On Tue, Mar 8, 2016 at 10:08 PM, Alexander Duyck
<alexander.du...@gmail.com> wrote:
> On Tue, Mar 8, 2016 at 9:50 PM, Joe Perches <j...@perches.com> wrote:
>> On Tue, 2016-03-08 at 21:23 -0800, Alexander Duyck wrote:
>>> On Tue, Mar 8, 2016 at 3:25 PM, Joe Perches <j...@perches.com> wrote:
>>> > On Tue, 2016-03-08 at 14:42 -0800, Alexander Duyck wrote:
>>> > > The code for csum_block_add was doing a funky byteswap to swap the even
>>> > > and
>>> > > odd bytes of the checksum if the offset was odd. Instead of doing this
>>> > > we
>>> > > can save ourselves some trouble and just shift by 8 as this should have
>>> > > the
>>> > > same effect in terms of the final checksum value and only requires one
>>> > > instruction.
>>> > 3 instructions?
>>> I was talking about just the one ror vs mov, shl, shr, and ,and, add.
>>>
>>> I assume when you say 3 you are including the test and either some
>>> form of conditional move or jump?
>>
>> Yeah, instruction count also depends on architecture (arm/x86/ppc...)
>
> Right. But the general idea is that rotate is an instruction most
> architectures have. I haven't heard of an instruction that swaps even
> and odd bytes of a 32 bit word.
>
Yes, I took a look inlining these.
#define rol32(V, X) ({ \
int word = V; \
if (__builtin_constant_p(X)) \
asm("roll $" #X ",%[word]\n\t" \
: [word] "=r" (word)); \
else \
asm("roll %%cl,%[word]\n\t" \
: [word] "=r" (word) \
: "c" (X)); \
word; \
})
With this I'm seeing a nice speedup in jhash which uses a lot of rol32s...
>>> > > diff --git a/include/net/checksum.h b/include/net/checksum.h
>> []
>>> > > @@ -88,8 +88,10 @@ static inline __wsum
>>> > > csum_block_add(__wsum csum, __wsum csum2, int offset)
>>> > > {
>>> > > u32 sum = (__force u32)csum2;
>>> > > - if (offset&1)
>>> > > - sum = ((sum&0xFF00FF)<<8)+((sum>>8)&0xFF00FF);
>>> > > +
>>> > > + if (offset & 1)
>>> > > + sum = (sum << 24) + (sum >> 8);
>>> > Maybe use ror32(sum, 8);
>>> I was actually thinking I could use something like this. I didn't
>>> realize it was even available.
>>
>> Now you know: bitops.h
>>
>>> > or maybe something like:
>>> >
>>> > {
>>> > u32 sum;
>>> >
>>> > /* rotated csum2 of odd offset will be the right checksum */
>>> > if (offset & 1)
>>> > sum = ror32((__force u32)csum2, 8);
>>> > else
>>> > sum = (__force u32)csum2;
>>> >
>>> Any specific reason for breaking it up like this? It seems like it
>>> was easier to just have sum be assigned first and then rotating it if
>>> needed. What is gained by splitting the assignment up over two
>>> different calls?
>>
>> It's only for reader clarity where a comment could be useful.
>> The compiler output shouldn't change.
>
> Okay, well I can add a one line comment about aligning to a 16b
> boundary for clarity.
>
> - Alex
