The following commit has been merged into the x86/core branch of tip:
Commit-ID: 2a144bcd661c4f0a503e03f9280e88854ac0bb37
Gitweb: https://git.kernel.org/tip/2a144bcd661c4f0a503e03f9280e88854ac0bb37
Author: Eric Dumazet <edumazet@xxxxxxxxxx>
AuthorDate: Thu, 25 Nov 2021 06:18:17 -08:00
Committer: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
CommitterDate: Tue, 30 Nov 2021 16:26:03 -08:00
x86/csum: Fix initial seed for odd buffers
When I folded do_csum() into csum_partial(), I missed that we
had to swap odd/even bytes from @sum argument.
This is because this swap will happen again at the end of the function.
[A, B, C, D] -> [B, A, D, C]
As far as Internet checksums (rfc 1071) are concerned, we can instead
rotate the whole 32bit value by 8 (or 24)
-> [D, A, B, C]
Note that I played with the idea of replacing this final swapping:
result = from32to16(result);
result = ((result >> 8) & 0xff) | ((result & 0xff) << 8);
With:
result = ror32(result, 8);
But while the generated code was definitely better for the odd case,
run time cost for the more likely even case was not better for gcc.
gcc is replacing a well predicted conditional branch
with a cmov instruction after a ror instruction which adds
a cost canceling the cmov gain.
Many thanks to Noah Goldstein for reporting this issue.
[ dhansen: * spelling: swaping => swapping
* updated Fixes commit ]
Cc: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Fixes: d31c3c683ee6 ("x86/csum: Rewrite/optimize csum_partial()")
Reported-by: Noah Goldstein <goldstein.w.n@xxxxxxxxx>
Signed-off-by: Eric Dumazet <edumazet@xxxxxxxxxx>
Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Link: https://lkml.kernel.org/r/20211125141817.3541501-1-eric.dumazet@xxxxxxxxx
---
arch/x86/lib/csum-partial_64.c | 1 +
1 file changed, 1 insertion(+)
diff --git a/arch/x86/lib/csum-partial_64.c b/arch/x86/lib/csum-partial_64.c
index 1eb8f2d..40b527b 100644
--- a/arch/x86/lib/csum-partial_64.c
+++ b/arch/x86/lib/csum-partial_64.c
@@ -41,6 +41,7 @@ __wsum csum_partial(const void *buff, int len, __wsum sum)
if (unlikely(odd)) {
if (unlikely(len == 0))
return sum;
+ temp64 = ror32((__force u32)sum, 8);
temp64 += (*(unsigned char *)buff << 8);
len--;
buff++;
|