Can I get the index of a value in the register by custom_lib? about behavioral-model HOT 4 CLOSED

You can define your own extern function or object, and implement this functionality in C++. There is an example here: https://github.com/p4lang/behavioral-model/tree/main/examples/custom_extern

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This example just show that how to manipulate basic data. If I want to do som operation in p4 register, how do I declaremy extern
p4 function define my c++ function

from behavioral-model.

From a bmv2 standpoint, you could write the following extern function:

void register_find(bm::Data &index, const bm::RegisterArray &ra, const bm::Data &v) {
	// look for value "v" in "ra", and set "index" appropriately
}
BM_REGISTER_EXTERN_FUNCTION(register_find, bm::Data &, const bm::RegisterArray &, const bm::Data &);

I am not sure what the P4 declaration would look like, and I am not sure if there is full compiler support (p4c with bmv2 backend) for this. But it is worth a try. Maybe something like this:

extern void register_find<T>(inout bit<32> index, in register<T> ra, in T v);

If this doesn't work, you will need to define your own register extern type (instead of using the standard v1model one), with the extra method that you need. This should be a good example: https://github.com/p4lang/behavioral-model/blob/main/targets/psa_switch/externs/psa_counter.h. You will need to do a similar thing, but with RegisterArray instead of CounterArray. An important thing to keep in mind is that bmv2 only supports registers which store binary strings (bit<X> / int<X> in P4). It doesn't support registers with an arbitrary value type (e.g. struct).

from behavioral-model.

Thanks for your reply. I've tried the first method, but the p4c compiler throws an error like this:

p4c-bm2-ss --emit-externs -o example.json example.p4
extern_lib/declaration.p4(19): [--Werror=type-error] error: ra: a parameter with type register cannot have a direction
extern void register_find<T>(inout bit<32> index, in register<T> ra, in T v);

after I change the declaration of register_find to extern void register_find<T>(inout bit<32> index, register<T> ra, in T v);. it throw an error example.p4(71): [--Werror=target-error] error: testRegister: Could not generate code for expression register_find(find_index, testRegister, 32w56);. This is most likely p4c does not support this;
But https://github.com/p4lang/behavioral-model/blob/main/targets/psa_switch/externs/psa_counter.h this example gives me a lot of help. I can define my own register and extend its function like this:

class MyRegister : public bm::ExternType{
  public:
   static constexpr bm::p4object_id_t spec_id = 0xffffffff;
    BM_EXTERN_ATTRIBUTES{
      BM_EXTERN_ATTRIBUTE_ADD(num);
      BM_EXTERN_ATTRIBUTE_ADD(width);
    }
  
  void init() override {
    _array = std::unique_ptr<bm::RegisterArray>(
        new bm::RegisterArray(get_name() + ".$impl",
                         spec_id,
                         num.get<size_t>(), width.get_int()));
  }

  void read(const bm::Data &index, bm::Data &value){
   // read array[index] to valeue
  }

  void write(const bm::Data &index, const bm::Data &value){
    // write value to array[index]
  }

  void find_index(const bm::Data &value, bm::Data &index){
   // find the index of value in array
  }

  private:
    bm::Data width;
    bm::Data num;
    std::unique_ptr<bm::RegisterArray> _array;
};

BM_REGISTER_EXTERN(MyRegister);
BM_REGISTER_EXTERN_METHOD(MyRegister, read, const bm::Data &, bm::Data &);
BM_REGISTER_EXTERN_METHOD(MyRegister, write, const bm::Data &,const bm::Data &);
BM_REGISTER_EXTERN_METHOD(MyRegister, find_index, const bm::Data &,bm::Data &);

and declare it in p4:

extern MyRegister<T>{
    MyRegister(T num, T width);
    void read(in T index, inout T value);
    void write(in T index, in T value);
    void find_index(in T value, inout T index);
}

Thank you again for your reply, it helped me a lot

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