Today's implementation of csum_shift() leads to branching based on
parity of 'offset'
000002f8 <csum_block_add>:
2f8: 70 a5 00 01 andi. r5,r5,1
2fc: 41 a2 00 08 beq 304 <csum_block_add+0xc>
300: 54 84 c0 3e rotlwi r4,r4,24
304: 7c 63 20 14 addc r3,r3,r4
308: 7c 63 01 94 addze r3,r3
30c: 4e 80 00 20 blr
Use first bit of 'offset' directly as input of the rotation instead of
branching.
000002f8 <csum_block_add>:
2f8: 54 a5 1f 38 rlwinm r5,r5,3,28,28
2fc: 20 a5 00 20 subfic r5,r5,32
300: 5c 84 28 3e rotlw r4,r4,r5
304: 7c 63 20 14 addc r3,r3,r4
308: 7c 63 01 94 addze r3,r3
30c: 4e 80 00 20 blr
And change to left shift instead of right shift to skip one more
instruction. This has no impact on the final sum.
000002f8 <csum_block_add>:
2f8: 54 a5 1f 38 rlwinm r5,r5,3,28,28
2fc: 5c 84 28 3e rotlw r4,r4,r5
300: 7c 63 20 14 addc r3,r3,r4
304: 7c 63 01 94 addze r3,r3
308: 4e 80 00 20 blr
Seems like only powerpc benefits from a branchless implementation.
Other main architectures like ARM or X86 get better code with
the generic implementation and its branch.
Signed-off-by: Christophe Leroy <christophe.le...@csgroup.eu>
---
arch/powerpc/include/asm/checksum.h | 7 +++++++
include/net/checksum.h | 2 ++
2 files changed, 9 insertions(+)
diff --git a/arch/powerpc/include/asm/checksum.h
b/arch/powerpc/include/asm/checksum.h
index 19eaa2b6d043..8321f6053a67 100644
--- a/arch/powerpc/include/asm/checksum.h
+++ b/arch/powerpc/include/asm/checksum.h
@@ -111,6 +111,13 @@ static __always_inline __wsum csum_add(__wsum csum, __wsum
addend)
#endif
}
+#define HAVE_ARCH_CSUM_SHIFT
+static __always_inline __wsum csum_shift(__wsum sum, int offset)
+{
+ /* rotate sum to align it with a 16b boundary */
+ return (__force __wsum)rol32((__force u32)sum, (offset & 1) << 3);
+}
+
/*
* This is a version of ip_compute_csum() optimized for IP headers,
* which always checksum on 4 octet boundaries. ihl is the number
diff --git a/include/net/checksum.h b/include/net/checksum.h
index 79c67f14c448..6bc783b7a06c 100644
--- a/include/net/checksum.h
+++ b/include/net/checksum.h
@@ -80,6 +80,7 @@ static __always_inline __sum16 csum16_sub(__sum16 csum,
__be16 addend)
return csum16_add(csum, ~addend);
}
+#ifndef HAVE_ARCH_CSUM_SHIFT
static __always_inline __wsum csum_shift(__wsum sum, int offset)
{
/* rotate sum to align it with a 16b boundary */
@@ -87,6 +88,7 @@ static __always_inline __wsum csum_shift(__wsum sum, int
offset)
return (__force __wsum)ror32((__force u32)sum, 8);
return sum;
}
+#endif
static __always_inline __wsum
csum_block_add(__wsum csum, __wsum csum2, int offset)
--
2.34.1
