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Date: Thu, 10 Dec 2015 19:21:19 +0100
From: Aurelien Jarno <aurelien@aurel32.net>
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Subject: Re: [Qemu-devel] [PATCH] tcg/arm: improve direct jump
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To: Richard Henderson <rth@twiddle.net>
Cc: TeLeMan <geleman@gmail.com>, qemu-devel@nongnu.org

On 2015-12-10 07:31, Richard Henderson wrote:
> On 12/10/2015 12:02 AM, Aurelien Jarno wrote:
> >Note: I don't really get the reason for the current 16MB limit. With the
> >standard branch instructions the offset is coded on 24 bits, but shifted
> >right by 2, which should give us a +/-32MB jumps, and therefore a 32MB
> >limit.
> 
> That might be me with the off-by-one error on the bit counting...

While the way to store the value has been changed a few times recently,
the original value dates from this commit:

commit 1cb0661e009267a5d060c4686f0857784a8da228
Author: balrog <balrog@c046a42c-6fe2-441c-8c8c-71466251a162>
Date:   Mon Dec 1 02:10:17 2008 +0000

    arm: Reserve code buffer in memory range reachable for pc-relative branch.
    
    Unfortunately this range is so narrow that I'm not sure if it makes more
    sense to always use memory load to pc kind of branch instead.
    
    
    git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@5844 c046a42c-6fe2-441c-8c8c-71466251a162

This doesn't fully explain the reason why 16MB and not 32MB.

Aurelien

-- 
Aurelien Jarno                          GPG: 4096R/1DDD8C9B
aurelien@aurel32.net                 http://www.aurel32.net