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83 lines
1.8 KiB
83 lines
1.8 KiB
/*
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* Compute 16-bit sum in ones' complement arithmetic (with end-around carry).
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* This sum is often used as a simple checksum in networking.
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*
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* Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
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* See https://llvm.org/LICENSE.txt for license information.
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* SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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*/
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#include "networking.h"
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#include "chksum_common.h"
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always_inline
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static inline uint32_t
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slurp_head32(const void **pptr, uint32_t *nbytes)
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{
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uint32_t sum = 0;
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Assert(*nbytes >= 4);
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uint32_t off = (uintptr_t) *pptr % 4;
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if (likely(off != 0))
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{
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/* Get rid of bytes 0..off-1 */
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const unsigned char *ptr32 = align_ptr(*pptr, 4);
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uint32_t mask = ~0U << (CHAR_BIT * off);
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sum = load32(ptr32) & mask;
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*pptr = ptr32 + 4;
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*nbytes -= 4 - off;
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}
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return sum;
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}
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/* Additional loop unrolling would help when not auto-vectorizing */
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unsigned short
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__chksum(const void *ptr, unsigned int nbytes)
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{
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bool swap = false;
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uint64_t sum = 0;
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if (nbytes > 300)
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{
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/* 4-byte align pointer */
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swap = (uintptr_t) ptr & 1;
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sum = slurp_head32(&ptr, &nbytes);
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}
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/* Else benefit of aligning not worth the overhead */
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/* Sum all 16-byte chunks */
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const char *cptr = ptr;
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for (uint32_t nquads = nbytes / 16; nquads != 0; nquads--)
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{
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uint64_t h0 = load32(cptr + 0);
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uint64_t h1 = load32(cptr + 4);
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uint64_t h2 = load32(cptr + 8);
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uint64_t h3 = load32(cptr + 12);
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sum += h0 + h1 + h2 + h3;
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cptr += 16;
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}
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nbytes %= 16;
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Assert(nbytes < 16);
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/* Handle any trailing 4-byte chunks */
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while (nbytes >= 4)
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{
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sum += load32(cptr);
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cptr += 4;
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nbytes -= 4;
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}
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Assert(nbytes < 4);
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if (nbytes & 2)
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{
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sum += load16(cptr);
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cptr += 2;
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}
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if (nbytes & 1)
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{
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sum += *(uint8_t *)cptr;
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}
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return fold_and_swap(sum, swap);
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}
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