From: Vineet Gupta <Vineet.Gupta1@synopsys.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Arnd Bergmann <arnd@arndb.de>,
Khalid Aziz <khalid.aziz@oracle.com>,
Andrey Konovalov <andreyknvl@google.com>,
Andrew Morton <akpm@linux-foundation.org>,
Peter Zijlstra <peterz@infradead.org>,
Christian Brauner <christian.brauner@ubuntu.com>,
Kees Cook <keescook@chromium.org>, Ingo Molnar <mingo@kernel.org>,
Aleksa Sarai <cyphar@cyphar.com>,
"open list:SYNOPSYS ARC ARCHITECTURE"
<linux-snps-arc@lists.infradead.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
linux-arch <linux-arch@vger.kernel.org>
Subject: Re: [RFC 4/4] ARC: uaccess: use optimized generic __strnlen_user/__strncpy_from_user
Date: Tue, 14 Jan 2020 22:14:31 +0000 [thread overview]
Message-ID: <67715aba-fa40-1f46-288d-391d086328ac@synopsys.com> (raw)
In-Reply-To: <CAHk-=wjX-c9YpPhbQ073UPnTvELNQCN49vqK1yY7JGuHSn5-ew@mail.gmail.com>
On 1/14/20 1:49 PM, Linus Torvalds wrote:
> On Tue, Jan 14, 2020 at 1:37 PM Vineet Gupta <Vineet.Gupta1@synopsys.com> 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
prev parent reply other threads:[~2020-01-14 22:14 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-01-14 20:08 [RFC 0/4] Switching ARC to optimized generic strncpy_from_user Vineet Gupta
2020-01-14 20:08 ` [RFC 1/4] asm-generic/uaccess: don't define inline functions if noinline lib/* in use Vineet Gupta
2020-01-14 20:57 ` Arnd Bergmann
2020-01-15 23:01 ` Vineet Gupta
2020-01-16 11:43 ` Arnd Bergmann
2020-01-14 21:32 ` Linus Torvalds
2020-01-15 9:08 ` Arnd Bergmann
2020-01-15 14:12 ` Al Viro
2020-01-15 14:21 ` Arnd Bergmann
2020-01-14 20:08 ` [RFC 2/4] lib/strncpy_from_user: Remove redundant user space pointer range check Vineet Gupta
2020-01-14 21:22 ` Linus Torvalds
2020-01-14 21:52 ` Vineet Gupta
2020-01-14 23:46 ` Al Viro
2020-01-15 14:42 ` Andrey Konovalov
2020-01-15 23:00 ` Vineet Gupta
2020-01-14 20:08 ` [RFC 3/4] ARC: uaccess: remove noinline variants of __strncpy_from_user() and friends Vineet Gupta
2020-01-14 20:08 ` [RFC 4/4] ARC: uaccess: use optimized generic __strnlen_user/__strncpy_from_user Vineet Gupta
2020-01-14 20:42 ` Arnd Bergmann
2020-01-14 21:36 ` Vineet Gupta
2020-01-14 21:49 ` Linus Torvalds
2020-01-14 22:14 ` Vineet Gupta [this message]
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