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Date:   Sat, 23 Oct 2021 20:32:29 +0200 (CEST)
From:   "Maciej W. Rozycki" <macro@...am.me.uk>
To:     Jinyang He <hejinyang@...ngson.cn>
cc:     Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        linux-mips@...r.kernel.org, linux-kernel@...r.kernel.org,
        Paul Burton <paulburton@...nel.org>,
        Jun-Ru Chang <jrjang@...ltek.com>
Subject: Re: [PATCH 4/4] MIPS: Add is_jr_ra_ins() to end the loop early

On Tue, 12 Jan 2021, Jinyang He wrote:

> For those leaf functions, they are likely to have no stack operations.
> Add is_jr_ra_ins() to determine whether jr ra has been touched before
> the frame_size is found. Without this patch, the get frame_size operation
> may be out of range and get the frame_size from the next nested function.
> 
> Signed-off-by: Jinyang He <hejinyang@...ngson.cn>
> ---
>  arch/mips/kernel/process.c | 34 +++++++++++++++++++++++++++++++++-
>  1 file changed, 33 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/mips/kernel/process.c b/arch/mips/kernel/process.c
> index bef8f8d..9e6f194 100644
> --- a/arch/mips/kernel/process.c
> +++ b/arch/mips/kernel/process.c
> @@ -205,6 +205,36 @@ struct mips_frame_info {
>  #define J_TARGET(pc,target)	\
>  		(((unsigned long)(pc) & 0xf0000000) | ((target) << 2))
>  
> +static inline int is_jr_ra_ins(union mips_instruction *ip)
> +{
> +#ifdef CONFIG_CPU_MICROMIPS
> +	/*
> +	 * jr16 ra
> +	 * jr ra
> +	 */
> +	if (mm_insn_16bit(ip->word >> 16)) {
> +		if (ip->mm16_r5_format.opcode == mm_pool16c_op &&
> +		    ip->mm16_r5_format.rt == mm_jr16_op &&
> +		    ip->mm16_r5_format.imm == 31)
> +			return 1;
> +		return 0;
> +	}
> +
> +	if (ip->r_format.opcode == mm_pool32a_op &&
> +	    ip->r_format.func == mm_pool32axf_op &&
> +	    ((ip->u_format.uimmediate >> 6) & GENMASK(9,0)) == mm_jalr_op &&
> +            ip->r_format.rs == 31)
> +		return 1;
> +	return 0;
> +#else

 Without looking into it much, this is likely missing the point, because 
while technically inteed JR and JR16 can be used with $ra in microMIPS 
machine code (there's JRS too), in reality either JRC or JRADDIUSP will.

 [Wading through e-mail recovered from mid-Jan linux-mips.org crash.]

  Maciej

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