Subject: Re: [PATCH v3] powerpc: Implement csum_ipv6_magic in assembly

From: Segher Boessenkool <[email-masked]>
Date: Thu, 24 May 2018 14:42:06 -0500

On Thu, May 24, 2018 at 08:20:16AM +0200, Christophe LEROY wrote:
> Le 23/05/2018 à 20:34, Segher Boessenkool a écrit :
> >On Tue, May 22, 2018 at 08:57:01AM +0200, Christophe Leroy wrote:
> >>+_GLOBAL(csum_ipv6_magic)
> >>+	lwz	r8, 0(r3)
> >>+	lwz	r9, 4(r3)
> >>+	lwz	r10, 8(r3)
> >>+	lwz	r11, 12(r3)
> >>+	addc	r0, r5, r6
> >>+	adde	r0, r0, r7
> >>+	adde	r0, r0, r8
> >>+	adde	r0, r0, r9
> >>+	adde	r0, r0, r10
> >>+	adde	r0, r0, r11
> >>+	lwz	r8, 0(r4)
> >>+	lwz	r9, 4(r4)
> >>+	lwz	r10, 8(r4)
> >>+	lwz	r11, 12(r4)
> >>+	adde	r0, r0, r8
> >>+	adde	r0, r0, r9
> >>+	adde	r0, r0, r10
> >>+	adde	r0, r0, r11
> >>+	addze	r0, r0
> >>+	rotlwi	r3, r0, 16
> >>+	add	r3, r0, r3
> >>+	not	r3, r3
> >>+	rlwinm	r3, r3, 16, 16, 31
> >>+	blr
> >>+EXPORT_SYMBOL(csum_ipv6_magic)
> >
> >Clustering the loads and carry insns together is pretty much the worst you
> >can do on most 32-bit CPUs.
> 
> Oh, really ? __csum_partial is written that way too.

I thought I told you about this before?  Maybe not.

> Right, now I tried interleaving the lwz and adde. I get no improvment at 
> all on a 885, but I get a 15% improvment on a 8321.

It won't likely help on single-issue cores (like the one 885 has), yes.

Segher