netdev - RE: [PATCH v2 net-next] net: Implement fast csum_partial for x86

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Date:	Wed, 6 Jan 2016 10:16:00 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Eric Dumazet' <eric.dumazet@...il.com>,
	Tom Herbert <tom@...bertland.com>
CC:	"davem@...emloft.net" <davem@...emloft.net>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"kernel-team@...com" <kernel-team@...com>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"hpa@...or.com" <hpa@...or.com>, "x86@...nel.org" <x86@...nel.org>
Subject: 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