Messages in this thread Patch in this message | | | From | Eric Dumazet <> | Subject | [PATCH] x86/csum: fix initial seed for odd buffers | Date | Thu, 25 Nov 2021 06:18:17 -0800 |
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From: Eric Dumazet <edumazet@google.com>
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 swaping:
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.
Fixes: df4554cebdaa ("x86/csum: Rewrite/optimize csum_partial()") Signed-off-by: Eric Dumazet <edumazet@google.com> Reported-by: Noah Goldstein <goldstein.w.n@gmail.com> Cc: Peter Zijlstra (Intel) <peterz@infradead.org> Cc: Alexander Duyck <alexanderduyck@fb.com> --- 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 1eb8f2d11f7c785be624eba315fe9ca7989fd56d..40b527ba1da1f74b5dbc51ddd97a9ecad22ee5ae 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++; -- 2.34.0.rc2.393.gf8c9666880-goog
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