how long to keep support for gcc on x86?

[Nathan Whitehorn]

On 01/13/13 14:48, John-Mark Gurney wrote:
> Peter Wemm wrote this message on Sun, Jan 13, 2013 at 14:26 -0800:
>> On Sun, Jan 13, 2013 at 12:29 PM, John-Mark Gurney <jmg at funkthat.com> wrote:
>>> Adrian Chadd wrote this message on Sat, Jan 12, 2013 at 23:44 -0800:
>>>>
>>>> People are still ironing out kinks/differences with clang. Anyone
>>>> saying otherwise is likely pushing an agenda. :-)
>>>>
>>>> Thus I think adding clang-only code to the system right now is very,
>>>> very premature. There still seem to be reasons to run systems on GCC
>>>> instead of clang.
>>>>
>>>> If you have a need for new instruction support, perhaps look at adding
>>>> it to our base GCC for the time being?
>>>
>>> I did look at it briefly, but I don't know gcc's internals, and it would
>>> take me 5+ hours to do it, while someone who does know gcc would take
>>> abount a half an hour (just a guess)...  I don't have the free time I
>>> used to, otherwise I would of done it by now..
>>
>> It seems to me that since clang is the default compiler for the
>> platforms that have AES-NI that the following could be done:
>>
>> * get the inline AES-NI stuff in and debugged and solid.
>> * .. without breaking the existing gcc-compatible code
>> * once the support is solid, decide what the appropriate thing to do for gcc is.
>>
>> .. so long as the existing code doesn't get broken.
>>
>> Trying to do backwards compatibility port to gcc with a moving target
>> has potential to be a work multiplier.
> 
> I already have a gcc compatible version of an improved AES-NI for
> amd64...  The real question is, do I improve things further by using
> intrinsics which means we can share code between amd64 and i386 and get
> great performance from both, or do I simply make a seperate version
> for i386 that is gcc compatible, but not as good performance...
> 
> Though a lot of this last little bit of performance questions isn't too
> useful since the overhead of the crypto framework and geom introduces
> a significant overhead already...
> 
> I'm not too interesting in creating AES-NI v2 module and having two
> versions that do the same thing just because of a compiler issue...
> 
> So I'm going to go with the plan of making an i386 and gcc compatible
> version... it'll still be a 4x+ performance over the existing code...
> This also means we could back port it to 9-stable if we wanted to...
> 
> Thanks for the input...
> 

This also raises the interesting question of whether we want to bother
supporting things like AES-NI on i386 at all. It's a legacy/embedded
architecture at this point, in my opinion, and the people who run it
probably don't care about fancy new features like this. A related
question is whether we want to have any clang-only features in the kernel...
-Nathan