RE: [PATCH] lib/x86: Optimise csum_partial of buffers that are not multiples of 8 bytes.

[David Laight <David.Laight@ACULAB.COM>]

From: David Laight <David.Laight@ACULAB.COM>
> Sent: 13 December 2021 18:01
> 
> Add in the trailing bytes first so that there is no need to worry
> about the sum exceeding 64 bits.

This is an alternate version that (mostly) compiles to reasonable code.
I've also booted a kernel with it - networking still works!

https://godbolt.org/z/K6vY31Gqs

I changed the while (len >= 64) loop into an
if (len >= 64) do (...) while(len >= 64) one.
But gcc makes a pigs breakfast of compiling it - it optimises
it so that it is while (ptr < lim) but adds a lot of code.
So I've done that by hand.
Then it still makes a meal of it because it refuses to take
'buff' from the final loop iteration.
An assignment to the limit helps.

Then there is the calculation of (8 - (len & 7)) * 8.
gcc prior to 9.2 just negate (len & 7) then use leal 56(,%rs1,8),%rcx.
But later ones and fail to notice.
Even given (64 + 8 * -(len & 7)) clang fails to use leal.

I'm not even sure the code clang generates is right:
(%rsi is (len & 7))
        movq    -8(%rsi,%rax), %rdx
        leal    (,%rsi,8), %ecx
        andb    $56, %cl
        negb    %cl
        shrq    %cl, %rdx

The 'negb' is the wrong size of the 'andb'.
It might be ok if it is assuming the cpu ignores the high 2 bits of %cl.
But that is a horrid assumption to be making.

	David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)