Date: Fri, 20 Jun 2003 20:35:12 +0100
From: John Bradford <john@grabjohn.com>
Message-Id: <200306201935.h5KJZCvc001455@81-2-122-30.bradfords.org.uk>
To: eli.carter@inet.com, john@grabjohn.com
Subject: Re: Crusoe's persistent translation on linux?
Cc: linux-kernel@vger.kernel.org, nuno.silva@vgertech.com, samphan@thai.com,
       torvalds@transmeta.com, vojtech@suse.cz
Sender: linux-kernel-owner@vger.kernel.org
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> From eli.carter@inet.com  Fri Jun 20 20:22:12 2003
> Envelope-To: john@bradfords.org.uk
> Date: Fri, 20 Jun 2003 11:49:22 -0500
> From: Eli Carter <eli.carter@inet.com>
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> To: John Bradford <john@grabjohn.com>
> CC: nuno.silva@vgertech.com, torvalds@transmeta.com,
>    linux-kernel@vger.kernel.org, samphan@thai.com, vojtech@suse.cz
> Subject: Re: Crusoe's persistent translation on linux?
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> John Bradford wrote:
> >>The translations are usually _better_ than statically compiled native 
> >>code (because the whole CPU is designed for speculation, and the static 
> >>compilers don't know how to do that), and thus going to native mode is not 
> >>necessarily a performance improvement.
> > 
> > 
> > Would it be possible, (with relevant documentation), to tune the code
> > morphing software for optimum performance of code generated by a
> > specific compiler, though?
> > 
> > If a particular version of GCC favours certain constructs and uses
> > particular sets of registers for a given piece of code, couldn't we
> > optimise for those cases, at the expense of others?  Maybe a
> > particular compiler doesn't use certain X86 instructions at all, and
> > these could be eliminated altogether?
> > 
> > It's not unlikely that an entirely open source system could have all
> > code compiled with the same compiler, and so maybe we can use this to
> > avoid implementing expensive corner cases in the CPU, because we're
> > never going to trigger them.
>
> Hmm... basically you want to trim the x86 instruction set to get closer 
> to RISC mentality.

Yep, that's what I was thinking.

> Interesting.  gcc may already do that to some extent 
> by not using the really complex instructions.  If that is the case, 
> dropping those instructions might give some room for testing some of its 
> possible benefits.

> I doubt restricting the registers used by some 
> instructions would help... I've heard comments that the x86 is 
> register-starved enough already.

Ah, no what I meant was something along the lines of the z80, (yeah,
it's been a while since I did assembler :-) ), where you had an
'accumulator', which you did all the complex operations in, and used
the BC, DE, HL, (you just have to love the name, it helped you
remember the endianness of the CPU), registers to do basic operations
in.

I was just thinking that if gcc favoured register BAR for operation
FOO, the CPU could do likewise, and if their was a performance hit for
using another register, we'd avoid it.  Probably less applicable to a
software based CPU like Cruesoe, though, and like you mentioned, X86
is register starved anyway.

John.
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