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

[Hubert Tong via llvm-dev]

On Mon, Apr 5, 2021 at 4:44 AM Harald van Dijk via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> 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?
>

LLVM uses triples to identify the host/target architecture (including the
difference between 32-bit and 64-bit variants of the "same architecture").
It cannot be said that config.guess must do the same (that would depend on
its users). That the selection, on AIX, between the 32-bit and 64-bit
variants happens later for an autoconf-based build is entirely possible. As
it is, we're able to do the selection early in the CMake-based build only
because we're using a compiler that respects the OBJECT_MODE environment
variable (which we also set up for Clang on AIX to do).


>
> > 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
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