[PATCH] riscv: lib: Implement optimized memchr function

[Palmer Dabbelt]

On Mon, 11 Dec 2023 07:25:15 PST (-0800), ivan.orlov at codethink.co.uk wrote:
> On 11/12/2023 15:08, Andrew Jones wrote:
>>> As you can see, the new function shows much better results even for
>>> the small arrays of 256 elements, therefore I believe it could be a
>>> useful addition to the existing riscv-specific string functions.
>>
>> Looks good, but do we want to maintain both this version and a zbb
>> version? I'd expect a zbb version to be even better.
>>
>
> Hi Andrew,
>
> Yes, ZBB analog would be much better, and if we use ZBB operations we
> could avoid the most part of bit magic happening there.
>
>>> +	add t1, x0, a2
>>
>> move t1, a2
>>
>> and for the remainder of the function s/x0/zero/
>>
>
> Alright, will be fixed in the next version.
>>> +	sltiu t2, a2, MIN_BORDER
>>> +	bnez t2, 6f
>>> +
>>> +	// get the number of bytes we should iterate before alignment
>>
>> I'm not sure, but I think even in assembly we prefer the /* */ comment
>> format.
>>
>>> +	andi t0, a0, SZREG - 1
>>> +	beqz t0, 4f
>>> +
>>> +	# get the SZREG - t0
>>
>> I'm 99% sure we don't want to use the # comment syntax.
>>
>>> +	xor t0, t0, SZREG - 1
>>
>> xori?
>>
>
> Hmm, I'm surprised that it is actually compilable... Yeah, should be fixed
>>> +	addi t0, t0, 1
>>> +
>>> +	sub a2, a2, t0
>>
>> nit: Looks a bit odd to put a blank line above the sub line above,
>> instead of above the below comment.
>>
>>> +	// iterate before alignment
>>> +1:
>>> +	beq t0, x0, 4f
>>> +	lbu t2, 0(a0)
>>> +	beq t2, a1, 3f
>>> +	addi t0, t0, -1
>>
>> This addi t0... isn't necessary if we do
>>
>
> Yeah, sounds reasonable, we can make it faster
>> 	add t0, a0, t0
>> 1:
>> 	beq a0, t0, 4f
>> 	...
>> 	...
>> 	addi a0, a0, 1
>> 	j 1b
>>
>>> +	addi a0, a0, 1
>>> +	j 1b
>>> +
>>> +2:
>>> +	// found a word. Iterate it until we find the target byte
>>> +	li t1, SZREG
>>> +	j 6f
>>
>> These instructions seem oddly placed among the rest.
>>
>>> +3:
>>> +	ret
>>
>> And this is an odd place to put this ret (after unconditional jump and
>> in the middle of the function). We can just put a label at the bottom ret.
>>
>
> I agree, thanks!
>>> +
>>> +4:
>>> +	// get the count remainder
>>> +	andi t1, a2, SZREG - 1
>>> +
>>> +	// align the count
>>> +	sub a2, a2, t1
>>> +
>>> +	// if we have no words to iterate, iterate the remainder
>>> +	beqz a2, 6f
>>> +
>>> +	// from 0xBA we will get 0xBABABABABABABABA
>>> +	li t3, REP_01
>>> +	mul t3, t3, a1
>>
>> I don't think we want to implement an optimized assembly function with
>> mul. We can just use a few shifts and ors.
>>
>> 	slli	t3, a1, 8
>> 	or	t3, t3, a1
>> 	slli	t4, t3, 16
>> 	or	t3, t4, t3
>> #if __riscv_xlen == 64
>> 	slli	t4, t3, 32
>> 	or	t3, t4, t3
>> #endif
>>
>
> Nice point, thanks! Will be optimized :)
>>> +
>>> +	add a2, a2, a0
>>> +
>>> +	li t4, REP_01
>>> +	li t5, REP_80
>>> +
>>> +5:
>>> +	REG_L t2, 0(a0)
>>> +
>>> +	// after this xor we will get one zero byte in the word if it contains the target byte
>>> +	xor t2, t2, t3
>>> +
>>> +	// word v contains the target byte if (v - 0x01010101) & (~v) & 0x80808080 is positive
>>
>> s/is positive/is not zero/
>>
>>> +	sub t0, t2, t4
>>> +
>>> +	not t2, t2
>>> +
>>> +	and t0, t0, t2
>>> +	and t0, t0, t5
>>> +
>>> +	bnez t0, 2b
>>> +	addi a0, a0, SZREG
>>> +	bne a0, a2, 5b
>>> +
>>> +6:
>>> +	// iterate the remainder
>>> +	beq t1, x0, 7f
>>> +	lbu t4, 0(a0)
>>> +	beq t4, a1, 3b
>>> +	addi a0, a0, 1
>>> +	addi t1, t1, -1
>>
>> Same comment as above about being able to drop the addi t1...
>>
>>> +	j 6b
>>> +
>>> +7:
>>> +	addi a0, x0, 0
>>
>> li a0, 0
>>
>>> +	ret
>>> +SYM_FUNC_END(memchr)
>>> +SYM_FUNC_ALIAS(__pi_memchr, memchr)
>>> --
>>> 2.34.1
>>>
>>
>> Thanks,
>> drew
>>
>
> Thanks a lot for the review!

Do you have a v2?  Sorry if I lost it.