[llvm-dev] Updating config.guess, license change

[Harald van Dijk via llvm-dev]

On 05/04/2021 00:24, Petr Hosek wrote:
> You can construct the triple from the variables CMake provides, we 
> already do this for some of the targets: 
> https://github.com/llvm/llvm-project/blob/00d5f1ecccc6d8ece9ac6dd19e9ad807c8a60097/llvm/cmake/modules/GetHostTriple.cmake 
> <https://github.com/llvm/llvm-project/blob/00d5f1ecccc6d8ece9ac6dd19e9ad807c8a60097/llvm/cmake/modules/GetHostTriple.cmake>

"Some of the targets" used to be only Windows, where we do not 
necessarily have a way to run config.guess at all. That seems like a 
perfectly good reason to me to add an exception for.

I do not know why <https://reviews.llvm.org/D74256> then also added 
logic for AIX there. Either config.guess is buggy, or it is not.

If config.guess is buggy, it is also buggy for every single 
autoconf-based project there is and will need to be fixed there as well. 
There has been no relevant change to upstream config.guess for AIX that 
I can see, but I may have missed something.

If config.guess is not buggy, the change is wrong.

Am I missing something here, perhaps some some scenario that LLVM 
supports but config.guess does not, that would make it correct to change 
that only in LLVM, and not in upstream config.guess?

> I'd prefer expanding GetHostTriple.cmake until it covers all the targets 
> we support. I don't think it'd necessarily require reimplementing all of 
> config.guess, since config.guess covers a lot of targets most of which 
> aren't supported by LLVM.

Not all, sure, but I do think you're underestimating how much work it 
will be, at least for an outside contributor who cannot ask people who 
have access to relevant systems to run some quick tests. Getting an 
overview of all targets supported by LLVM, the values the relevant CMake 
variables will be set to, and the triples that config.guess figures out 
for those systems, that would still be a massive amount of work. Once we 
have that, writing it up in CMake should be fairly easy, sure, but 
that's not where the main work will be. All that just to get a result we 
can get without any extra effort already.

Cheers,
Harald van Dijk