Date: Sun, 13 Jan 2013 14:53:54 -0800 From: Nathan Whitehorn <nwhitehorn@freebsd.org> To: freebsd-arch@freebsd.org Subject: Re: how long to keep support for gcc on x86? Message-ID: <50F33B02.6040303@freebsd.org> In-Reply-To: <20130113224800.GS1410@funkthat.com> References: <20130112233147.GK1410@funkthat.com> <20130113014242.GA61609@troutmask.apl.washington.edu> <CAJ-VmomrSFXcZg%2BKj6C2ARhpmjB9hxZATYJyRZB7-eRrcBLprg@mail.gmail.com> <20130113053725.GL1410@funkthat.com> <CAJ-VmomGKayr-1VucfwgodhXEHrXxx8r=9crHZJf74iVKZyTmQ@mail.gmail.com> <20130113202952.GO1410@funkthat.com> <CAGE5yCpB8dHLn0TaW=r0Ov39owOQVi=X5FFw%2BuQ=qZ9zYi5anA@mail.gmail.com> <20130113224800.GS1410@funkthat.com>
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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@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
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