Adding a MACHINE_ARCH note

[Warner Losh]

On Jul 10, 2013, at 1:03 AM, Konstantin Belousov wrote:

> On Wed, Jul 10, 2013 at 08:54:05AM +0200, Dimitry Andric wrote:
>> On Jul 10, 2013, at 03:08, Peter Wemm <peter at wemm.org> wrote:
>>> On Tue, Jul 9, 2013 at 5:56 PM, Adrian Chadd <adrian at freebsd.org> wrote:
>>>> ... boy I'd like to see this particular x86 hiccup fixed before this
>>>> stuff is mainstream.
>>> 
>>> I'm not entirely sure how much support there is behind "x32".  I don't
>>> know if its much more than an academic curiosity or if there's real
>>> demand for it.
>> 
>> It seems to be driven by Intel and Google.  The idea is that for some
>> applications (or maybe even most :), an ILP32 model will perform better.
>> Quoting from one of the presentations:
>> 
>> On Core i7 2600K 3.40GHz:
>> - Improved SPEC CPU 2K/2006 INT geomean by 7-10% over ia32 and 5-8% over
>>  Intel64.
>> - Improved SPEC CPU 2K/2006 FP geomean by 5-11% over ia32.
>> - Very little changes in SPEC CPU 2K/2006 FP geomean, comparing against
>>  Intel64.
>> - Comparing against ia32 PIC, x32 PIC:
>>  - Improved SPEC CPU 2K INT by another 10%.
>>  - Improved SPEC CPU 2K FP by another 3%.
>>  - Improved SPEC CPU 2006 INT by another 6%
>>  - Improved SPEC CPU 2006 FP by another 2%.
>> 
>> As to how often it is actually used in practice, I am not sure.
>> 
>> 
>>> gcc-4.8 and clang have it, or have patches for it.
>> 
>> You also need a fairly recent binutils.  And kernel + libc support...
>> It is probably not a trivial task. :-)
> 
> You definitely need a support from libc, libthr and rtld.
> I am not convinced that the kernel modifications are needed,
> except for the image activator to recognize new ELF ids. In
> other words, I believe it is better to put shims into libc in
> the long run.

For MIPS N32, you also need to make sure that the system calls grok the ABI. You're going to give up some of the performance gains by shimming, plus you run the risk of mixing x32 with amd64 code, which would be bad. Attractive as it may sound to try to put it all in libc, I'm not convinced that would work out in the end.

Warner