Re: [Qemu-devel] 0.9.0 and svn don't build with -march=pentium2 etc.; was: Latest SVN fails to build on Fedora Core 6 (same with 0.9.0)

From: Sunil Amitkumar Janki <psychicistnonconformist@gmail.com>
To: qemu-devel@nongnu.org
Subject: Re: [Qemu-devel] 0.9.0 and svn don't build with -march=pentium2	etc.; was: Latest SVN fails to build on Fedora Core 6 (same with 0.9.0)
Date: Sat, 24 Mar 2007 13:32:39 +0100	[thread overview]
Message-ID: <46051A67.6060300@gmail.com> (raw)
In-Reply-To: <20070323211124.4d7d7b79@neuling>

Thomas Orgis wrote:
> Am Fri, 23 Mar 2007 15:45:49 +0000
> schrieb Paul Brook <paul@codesourcery.com>: 
>
>   
>>> I do not understand enough of QEMU yet, but I have checked out CVS and
>>> am reading through its source. When I understand more I hope we can fix
>>> this long-standing annoyance.
>>>       
>> This is GCC PR16185 <http://gcc.gnu.org/PR16185>.
>>     
>
> Hm, I think I stumbled over this before... so it is still valid.
>
>   
>> There's no fundamental reason why some options trigger it, and others don't. 
>> It trigerrs fairly randomly depending on the code generation choices gcc 
>> happens to make.
>>     
>
> So it is indeed the case that nothing particular in qemu triggers it,
> just the general grab on the registers.
> And it is something that won't be fixed anytime soon (in gcc), apart
> from ia32 going out of use...
> Personally, I started using qemu with my Pentium-M 1.4GHz laptop... so
> 32bit is the way there for a few years.
> So I'll check for every new qemu release and gcc version if it perhaps
> builds with normal optimization settings... or one identifies a spot to
> release a register for x86 where it doesn't hurt qemu (since it's 
> always softmmu_template.h with helper.c).
>
> Anyway, this sounds more and more like a FAQ entry.
> Perhaps it should be mentioned in qemu docs.
>
>   
>> The problem is that x86 is chronically short of registers. qemu makes this 
>> significantly worse by telling gcc it can't use most of them. gcc simply 
>> isn't designed to work in these extreme situations.
>>     
>
> Yep, x86 is chronically short of many things and loaded with others.
> At work I secured two good old XP1000 alphas for myself...
> they're not really as fast as my laptop, but register starvation is nothing
> I expect to happen there. They got .. hm, what's the correct term... 
> a shitload of registers! ;-)
> x86 really was the worst arch to become market leader for home computing.
> On the other hand that leadership was what made the arch mutate in the
> ways it did.
>
>
> Thomas.
>
>
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>
>   
As far as X86 is concerned i386/i486/i586 are very different from later 
generation
processors. I am wondering whether another host and target architecture 
could be
created called i686 that makes use of something like MMX or other 
registers in Intel
Pentium II/III/4 and AMD Athlon to negate the lack of general purpose 
registers. In a
certain sense i486 compatibility holds back the options you have for 
optimisation.

The fact that QEMU works and can be optimised on x86_64 is the only 
saving grace
for the architecture, that is still suffering from a lack of registers 
compared to any
other architecture.

Anyhow, I expect 32-bit hardware to gradually die because of wear and 
tear in the
next few years and the replacement will be 64-bit hardware so the 
problem will solve
itself that way.

Sunil.