It is possible to avoid ODR violations :) We put the per-target code into unique namespaces, and export a function pointer to them.

You can do many thing with macros and inline namespaces but I believe they run into problems when modules come into play. Can you compile the same code twice, with different flags with modules?

We use pragma target instead of compiler flags :)

I don't think we understand each other.

We want to take one function and compile it twice:

``` namespace MEGA_MACRO {

void foo(std::span<int> s) { super_awesome_platform_specific_thing(s); }

} // namespace MEGA_MACRO ```

Whatever you do - the code above has to be written once but compiled twice. In one file/in many files - doesn't matter.

My point is - I don't think you can compile that code twice if you support modules.

I think I do understand, this is exactly what we do. (MEGA_MACRO == HWY_NAMESPACE)

Then we have a table of function pointers to &AVX2::foo, &AVX3::foo etc. As long as the module exports one single thing, which either calls into or exports this table, I do not see how it is incompatible with building your project using modules enabled?

(The way we compile the code twice is to re-include our source file, taking care that only the SIMD parts are actually seen by the compiler, and stuff like the module exports would only be compiled once.)

> is to re-include our source file

Yeah - that means your source file is never a module. We would really like eve to be modularized, the CI times are unbearable.

I'd love to be proven wrong here, that'd be amazing. But I don't think google highway can be modularized.

What leads you to that conclusion? It is still possible to use #include in module implementations. We can use that to make the module implementation look like your example.

Thus it ought to be possible, though I have not yet tried it.

Well.

You have a file, something like: load.h

You need to include it multiple times, compiled with different flags.

So - it's never going to be in load.cxx or whatever that's called.

As mentioned ("re-include our source file"), we are indeed able to put the SIMD code, as well as the self-#include of itself, in a load.cxx TU.

Here is an example: https://github.com/google/gemma.cpp/blob/9dfe2a76be63bcfe679...

I don't think this works if your files are modules.

Let's stop here, it doesn't seem like we understand each other.