linux-kernel - Re: [PATCH 2/4] MIPS: microMIPS: Fix decoding of addiusp instruction

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Date:   Wed, 1 Mar 2017 13:51:19 +0000
From:   Matt Redfearn <matt.redfearn@...tec.com>
To:     "Maciej W. Rozycki" <macro@...tec.com>
CC:     Ralf Baechle <ralf@...ux-mips.org>, <linux-mips@...ux-mips.org>,
        Marcin Nowakowski <marcin.nowakowski@...tec.com>,
        <linux-kernel@...r.kernel.org>,
        Paul Burton <paul.burton@...tec.com>
Subject: Re: [PATCH 2/4] MIPS: microMIPS: Fix decoding of addiusp instruction

Hi Maciej,


On 28/02/17 22:04, Maciej W. Rozycki wrote:
> On Tue, 28 Feb 2017, Matt Redfearn wrote:
>
>> diff --git a/arch/mips/kernel/process.c b/arch/mips/kernel/process.c
>> index 5b1e932ae973..6ba5b775579c 100644
>> --- a/arch/mips/kernel/process.c
>> +++ b/arch/mips/kernel/process.c
>> @@ -386,8 +386,9 @@ static int get_frame_info(struct mips_frame_info *info)
>>   
>>   					if (ip->halfword[0] & mm_addiusp_func)
>>   					{
>> -						tmp = (((ip->halfword[0] >> 1) & 0x1ff) << 2);
>> -						info->frame_size = -(signed short)(tmp | ((tmp & 0x100) ? 0xfe00 : 0));
>> +						tmp = (ip->halfword[0] >> 1) & 0x1ff;
>> +						tmp = tmp | ((tmp & 0x100) ? 0xfe00 : 0);
>> +						info->frame_size = -(signed short)(tmp << 2);
>   Ugh, this is unreadable -- can you please figure out a way to fit it in
> 79 columns?  Perhaps by factoring this piece out?

Yeah, it's not pretty. I've got a v2 which refactors this into 
is_sp_move_ins, which makes it work the same way as is_ra_save_ins and 
perform the immediate interpretation there, instead.
But I've kept that as a separate patch so as to keep the functional fix 
and refactor separate.

>
>   Also this:
>
> 	tmp = (ip->halfword[0] >> 1) & 0x1ff;
> 	tmp = tmp | ((tmp & 0x100) ? 0xfe00 : 0);
>
> will likely result in better code without the conditional, e.g.:
>
> 	tmp = (((ip->halfword[0] >> 1) & 0x1ff) ^ 0x100) - 0x100;
>
> (the usual way to sign-extend).
>
>    Maciej

Yes, that looks nicer.

Thanks,
Matt