Letting go of C++11


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This post is about Nuitka the Python compiler started out using C++0x which is now C++11, and then chose to stop it.

In the Beginning

Very early on, when I considered how to generate code from the node tree, in a way, that mistakes should practically be impossible to make, I made the fundamental decision, that every Python expression, which produces temporary variables, should become an expression in the generated code too.

Note

That is my choice, I think it keeps code generation more simple, and easier to understand. There may come a separate post about how that played out.

That decision meant some trouble. Certain things were not easy, but generally, it was achievable for g++ relatively quickly, and then lots of helper functions would be needed. Think of MAKE_TUPLE and MAKE_DICT, but also other stuff needed that. Calling a Python built-in with variable number of parameters e.g. could be implemented that way easily.

Other nice things were enum classes, and generally good stuff. It was really quick to get Nuitka code generation off the ground this way.

Reality Strikes

But then, as time went on, I found that the order of evaluation was becoming an issue. It became apparent that for more and more things, I needed to reverse it, so it works. Porting to ARM, it then became clear, that it needs to be the other way around for that platform. And checking out clang, which is also a C++11 compiler, I noticed, this one yet uses a different one.

So, for normal functions, I found a solution that involves the pre-processor to reverse or not, both function definition and call sites, and then it is already correct.

This of course, doesn't work for C++11 variadic functions. So, there came a point, where I had to realize, that each of its uses was more or less causing evaluation order bugs. So that most of their uses were already removed. And so I basically knew they couldn't stay that way.

Other Features

Also, things I initially assumed, e.g. that lambda functions of C++11 may prove useful, or even "auto", didn't turn out to be true. There seemingly is a wealth of new features, besides variadic templates that I didn't see how Nuitka would benefit from it at all.

New Wishes

Then, at Europython, I realized, that Android is still stuck with g++-4.4 and as such, that an important target platform will be unavailable to me. This platform will become even more important, as I intend to buy an device now.

Biting it

So what I did, was to remove all variadic functions and instead generate code for them as necessary. I just need to trace the used argument counts, and then provide those, simple enough.

Also, other things like deleted copy constructors, and so on, I had to give up on these a bit.

This change was probably suited to remove subtle evaluation order problems, although I don't recall seeing them.

The Present

The current stable release still requires C++11, but the next release will work on g++-4.4 and compiles fine with MSVC from Visual Studio 2008, although at this time, there is still the issue of generators not working yet, but I believe that ought to be solvable.

The new requirement is only C++03, which means, there is a good chance that supporting Android will become feasible. I know there is interest from App developers, because there, even the relatively unimportant 2x speedup, that Nuitka might give for some code, may matter.

Conclusion

So that is a detour, I have taken, expanding the base of Nuitka even further. I felt, this was important enough to write down the history part of it.

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