Adding a MACHINE_ARCH note

[Bruce Evans]

On Wed, 10 Jul 2013, 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.

ISTR reading a claim that pure 64-bit mode was mainly a marketing ploy
by AMD.  It distinguished them from Intel and let them get to market
faster.  The result was a marketing win but a not so good instruction
set architecture.  This was contrasted with sparc64 (Sun?), where
32-bit applications remained the default and the differences were
smaller (no mode switch?).

Bruce