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230 lines
6.9 KiB
Go
230 lines
6.9 KiB
Go
// Copyright 2011 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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// +build !appengine,!gccgo
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// AMD64-specific hardware-assisted CRC32 algorithms. See crc32.go for a
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// description of the interface that each architecture-specific file
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// implements.
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package crc32
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import "unsafe"
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// This file contains the code to call the SSE 4.2 version of the Castagnoli
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// and IEEE CRC.
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// haveSSE41/haveSSE42/haveCLMUL are defined in crc_amd64.s and use
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// CPUID to test for SSE 4.1, 4.2 and CLMUL support.
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func haveSSE41() bool
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func haveSSE42() bool
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func haveCLMUL() bool
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// castagnoliSSE42 is defined in crc32_amd64.s and uses the SSE4.2 CRC32
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// instruction.
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//go:noescape
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func castagnoliSSE42(crc uint32, p []byte) uint32
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// castagnoliSSE42Triple is defined in crc32_amd64.s and uses the SSE4.2 CRC32
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// instruction.
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//go:noescape
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func castagnoliSSE42Triple(
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crcA, crcB, crcC uint32,
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a, b, c []byte,
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rounds uint32,
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) (retA uint32, retB uint32, retC uint32)
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// ieeeCLMUL is defined in crc_amd64.s and uses the PCLMULQDQ
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// instruction as well as SSE 4.1.
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//go:noescape
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func ieeeCLMUL(crc uint32, p []byte) uint32
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var sse42 = haveSSE42()
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var useFastIEEE = haveCLMUL() && haveSSE41()
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const castagnoliK1 = 168
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const castagnoliK2 = 1344
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type sse42Table [4]Table
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var castagnoliSSE42TableK1 *sse42Table
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var castagnoliSSE42TableK2 *sse42Table
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func archAvailableCastagnoli() bool {
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return sse42
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}
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func archInitCastagnoli() {
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if !sse42 {
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panic("arch-specific Castagnoli not available")
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}
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castagnoliSSE42TableK1 = new(sse42Table)
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castagnoliSSE42TableK2 = new(sse42Table)
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// See description in updateCastagnoli.
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// t[0][i] = CRC(i000, O)
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// t[1][i] = CRC(0i00, O)
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// t[2][i] = CRC(00i0, O)
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// t[3][i] = CRC(000i, O)
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// where O is a sequence of K zeros.
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var tmp [castagnoliK2]byte
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for b := 0; b < 4; b++ {
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for i := 0; i < 256; i++ {
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val := uint32(i) << uint32(b*8)
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castagnoliSSE42TableK1[b][i] = castagnoliSSE42(val, tmp[:castagnoliK1])
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castagnoliSSE42TableK2[b][i] = castagnoliSSE42(val, tmp[:])
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}
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}
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}
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// castagnoliShift computes the CRC32-C of K1 or K2 zeroes (depending on the
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// table given) with the given initial crc value. This corresponds to
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// CRC(crc, O) in the description in updateCastagnoli.
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func castagnoliShift(table *sse42Table, crc uint32) uint32 {
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return table[3][crc>>24] ^
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table[2][(crc>>16)&0xFF] ^
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table[1][(crc>>8)&0xFF] ^
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table[0][crc&0xFF]
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}
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func archUpdateCastagnoli(crc uint32, p []byte) uint32 {
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if !sse42 {
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panic("not available")
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}
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// This method is inspired from the algorithm in Intel's white paper:
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// "Fast CRC Computation for iSCSI Polynomial Using CRC32 Instruction"
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// The same strategy of splitting the buffer in three is used but the
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// combining calculation is different; the complete derivation is explained
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// below.
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//
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// -- The basic idea --
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//
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// The CRC32 instruction (available in SSE4.2) can process 8 bytes at a
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// time. In recent Intel architectures the instruction takes 3 cycles;
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// however the processor can pipeline up to three instructions if they
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// don't depend on each other.
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//
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// Roughly this means that we can process three buffers in about the same
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// time we can process one buffer.
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//
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// The idea is then to split the buffer in three, CRC the three pieces
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// separately and then combine the results.
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//
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// Combining the results requires precomputed tables, so we must choose a
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// fixed buffer length to optimize. The longer the length, the faster; but
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// only buffers longer than this length will use the optimization. We choose
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// two cutoffs and compute tables for both:
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// - one around 512: 168*3=504
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// - one around 4KB: 1344*3=4032
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//
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// -- The nitty gritty --
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//
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// Let CRC(I, X) be the non-inverted CRC32-C of the sequence X (with
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// initial non-inverted CRC I). This function has the following properties:
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// (a) CRC(I, AB) = CRC(CRC(I, A), B)
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// (b) CRC(I, A xor B) = CRC(I, A) xor CRC(0, B)
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//
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// Say we want to compute CRC(I, ABC) where A, B, C are three sequences of
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// K bytes each, where K is a fixed constant. Let O be the sequence of K zero
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// bytes.
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//
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// CRC(I, ABC) = CRC(I, ABO xor C)
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// = CRC(I, ABO) xor CRC(0, C)
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// = CRC(CRC(I, AB), O) xor CRC(0, C)
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// = CRC(CRC(I, AO xor B), O) xor CRC(0, C)
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// = CRC(CRC(I, AO) xor CRC(0, B), O) xor CRC(0, C)
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// = CRC(CRC(CRC(I, A), O) xor CRC(0, B), O) xor CRC(0, C)
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//
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// The castagnoliSSE42Triple function can compute CRC(I, A), CRC(0, B),
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// and CRC(0, C) efficiently. We just need to find a way to quickly compute
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// CRC(uvwx, O) given a 4-byte initial value uvwx. We can precompute these
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// values; since we can't have a 32-bit table, we break it up into four
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// 8-bit tables:
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//
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// CRC(uvwx, O) = CRC(u000, O) xor
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// CRC(0v00, O) xor
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// CRC(00w0, O) xor
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// CRC(000x, O)
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//
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// We can compute tables corresponding to the four terms for all 8-bit
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// values.
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crc = ^crc
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// If a buffer is long enough to use the optimization, process the first few
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// bytes to align the buffer to an 8 byte boundary (if necessary).
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if len(p) >= castagnoliK1*3 {
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delta := int(uintptr(unsafe.Pointer(&p[0])) & 7)
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if delta != 0 {
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delta = 8 - delta
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crc = castagnoliSSE42(crc, p[:delta])
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p = p[delta:]
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}
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}
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// Process 3*K2 at a time.
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for len(p) >= castagnoliK2*3 {
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// Compute CRC(I, A), CRC(0, B), and CRC(0, C).
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crcA, crcB, crcC := castagnoliSSE42Triple(
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crc, 0, 0,
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p, p[castagnoliK2:], p[castagnoliK2*2:],
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castagnoliK2/24)
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// CRC(I, AB) = CRC(CRC(I, A), O) xor CRC(0, B)
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crcAB := castagnoliShift(castagnoliSSE42TableK2, crcA) ^ crcB
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// CRC(I, ABC) = CRC(CRC(I, AB), O) xor CRC(0, C)
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crc = castagnoliShift(castagnoliSSE42TableK2, crcAB) ^ crcC
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p = p[castagnoliK2*3:]
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}
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// Process 3*K1 at a time.
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for len(p) >= castagnoliK1*3 {
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// Compute CRC(I, A), CRC(0, B), and CRC(0, C).
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crcA, crcB, crcC := castagnoliSSE42Triple(
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crc, 0, 0,
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p, p[castagnoliK1:], p[castagnoliK1*2:],
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castagnoliK1/24)
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// CRC(I, AB) = CRC(CRC(I, A), O) xor CRC(0, B)
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crcAB := castagnoliShift(castagnoliSSE42TableK1, crcA) ^ crcB
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// CRC(I, ABC) = CRC(CRC(I, AB), O) xor CRC(0, C)
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crc = castagnoliShift(castagnoliSSE42TableK1, crcAB) ^ crcC
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p = p[castagnoliK1*3:]
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}
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// Use the simple implementation for what's left.
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crc = castagnoliSSE42(crc, p)
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return ^crc
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}
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func archAvailableIEEE() bool {
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return useFastIEEE
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}
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var archIeeeTable8 *slicing8Table
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func archInitIEEE() {
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if !useFastIEEE {
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panic("not available")
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}
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// We still use slicing-by-8 for small buffers.
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archIeeeTable8 = slicingMakeTable(IEEE)
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}
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func archUpdateIEEE(crc uint32, p []byte) uint32 {
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if !useFastIEEE {
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panic("not available")
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}
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if len(p) >= 64 {
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left := len(p) & 15
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do := len(p) - left
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crc = ^ieeeCLMUL(^crc, p[:do])
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p = p[do:]
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}
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if len(p) == 0 {
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return crc
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}
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return slicingUpdate(crc, archIeeeTable8, p)
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}
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