Re: [RFC 4/4] ARC: uaccess: use optimized generic __strnlen_user/__strncpy_from_user

[Vineet Gupta]

On 1/14/20 1:49 PM, Linus Torvalds wrote:
> On Tue, Jan 14, 2020 at 1:37 PM Vineet Gupta <Vineet.Gupta1@xxxxxxxxxxxx> wrote:
>>
>> On 1/14/20 12:42 PM, Arnd Bergmann wrote:
>>>
>>> What's wrong with the generic version on little-endian? Any
>>> chance you can find a way to make it work as well for you as
>>> this copy?
>>
>> find_zero() by default doesn't use pop count instructions.
> 
> Don't you think the generic find_zero() is likely just as fast as the
> pop count instruction? On 32-bit, I think it's like a shift and a mask
> and a couple of additions.

You are right that in grand scheme things it may be less than noise.

ARC pop count version

# 	bits = (bits - 1) & ~bits;
#  	return bits >> 7;

	sub r0,r6,1
	bic r6,r0,r6
	lsr r0,r6,7

# 	return fls(mask) >> 3;

	fls.f	r0, r0
	add.nz	r0, r0, 1
	asr r5,r0,3

	j_s.d [blink]

Generic version

# 	bits = (bits - 1) & ~bits;
#  	return bits >> 7;

	sub r5,r6,1
	bic r6,r5,r6
	lsr r5,r6,7

#  	unsigned long a = (0x0ff0001+mask) >> 23;
# 	return a & mask;

	add r0,r5,0x0ff0001	<-- this is 8 byte instruction though
	lsr_s r0,r0,23
	and r5,r5,r0

	j_s.d [blink]


But its the usual itch/inclination of arch people to try and use the specific
instruction if available.

> 
> The 64-bit case has a multiply that is likely expensive unless you
> have a good multiplication unit (but what 64-bit architecture
> doesn't?), but the generic 32-bit LE code should already be pretty
> close to optimal, and it might not be worth it to worry about it.
> 
> (The big-endian case is very different, and architectures really can
> do much better. But LE allows for bit tricks using the carry chain)

-Vineet