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

[Christophe LEROY]

Le 23/05/2018 à 20:34, Segher Boessenkool a écrit :
On Tue, May 22, 2018 at 08:57:01AM +0200, Christophe Leroy wrote:
The generic csum_ipv6_magic() generates a pretty bad result

<snip>

Please try with a more recent compiler, what you used is pretty ancient.
It's not like recent compilers do great on this either, but it's not
*that* bad anymore ;-)

--- a/arch/powerpc/lib/checksum_32.S
+++ b/arch/powerpc/lib/checksum_32.S
@@ -293,3 +293,36 @@ dst_error:
        EX_TABLE(51b, dst_error);
  
  EXPORT_SYMBOL(csum_partial_copy_generic)
+
+/*
+ * static inline __sum16 csum_ipv6_magic(const struct in6_addr *saddr,
+ *                                   const struct in6_addr *daddr,
+ *                                   __u32 len, __u8 proto, __wsum sum)
+ */
+
+_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.

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.

Christophe



Segher