netdev - Re: [PATCH v2] xfrm: branchless addr4

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Message-ID: <20170327103719.GI23004@secunet.com>
Date:   Mon, 27 Mar 2017 12:37:19 +0200
From:   Steffen Klassert <steffen.klassert@...unet.com>
To:     Alexey Dobriyan <adobriyan@...il.com>
CC:     "herbert@...dor.apana.org.au" <herbert@...dor.apana.org.au>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        <David.Laight@...LAB.COM>
Subject: Re: [PATCH v2] xfrm: branchless addr4_match() on 64-bit

On Sat, Mar 25, 2017 at 07:41:17PM +0300, Alexey Dobriyan wrote:
> Current addr4_match() code has special test for /0 prefixes because of
> standard required undefined behaviour. However, it is possible to omit
> it on 64-bit because shifting can be done within a 64-bit register and
> then truncated to the expected value (which is 0 mask).
> 
> Implicit truncation by htonl() fits nicely into R32-within-R64 model
> on x86-64.
> 
> Space savings: none (coincidence)
> Branch savings: 1
> 
> Before:
> 
> 	movzx  eax,BYTE PTR [rdi+0x2a]		# ->prefixlen_d
> 	test   al,al
> 	jne    xfrm_selector_match + 0x23f
> 		...
> 	movzx  eax,BYTE PTR [rbx+0x2b]		# ->prefixlen_s
> 	test   al,al
> 	je     xfrm_selector_match + 0x1c7
> 
> After (no branches):
> 
> 	mov    r8d,0x20
> 	mov    rdx,0xffffffffffffffff
> 	mov    esi,DWORD PTR [rsi+0x2c]
> 	mov    ecx,r8d
> 	sub    cl,BYTE PTR [rdi+0x2a]
> 	xor    esi,DWORD PTR [rbx]
> 	mov    rdi,rdx
> 	xor    eax,eax
> 	shl    rdi,cl
> 	bswap  edi
> 
> Signed-off-by: Alexey Dobriyan <adobriyan@...il.com>

Also applied to ipsec-next, thanks for the patches Alexey!