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

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Date:   Tue, 28 Feb 2017 22:04:10 +0000
From:   "Maciej W. Rozycki" <macro@...tec.com>
To:     Matt Redfearn <matt.redfearn@...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

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?

 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