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Faster BCD into integer parsing

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This commit is contained in:
Niels Dossche 2024-05-20 15:10:58 +02:00
parent 04a34c3433
commit 4e99bb5935
1 changed files with 64 additions and 8 deletions
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72
ext/bcmath/libbcmath/src/recmul.c
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@ -58,12 +58,68 @@ static inline void bc_digits_adjustment(BC_UINT_T *prod_uint, size_t prod_arr_si
}
}
/* This is based on the technique described in https://kholdstare.github.io/technical/2020/05/26/faster-integer-parsing.html.
* This function transforms AABBCCDD into 1000 * AA + 100 * BB + 10 * CC + DD,
* with the caveat that all components must be in the interval [0, 25] to prevent overflow
* due to the multiplication by power of 10 (10 * 25 = 250 is the largest number that fits in a byte).
* The advantage of this method instead of using shifts + 3 multiplications is that this is cheaper
* due to its divide-and-conquer nature.
*/
#if SIZEOF_SIZE_T == 4
static uint32_t bc_parse_chunk_chars(const char *str)
{
uint32_t tmp;
memcpy(&tmp, str, sizeof(tmp));
#if !BC_LITTLE_ENDIAN
tmp = BC_BSWAP(tmp);
#endif
uint32_t lower_digits = (tmp & 0x0f000f00) >> 8;
uint32_t upper_digits = (tmp & 0x000f000f) * 10;
tmp = lower_digits + upper_digits;
lower_digits = (tmp & 0x00ff0000) >> 16;
upper_digits = (tmp & 0x000000ff) * 100;
return lower_digits + upper_digits;
}
#elif SIZEOF_SIZE_T == 8
static uint64_t bc_parse_chunk_chars(const char *str)
{
uint64_t tmp;
memcpy(&tmp, str, sizeof(tmp));
#if !BC_LITTLE_ENDIAN
tmp = BC_BSWAP(tmp);
#endif
uint64_t lower_digits = (tmp & 0x0f000f000f000f00) >> 8;
uint64_t upper_digits = (tmp & 0x000f000f000f000f) * 10;
tmp = lower_digits + upper_digits;
lower_digits = (tmp & 0x00ff000000ff0000) >> 16;
upper_digits = (tmp & 0x000000ff000000ff) * 100;
tmp = lower_digits + upper_digits;
lower_digits = (tmp & 0x0000ffff00000000) >> 32;
upper_digits = (tmp & 0x000000000000ffff) * 10000;
return lower_digits + upper_digits;
}
#endif
/*
* Converts BCD to uint, going backwards from pointer n by the number of
* characters specified by len.
*/
static inline BC_UINT_T bc_partial_convert_to_uint(const char *n, size_t len)
{
if (len == BC_MUL_UINT_DIGITS) {
return bc_parse_chunk_chars(n - BC_MUL_UINT_DIGITS + 1);
}
BC_UINT_T num = 0;
BC_UINT_T base = 1;
@ -226,14 +282,14 @@ static void bc_standard_mul(bc_num n1, size_t n1len, bc_num n2, size_t n2len, bc
char *pend = pptr + prodlen - 1;
i = 0;
while (i < prod_arr_size - 1) {
if (BC_MUL_UINT_DIGITS == 4) {
bc_write_bcd_representation(prod_uint[i], pend - 3);
pend -= 4;
} else {
bc_write_bcd_representation(prod_uint[i] / 10000, pend - 7);
bc_write_bcd_representation(prod_uint[i] % 10000, pend - 3);
pend -= 8;
}
#if BC_MUL_UINT_DIGITS == 4
bc_write_bcd_representation(prod_uint[i], pend - 3);
pend -= 4;
#else
bc_write_bcd_representation(prod_uint[i] / 10000, pend - 7);
bc_write_bcd_representation(prod_uint[i] % 10000, pend - 3);
pend -= 8;
#endif
i++;
}