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From: Richard Henderson <rth@twiddle.net>
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Date: Fri, 10 Jul 2015 07:48:31 +0100
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Subject: Re: [Qemu-devel] [PATCH 05/10] target-i386: Access segs via TCG
	registers
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To: Paolo Bonzini <pbonzini@redhat.com>, qemu-devel@nongnu.org

On 07/09/2015 02:30 PM, Paolo Bonzini wrote:
>
>
> On 09/07/2015 09:15, Richard Henderson wrote:
>> Having segs[].base as a register significantly improves code
>> generation for real and protected modes, particularly for TBs
>> that have multiple memory references where the segment base
>> can be held in a hard register through the TB.
>
> On top of this, "is base 0" could be cached in HFLAGS, and CS.BASE does
> not need to be in a register at all.  Or, to avoid wasting too many
> HFLAGS, "is DS/ES/SS base equal to CS base" since the CS base is known
> and usually all four of them (unlike FS and GS) are zero.

We do have CS/DS/ES/SS base not-equal to zero in hflags -- this is 
HF_ADDSEG_SHIFT.  You're right about CS.BASE being known via the TB though, I 
hadn't thought about that.

I'm not sure that CS=DS/ES/SS really helps much 16-bit code though; afaik 
except for the smallest COMs they're always different.


r~