Re: Do we still care about compilers without __seg_fs and __seg_gs support??

["H. Peter Anvin" <hpa@zytor.com>]

On 2025-12-19 16:24, Nathan Chancellor wrote:
> On Fri, Dec 19, 2025 at 03:24:21PM -0800, H. Peter Anvin wrote:
>> As of Linux 6.16, we require:
>>
>> 	gcc 8.1 or higher
>> 	clang 15.0.0 or higher
>>
>> If my reading of the release notes is correct, then both versions *should*
>> supported __seg_fs and __seg_gs, but we have:
>>
>> config CC_HAS_NAMED_AS
>>         def_bool $(success,echo 'int __seg_fs fs; int __seg_gs gs;' | $(CC) -x
>> c - -S -o /dev/null)
>>         depends on CC_IS_GCC
>>
>> We don't even try on clang.
>>
>> Being able to actually rely on the compiler for this would make a lot of
>> things cleaner.  For one thing, I'm trying to untangle a bunch of ugliness in
>> the code sharing between realmode and proper flat mode code...
>>
>> Uros, you seem to have touched this code as recently as earlier this year; any
>> thoughts?
>>
>> What about the LLVM people, any insights?
> 
> Trying to use __seg_fs or __seg_gs in certain cases crashes the X86
> backend.
> 
>   https://github.com/llvm/llvm-project/issues/93449
> 
> Is there anyone on AMD or Intel's LLVM teams that could look into
> solving that? Nick pinged a couple of Intel's folks but it does not look
> 

It looks to me that this is specifically related to static initializers, or is
there something else here that I'm not sure about?

I'm asking because it might still allow at least the boot code improvements,
and/or have some other less painful workaround than carrying these hacks.

As far as I can tell, on x86 gcc will not change the value of the pointer when
cast to a different address space and I believe Linux expects this behavior.

	-hpa