clang time-trace 'InstantiateFunction' problem

[leaflinfeng]

I was using clang time-trace to analyze the compilation time of our project. Some cpp file does only include 'json.hpp' indirectly, without using any json class. But I found that many nlohmann json's related 'InstantiateFunction' in the time-trace events, and the cumulative time is very high. I wonder why? And how to avoid this？ thx 🙏

The minimum cpp demo :

#include <iostream>
#include "nlohmann/json.hpp"

int main()
{
    std::cout << "hello world" << std::endl;
}

clang time-trace :

{
    "traceEvents": [
        {
            "pid": 7685,
            "tid": 259,
            "ph": "X",
            "ts": 698177,
            "dur": 30477,
            "name": "InstantiateFunction",
            "args": {
                "detail": "nlohmann::basic_json<>::json_value::destroy"
            }
        },
        {
            "pid": 7685,
            "tid": 259,
            "ph": "X",
            "ts": 680277,
            "dur": 48378,
            "name": "InstantiateFunction",
            "args": {
                "detail": "nlohmann::basic_json<>::basic_json"
            }
        },
        {
            "pid": 7685,
            "tid": 259,
            "ph": "X",
            "ts": 679317,
            "dur": 49499,
            "name": "InstantiateFunction",
            "args": {
                "detail": "nlohmann::basic_json<>::basic_json"
            }
        },
       //  many other InstantiateFunction
}

[nlohmann]

I cannot reproduce this with a debugger. Can you share the call to clang?

[leaflinfeng]

I cannot reproduce this with a debugger. Can you share the call to clang?

main.cpp

#include <iostream>
#include "nlohmann/json.hpp"

int main() {
    std::cout << "hello world" << std::endl;
}

I used the following command on macos :
clang -x c++ -ftime-trace -std=c++17 -c main.cpp -o main.o

[nlohmann]

I can create the JSON file now, thank you. Though I have no idea how to interpret the results. I would of course assume there is effort for the compiler to parse the included header.