[v12,1/5] asm-generic: Improve csum_fold

Commit Message

Charlie Jenkins Dec. 13, 2023, 1:18 a.m. UTC

This csum_fold implementation introduced into arch/arc by Vineet Gupta
is better than the default implementation on at least arc, x86, and
riscv. Using GCC trunk and compiling non-inlined version, this
implementation has 41.6667%, 25% fewer instructions on riscv64, x86-64
respectively with -O3 optimization. Most implmentations override this
default in asm, but this should be more performant than all of those
other implementations except for arm which has barrel shifting and
sparc32 which has a carry flag.

Signed-off-by: Charlie Jenkins <[e-mail]>
Reviewed-by: David Laight <[e-mail]>
---
 include/asm-generic/checksum.h | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

Comments

Arnd Bergmann Dec. 13, 2023, 7:52 a.m. UTC | #1

On Wed, Dec 13, 2023, at 02:18, Charlie Jenkins wrote:
> This csum_fold implementation introduced into arch/arc by Vineet Gupta
> is better than the default implementation on at least arc, x86, and
> riscv. Using GCC trunk and compiling non-inlined version, this
> implementation has 41.6667%, 25% fewer instructions on riscv64, x86-64
> respectively with -O3 optimization. Most implmentations override this
> default in asm, but this should be more performant than all of those
> other implementations except for arm which has barrel shifting and
> sparc32 which has a carry flag.
>
> Signed-off-by: Charlie Jenkins <[e-mail]>

Acked-by: Arnd Bergmann <[e-mail]>

Patch

diff --git a/include/asm-generic/checksum.h b/include/asm-generic/checksum.h
index 43e18db89c14..ad928cce268b 100644
--- a/include/asm-generic/checksum.h
+++ b/include/asm-generic/checksum.h
@@ -2,6 +2,8 @@ 
 #ifndef __ASM_GENERIC_CHECKSUM_H
 #define __ASM_GENERIC_CHECKSUM_H
 
+#include <linux/bitops.h>
+
 /*
  * computes the checksum of a memory block at buff, length len,
  * and adds in "sum" (32-bit)
@@ -31,9 +33,7 @@  extern __sum16 ip_fast_csum(const void *iph, unsigned int ihl);
 static inline __sum16 csum_fold(__wsum csum)
 {
 	u32 sum = (__force u32)csum;
-	sum = (sum & 0xffff) + (sum >> 16);
-	sum = (sum & 0xffff) + (sum >> 16);
-	return (__force __sum16)~sum;
+	return (__force __sum16)((~sum - ror32(sum, 16)) >> 16);
 }
 #endif