liblzma: Add crc_clmul_consts_gen.c

It's a standalone program that prints the required constants.
It's won't be a part of the normal build of the package.
This commit is contained in:
Lasse Collin 2024-06-10 14:45:44 +03:00
parent 71b147aab7
commit 9f5fc17e32
2 changed files with 161 additions and 0 deletions

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@ -5,6 +5,7 @@
## currently crc32 is always enabled. ## currently crc32 is always enabled.
EXTRA_DIST += \ EXTRA_DIST += \
check/crc_clmul_consts_gen.c \
check/crc32_tablegen.c \ check/crc32_tablegen.c \
check/crc64_tablegen.c check/crc64_tablegen.c

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@ -0,0 +1,160 @@
// SPDX-License-Identifier: 0BSD
///////////////////////////////////////////////////////////////////////////////
//
/// \file crc_clmul_consts_gen.c
/// \brief Generate constants for CLMUL CRC code
///
/// Compiling: gcc -std=c99 -o crc_clmul_consts_gen crc_clmul_consts_gen.c
///
/// This is for CRCs that use reversed bit order (bit reflection).
/// The same CLMUL CRC code can be used with CRC64 and smaller ones like
/// CRC32 apart from one special case: CRC64 needs an extra step in the
/// Barrett reduction to handle the 65th bit; the smaller ones don't.
/// Otherwise it's enough to just change the polynomial and the derived
/// constants and use the same code.
///
/// See the Intel white paper "Fast CRC Computation for Generic Polynomials
/// Using PCLMULQDQ Instruction" from 2009.
//
// Author: Lasse Collin
//
///////////////////////////////////////////////////////////////////////////////
#include <inttypes.h>
#include <stdio.h>
/// CRC32 (Ethernet) polynomial in reversed representation
static const uint64_t p32 = 0xedb88320;
// CRC64 (ECMA-182) polynomial in reversed representation
static const uint64_t p64 = 0xc96c5795d7870f42;
/// Calculates floor(x^128 / p) where p is a CRC64 polynomial in
/// reversed representation. The result is in reversed representation too.
static uint64_t
calc_cldiv(uint64_t p)
{
// Quotient
uint64_t q = 0;
// Align the x^64 term with the x^128 (the implied high bits of the
// divisor and the dividend) and do the first step of polynomial long
// division, calculating the first remainder. The variable q remains
// zero because the highest bit of the quotient is an implied bit 1
// (we kind of set q = 1 << -1).
uint64_t r = p;
// Then process the remaining 64 terms. Note that r has no implied
// high bit, only q and p do. (And remember that a high bit in the
// polynomial is stored at a low bit in the variable due to the
// reversed bit order.)
for (unsigned i = 0; i < 64; ++i) {
q |= (r & 1) << i;
r = (r >> 1) ^ (r & 1 ? p : 0);
}
return q;
}
/// Calculate the remainder of carryless division:
///
/// x^(bits + n - 1) % p, where n=64 (for CRC64)
///
/// p must be in reversed representation which omits the bit of
/// the highest term of the polynomial. Instead, it is an implied bit
/// at kind of like "1 << -1" position, as if it had just been shifted out.
///
/// The return value is in the reversed bit order. (There are no implied bits.)
static uint64_t
calc_clrem(uint64_t p, unsigned bits)
{
// Do the first step of polynomial long division.
uint64_t r = p;
// Then process the remaining terms. Start with i = 1 instead of i = 0
// to account for the -1 in x^(bits + n - 1). This -1 is convenient
// with the reversed bit order. See the "Bit-Reflection" section in
// the Intel white paper.
for (unsigned i = 1; i < bits; ++i)
r = (r >> 1) ^ (r & 1 ? p : 0);
return r;
}
extern int
main(void)
{
puts("// CRC64");
// The order of the two 64-bit constants in a vector don't matter.
// It feels logical to put them in this order as it matches the
// order in which the input bytes are read.
printf("const __m128i fold512 = _mm_set_epi64x("
"0x%016" PRIx64 ", 0x%016" PRIx64 ");\n",
calc_clrem(p64, 4 * 128 - 64),
calc_clrem(p64, 4 * 128));
printf("const __m128i fold128 = _mm_set_epi64x("
"0x%016" PRIx64 ", 0x%016" PRIx64 ");\n",
calc_clrem(p64, 128 - 64),
calc_clrem(p64, 128));
// When we multiply by mu, we care about the high bits of the result
// (in reversed bit order!). It doesn't matter that the low bit gets
// shifted out because the affected output bits will be ignored.
// Below we add the implied high bit with "| 1" after the shifting
// so that the high bits of the multiplication will be correct.
//
// p64 is shifted left by one so that the final multiplication
// in Barrett reduction won't be misaligned by one bit. We could
// use "(p64 << 1) | 1" instead of "p64 << 1" too but it makes
// no difference as that bit won't affect the relevant output bits
// (we only care about the lowest 64 bits of the result, that is,
// lowest in the reversed bit order).
//
// NOTE: The 65rd bit of p64 gets shifted out. It needs to be
// compensated with 64-bit shift and xor in the CRC64 code.
printf("const __m128i mu_p = _mm_set_epi64x("
"0x%016" PRIx64 ", 0x%016" PRIx64 ");\n",
(calc_cldiv(p64) << 1) | 1,
p64 << 1);
puts("");
puts("// CRC32");
printf("const __m128i fold512 = _mm_set_epi64x("
"0x%08" PRIx64 ", 0x%08" PRIx64 ");\n",
calc_clrem(p32, 4 * 128 - 64),
calc_clrem(p32, 4 * 128));
printf("const __m128i fold128 = _mm_set_epi64x("
"0x%08" PRIx64 ", 0x%08" PRIx64 ");\n",
calc_clrem(p32, 128 - 64),
calc_clrem(p32, 128));
// CRC32 calculation is done by modulus scaling it to a CRC64.
// Since the CRC is in reversed representation, only the mu
// constant changes with the modulus scaling. This method avoids
// one additional constant and one additional clmul in the final
// reduction steps, making the code both simpler and faster.
//
// p32 is shifted left by one so that the final multiplication
// in Barrett reduction won't be misaligned by one bit. We could
// use "(p32 << 1) | 1" instead of "p32 << 1" too but it makes
// no difference as that bit won't affect the relevant output bits.
//
// NOTE: The 33-bit value fits in 64 bits so, unlike with CRC64,
// there is no need to compensate for any missing bits in the code.
printf("const __m128i mu_p = _mm_set_epi64x("
"0x%016" PRIx64 ", 0x%" PRIx64 ");\n",
(calc_cldiv(p32) << 1) | 1,
p32 << 1);
return 0;
}