Is it possible to build a constexpr data structure across several source files?

Question

I'm trying to build an intrusive profiler in which tags are hand placed. It would be very nice if those tags could be translated into integers that directly index into a data structure at compile time so runtime code for the profile tag was as small as possible (perhaps appending an integer identifier to a ring buffer, for example).

Is it possible to use constexpr to do this completely at compile time? Needless to say, the tags will be occurring in several different compilation units.

I've discovered that taking the address of an extern is constexpr friendly in C++17, but I haven't thought of a way to make this do what I want.

Is there a way to do what I want, or is it simply not possible? The main hurdle I see is that it seems like it would require some compile time state to transferred between compilation units. Using the extern trick, you can kind of accomplish this, but I'm not sure the state that's being shared is useful for what I want to do.

Here's an example of what I'm talking about:

file1.cpp:

#include <ProfilerThing.h>

#define xstr(a) str(a)
#define str(a) $a

int function1()
{
    static constexpr auto token1 = MakeGlobalToken(__PRETTY_FUNCTION__ " at " __FILE__ ": " xstr(__LINE__));
    BeginProfile(token1);

    EndProfile(token1);
}

file2.cpp:

int function2()
{
    static constexpr auto token2 = MakeGlobalToken(__PRETTY_FUNCTION__ " at " __FILE__ ": " xstr(__LINE__));
    BeginProfile(token2);

    EndProfile(token2);
}

What would MakeGlobalToken have to look like? Is it even possible to write? token1 and token2 have to be unique. And, ideally, they would be indexes into some sort of data structure.

Can you provide a sample usage? The description isn't entirely clear to me. — Passer By, Sep 10 '18 at 04:05
Are you perhaps looking for `inline constexpr` variables (https://stackoverflow.com/questions/38043442/how-do-inline-variables-work)? — cantordust, Sep 10 '18 at 04:26
unclear what you are asking: what are you profiling:number of instances in use or? do you want to tag instances or types or...? — NoSenseEtAl, Sep 10 '18 at 07:35
does this help you ? https://www.youtube.com/watch?v=gI6Qtn4US9E — NoSenseEtAl, Sep 10 '18 at 15:58
@NoSenseEtAl - I'll have to watch that video all the way through, but from just the title it looks very promising. — Omnifarious, Sep 10 '18 at 16:58
@NoSenseEtAl - Almost. That only works within a single compilation unit I think though. — Omnifarious, Sep 10 '18 at 17:53

score 2 · Answer 1 · answered Sep 10 '18 at 13:11

2

The first thing that comes to mind is addresses. They are guaranteed to be unique and are readily hashable.

template<auto>
struct token
{
    inline static struct {} dummy;
};

template<auto x>
constexpr void* MakeGlobalToken()
{
    return &token<x>::dummy;
}

Used as

inline std::unordered_map<void*, int> logger;

void BeginProfile(void* token)
{
    logger[token]++;
}

void EndProfile(void* token)
{
    logger[token]++;
}

int function1()
{
    static constexpr auto token1 = MakeGlobalToken<function1>();
    BeginProfile(token1);

    EndProfile(token1);
}

int function2()
{
    static constexpr auto token2 = MakeGlobalToken<function2>();
    BeginProfile(token2);

    EndProfile(token2);
}

answered Sep 10 '18 at 13:11

Passer By

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I was thinking about addresses too. The problem is that it would be really nice to squirrel away data about the profile point at compile time too. – Omnifarious Sep 10 '18 at 13:33
@Omnifarious What do you mean? You could always write `BeginProfile` if that's what you're asking. – Passer By Sep 11 '18 at 01:43
I mean store a string with the function name and line number. – Omnifarious Sep 11 '18 at 03:52

Is it possible to build a constexpr data structure across several source files?

1 Answers1