LLVM.如何根据结构名称访问结构域? [英] LLVM. How to access to struct fields based on their names?

问题描述

我在C ++中几乎没有示例代码:

I have little example code in C++:

struct RecordTest
{
    int value1;
    int value2;
};

void test()
{
    RecordTest rt;
    rt.value1 = 15;
    rt.value2 = 75;
}

和LLVM 3.4 IR:

and LLVM 3.4 IR for it:

%struct.RecordTest = type { i32, i32 }

; Function Attrs: nounwind
define void @_Z4testv() #0 {
entry:
  %rt = alloca %struct.RecordTest, align 4
  %value1 = getelementptr inbounds %struct.RecordTest* %rt, i32 0, i32 0
  store i32 15, i32* %value1, align 4
  %value2 = getelementptr inbounds %struct.RecordTest* %rt, i32 0, i32 1
  store i32 75, i32* %value2, align 4
  ret void
}

和一个很简单的问题:如何访问RecordTest字段(在解析.cpp时)，而没有它们的索引，而仅使用名称(value1和value2)?

and a pretty easy question: How can I access to RecordTest fields (when I parsing .cpp), without their indexes, with only names (value1 and value2)?

我只知道一种方法(来自llc -march=cpp)-具有索引:

I know only one way (from llc -march=cpp) - with indexes:

  AllocaInst* ptr_rt = new AllocaInst(StructTy_struct_RecordTest, "rt", label_entry);
  ptr_rt->setAlignment(4);
  std::vector<Value*> ptr_value1_indices;
  ptr_value1_indices.push_back(const_int32_6);
  ptr_value1_indices.push_back(const_int32_6);
  Instruction* ptr_value1 = GetElementPtrInst::Create(ptr_rt, ptr_value1_indices, "value1", label_entry);
  StoreInst* void_9 = new StoreInst(const_int32_7, ptr_value1, false, label_entry);
  void_9->setAlignment(4);
  std::vector<Value*> ptr_value2_indices;
  ptr_value2_indices.push_back(const_int32_6);
  ptr_value2_indices.push_back(const_int32_5);
  Instruction* ptr_value2 = GetElementPtrInst::Create(ptr_rt, ptr_value2_indices, "value2", label_entry);
  StoreInst* void_10 = new StoreInst(const_int32_8, ptr_value2, false, label_entry);
  void_10->setAlignment(4);

因此，如果我不知道字段的索引(上面的代码中的const_int32_5和const_int32_6)，我可以从C ++转换为LLVM IR吗?

So, can I translate from C++ to LLVM IR, if I don't know the indexes of the fields (const_int32_5 and const_int32_6 in code above) ?

UPD ===============================

UPD================================

因此，我们无法访问字段名称.如果我们需要它(如果解析.cpp，我们也会这样做)， 我们可以这样写:

So, we can't access to field names. And if we need it (and we do, if we parse .cpp), we can write something like this:

 // It can be some kind of singletone
static std::map<std::string, std::vector<std::string>> mymap;

// Some function, where we first time meet RecordTest
    std::vector<std::string> fieldNames;
    fieldNames.push_back("value1");
    fieldNames.push_back("value2");
    mymap["RecordTest"] = fieldNames;

// Some function, where we need to access to RecordTest field
    std::vector<std::string> fieldNamesAgain = mymap.find("RecordTest")->second;
    std::string fieldName = "value1";
    int idxValue1 = -1;
    for (int i = 0, e = fieldNamesAgain.size(); i < e; i++) // little ugly search
    {
        if (fieldName == fieldNamesAgain[i])
        {
                // we get field index, and now we can build code
                // as in example above (llc -march=cpp)
            idxValue1 = i;  
            break;
        }
    }

这是对的吗?

推荐答案

您不能按名称(仅按索引)访问结构的字段.当您使用Clang编译时，此信息通常不存在.

You cannot access the fields of the struct by name, only by index. This information is just normally not there when you compile with Clang.

对此有一个例外，那就是您编译了调试信息.在这种情况下，您将拥有有关类型的大量数据；具体来说，您将获得字段的顺序，以及每个字段的元数据条目，其中包含该字段的名称(以及其他有用的内容，例如从类型开头的偏移量).

There is one exception to this, and this is if you compiled with debug information. In that case, you'll have ample data about the type; specifically, you'll get the order of the fields, along with a metadata entry for each field which contains its name (and other useful stuff, such as its offset from the beginning of the type).

在源代码级调试指南中了解有关此内容的详细信息-特别是，请参见关于结构编码的这一部分，其示例非常好.

Read more about this on the Source Level Debugging guide - and particularly, see this section about struct encoding, with its very nice example.

查看 DebugInfo.h 中的类，以帮助查询调试信息，尽管我认为您还是必须手动进行一些挖掘.

Take a look at DebugInfo.h for classes to help on querying debug info, though I think you're going to have to do some manually digging anyway.

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