[llvm-dev] RFC: Deprecate config.guess

[Tom Stellard via llvm-dev]

On 9/16/21 9:59 AM, Fāng-ruì Sòng wrote:
> On Thu, Sep 16, 2021 at 3:42 AM Renato Golin <rengolin at gmail.com> wrote:
>>
>> On Wed, 15 Sept 2021 at 20:32, Fāng-ruì Sòng <maskray at google.com> wrote:
>>>
>>> The nature of such a change is that only when I flip the default,
>>> users will actually notice.
>>
>>
>> Getting a different triple may be worse than the wrong triple.
>>
>> The wrong triple crashes something and is easy to identify. A different, but valid, triple may bring slight architectural changes down the line that are difficult to spot.
>>
>> Changing the triple on the front end isn't always trivial either.
>>
>> There's a whole dance of changing triples in the clang driver that defies logic sometimes. The command line options to the actual front-end depend on the triple and the path taken, which in turn, can change code generation in unpredictable ways down the line.
>>
>>> Users of alternative compilers have already contributed relevant logic
>>> to llvm/cmake/modules/GetHostTriple.cmake ,
>>> and we should just focus on gcc/clang which are more relevant for the
>>> existing config.guess use cases.
>>> I believe {gcc,clang} -dumpmachine is correct in more cases than config.guess .
>>
>>
>> Not only that, but using the compiler driver to "predict" what the compiler front-end needs is the obvious thing to do.
>>
>> I'm not against the change, I think we should have done this a long time ago, but I think we can give people some grace time to test out on their sides, especially downstream people and less popular platforms that still use clang/gcc to build LLVM.
> 
> I'm glad we are on the same page for the direction, but I don't think
> giving more grace time would be beneficial.
> 
> *-suse-*, *-redhat-*, *-linux-musl triples are already incorrect.
> It led to unnecessary riscv-* changes such as
> https://reviews.llvm.org/D63497 , https://reviews.llvm.org/D74399.
> For less popular platforms, config.guess really caused more harm than
> its benefit.
> Some of the less popular platforms (I happen to like exploring such
> less popular platforms sometimes) may need LLVM_DEFAULT_TARGET_TRIPLE
> and LLVM_HOST_TRIPLE to cancel the config.guess harm.
> In https://reviews.llvm.org/D109837 , Tom mentioned that
> llvm/cmake/modules/GetHostTriple.cmake can just notify the user about
> fetching the latest config.guess .
> This will give the less popular compilers (technically, llvm-project
> doesn't even support such compilers
> https://llvm.org/docs/GettingStarted.html#host-c-toolchain-both-compiler-and-standard-library)
> transition period.
> 

We've been iterating on the patch and I've come to agree with Fāng-ruì
that it would be better to just remove config.guess.  config.guess is not
used on Windows and our new detection code works with clang and gcc, so
that is going to cover a majority of the cases already.  I don't see much
advantage to keeping config.guess around as a fallback, when we are just
goint to remove it at some point anyway.

-Tom

> 
> 
>> Giving them a CMake flag to test out for a few weeks wouldn't hurt before we turn it on by default.
>>
>> cheers,
>> --renato
>