2018-04-10 06:42:49

by Christophe Leroy

Subject: [PATCH] powerpc/64: optimises from64to32()

The current implementation of from64to32() gives a poor result:

0000000000000270 <.from64to32>:
270: 38 00 ff ff li r0,-1
274: 78 69 00 22 rldicl r9,r3,32,32
278: 78 00 00 20 clrldi r0,r0,32
27c: 7c 60 00 38 and r0,r3,r0
280: 7c 09 02 14 add r0,r9,r0
284: 78 09 00 22 rldicl r9,r0,32,32
288: 7c 00 4a 14 add r0,r0,r9
28c: 78 03 00 20 clrldi r3,r0,32
290: 4e 80 00 20 blr

This patch modifies from64to32() to operate in the same
spirit as csum_fold()

It swaps the two 32-bit halves of sum then it adds it with the
unswapped sum. If there is a carry from adding the two 32-bit halves,
it will carry from the lower half into the upper half, giving us the
correct sum in the upper half.

The resulting code is:

0000000000000260 <.from64to32>:
260: 78 60 00 02 rotldi r0,r3,32
264: 7c 60 1a 14 add r3,r0,r3
268: 78 63 00 22 rldicl r3,r3,32,32
26c: 4e 80 00 20 blr

Signed-off-by: Christophe Leroy <[email protected]>
---
arch/powerpc/include/asm/checksum.h | 7 ++-----
1 file changed, 2 insertions(+), 5 deletions(-)

diff --git a/arch/powerpc/include/asm/checksum.h b/arch/powerpc/include/asm/checksum.h
index 4e63787dc3be..54065caa40b3 100644
--- a/arch/powerpc/include/asm/checksum.h
+++ b/arch/powerpc/include/asm/checksum.h
@@ -12,6 +12,7 @@
#ifdef CONFIG_GENERIC_CSUM
#include <asm-generic/checksum.h>
#else
+#include <linux/bitops.h>
/*
* Computes the checksum of a memory block at src, length len,
* and adds in "sum" (32-bit), while copying the block to dst.
@@ -55,11 +56,7 @@ static inline __sum16 csum_fold(__wsum sum)

static inline u32 from64to32(u64 x)
{
- /* add up 32-bit and 32-bit for 32+c bit */
- x = (x & 0xffffffff) + (x >> 32);
- /* add up carry.. */
- x = (x & 0xffffffff) + (x >> 32);
- return (u32)x;
+ return (x + ror64(x, 32)) >> 32;
}

static inline __wsum csum_tcpudp_nofold(__be32 saddr, __be32 daddr, __u32 len,
--
2.13.3

2018-06-04 14:18:12

by Michael Ellerman

Subject: Re: powerpc/64: optimises from64to32()

On Tue, 2018-04-10 at 06:34:35 UTC, Christophe Leroy wrote:
> The current implementation of from64to32() gives a poor result:
>
> 0000000000000270 <.from64to32>:
> 270: 38 00 ff ff li r0,-1
> 274: 78 69 00 22 rldicl r9,r3,32,32
> 278: 78 00 00 20 clrldi r0,r0,32
> 27c: 7c 60 00 38 and r0,r3,r0
> 280: 7c 09 02 14 add r0,r9,r0
> 284: 78 09 00 22 rldicl r9,r0,32,32
> 288: 7c 00 4a 14 add r0,r0,r9
> 28c: 78 03 00 20 clrldi r3,r0,32
> 290: 4e 80 00 20 blr
>
> This patch modifies from64to32() to operate in the same
> spirit as csum_fold()
>
> It swaps the two 32-bit halves of sum then it adds it with the
> unswapped sum. If there is a carry from adding the two 32-bit halves,
> it will carry from the lower half into the upper half, giving us the
> correct sum in the upper half.
>
> The resulting code is:
>
> 0000000000000260 <.from64to32>:
> 260: 78 60 00 02 rotldi r0,r3,32
> 264: 7c 60 1a 14 add r3,r0,r3
> 268: 78 63 00 22 rldicl r3,r3,32,32
> 26c: 4e 80 00 20 blr
>
> Signed-off-by: Christophe Leroy <[email protected]>

Applied to powerpc next, thanks.

https://git.kernel.org/powerpc/c/55a0edf083022e402042255a0afb03

cheers