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

From: Eric Dumazet

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'.

	David