On Tue, Mar 8, 2016 at 9:50 PM, Joe Perches <[email protected]> wrote:
> On Tue, 2016-03-08 at 21:23 -0800, Alexander Duyck wrote:
>> On Tue, Mar 8, 2016 at 3:25 PM, Joe Perches <[email protected]> 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.
>> > > 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
