Re: [PATCH v2 net-next] net: Implement fast csum_partial for x86_64

On Wed, 2016-01-06 at 10:16 +0000, David Laight wrote:

From: Eric Dumazet

quoted

Sent: 05 January 2016 22:19
To: Tom Herbert
You might add a comment telling the '4' comes from length of 'adcq
6*8(%rdi),%rax' instruction, and that the 'nop' is to compensate that
'adcq    0*8(%rdi),%rax' is using 3 bytes instead.

We also could use .byte 0x48, 0x13, 0x47, 0x00 to force a 4 bytes
instruction and remove the nop.


+       lea     20f(, %rcx, 4), %r11
+       clc
+       jmp     *%r11
+
+.align 8
+       adcq    6*8(%rdi),%rax
+       adcq    5*8(%rdi),%rax
+       adcq    4*8(%rdi),%rax
+       adcq    3*8(%rdi),%rax
+       adcq    2*8(%rdi),%rax
+       adcq    1*8(%rdi),%rax
+       adcq    0*8(%rdi),%rax // could force a 4 byte instruction (.byte 0x48, 0x13, 0x47, 0x00)
+       nop
+20:    /* #quads % 8 jump table base */

Or move label at the top (after the .align) and adjust the maths.
You could add a second label after the first adcq and use the
difference between them for the '4'.

Not really.

We could not use the trick it the length was 5.

Only 1, 2, 4 and 8 are supported.