linux-kernel - Re: [PATCH 4/4] MIPS: Add is_jr_ra

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Date:   Mon, 25 Oct 2021 10:11:35 +0800
From:   Jinyang He <hejinyang@...ngson.cn>
To:     "Maciej W. Rozycki" <macro@...am.me.uk>
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 10/24/2021 02:32 AM, Maciej W. Rozycki wrote:

> 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

You are right, I missed the point. I have refered to microMIPS64
Instruction Set. JRC and JRADDIUSP also can be used with $ra in
microMIPS. I am not sure what compiler do at epilogue. But when
we call $ra without saving $ra, it means return, I think.
I will fix it later.

Thanks,
Jinyang.