        Result := FBuilder.CreateVar(Ident, BaseTypeLLVMTypeMap[BaseType]);
    end;
end;

Hope you understand it :)