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Message-ID: <504E939B.5060209@gmail.com>
Date:   Mon, 10 Sep 2012 18:27:55 -0700
From:   David Daney <ddaney.cavm@gmail.com>
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To:     "Kevin D. Kissell" <kevink@paralogos.com>
CC:     linux-mips@linux-mips.org
Subject: Re: Is r25 saved across syscalls?
References: <20120909193008.GA15157@brightrain.aerifal.cx> <20120910170830.GB24448@linux-mips.org> <20120910172248.GN27715@brightrain.aerifal.cx> <alpine.LFD.2.00.1209110059580.8926@eddie.linux-mips.org> <504E8E27.5030904@paralogos.com>
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On 09/10/2012 06:04 PM, Kevin D. Kissell wrote:
> On 09/10/2012 05:29 PM, Maciej W. Rozycki wrote:
>> I do wonder however why we have these instructions to save/restore $25
>> in SAVE_SOME/RESTORE_SOME. This dates back to 2.4 at the very least.
>> Ralf, any insights?
> Hi, guys.  Maybe the fact that it's used for dispatching PIC calls has
> something to do with it?
>

I don't think so.  It is call clobbered in all Linux ABIs.  So there is 
never a need to save it.  It even has the pseudonym of $t9 indicating 
that it will get clobbered.

The other call clobbered registers that happen to be saved are for the 
nefarious uses of the kernel itself (a0..a6 for system call restarting), 
I think this is just left over cruft.

David Daney


> /K.
>
>
>
>