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optimize uint16 parsing

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Signed-off-by: Shikhar <shikharish05@gmail.com>
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Shikhar 2026-01-01 08:29:14 +05:30
parent 36d3441dc0
commit d0af1cfdbd
1 changed files with 116 additions and 0 deletions
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116
include/fast_float/ascii_number.h
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@ -68,6 +68,26 @@ read8_to_u64(UC const *chars) {
return val;
}
// Read 4 UC into a u32. Truncates UC if not char.
template <typename UC>
fastfloat_really_inline FASTFLOAT_CONSTEXPR20 uint32_t
read4_to_u32(UC const *chars) {
if (cpp20_and_in_constexpr() || !std::is_same<UC, char>::value) {
uint32_t val = 0;
for (int i = 0; i < 4; ++i) {
val |= uint32_t(uint8_t(*chars)) << (i * 8);
++chars;
}
return val;
}
uint32_t val;
::memcpy(&val, chars, sizeof(uint32_t));
#if FASTFLOAT_IS_BIG_ENDIAN == 1
val = byteswap(val);
#endif
return val;
}
#ifdef FASTFLOAT_SSE2
fastfloat_really_inline uint64_t simd_read8_to_u64(__m128i const data) {
@ -149,6 +169,13 @@ is_made_of_eight_digits_fast(uint64_t val) noexcept {
0x8080808080808080));
}
fastfloat_really_inline FASTFLOAT_CONSTEXPR14 uint32_t
parse_four_digits_unrolled(uint32_t val) noexcept {
val -= 0x30303030;
val = (val * 10) + (val >> 8);
return (((val & 0x00FF00FF) * 0x00640001) >> 16) & 0xFFFF;
}
#ifdef FASTFLOAT_HAS_SIMD
// Call this if chars might not be 8 digits.
@ -606,6 +633,95 @@ parse_int_string(UC const *p, UC const *pend, T &value,
}
}
FASTFLOAT_IF_CONSTEXPR17((std::is_same<T, std::uint16_t>::value)) {
if (base == 10) {
const size_t len = size_t(pend - p);
if (len == 0) {
if (has_leading_zeros) {
value = 0;
answer.ec = std::errc();
answer.ptr = p;
} else {
answer.ec = std::errc::invalid_argument;
answer.ptr = first;
}
return answer;
}
uint32_t digits;
if (len >= 4) {
digits = read4_to_u32(p);
} else {
uint32_t b0 = uint32_t(uint8_t(p[0]));
uint32_t b1 = (len > 1) ? uint32_t(uint8_t(p[1])) : 0xFFu;
uint32_t b2 = (len > 2) ? uint32_t(uint8_t(p[2])) : 0xFFu;
digits = b0 | (b1 << 8) | (b2 << 16) | (0xFFu << 24);
}
uint32_t magic =
((digits + 0x46464646u) | (digits - 0x30303030u)) & 0x80808080u;
uint32_t nd = (magic == 0) ? 4u : (uint32_t(countr_zero_32(magic)) >> 3);
if (nd == 0) {
if (has_leading_zeros) {
value = 0;
answer.ec = std::errc();
answer.ptr = p;
return answer;
}
answer.ec = std::errc::invalid_argument;
answer.ptr = first;
return answer;
}
if (nd < 4) {
// mask out non-digit bytes and replace with '0' (0x30)
uint32_t mask = 0xFFFFFFFFu >> ((4u - nd) * 8u);
uint32_t padded = (digits & mask) | (~mask & 0x30303030u);
uint32_t v = parse_four_digits_unrolled(padded);
static constexpr uint32_t divs[] = {0, 1000, 100, 10};
value = (uint16_t)(v / divs[nd]);
answer.ec = std::errc();
answer.ptr = p + nd;
return answer;
}
uint32_t v = parse_four_digits_unrolled(digits);
uint32_t d4 = (len > 4) ? uint32_t(p[4] - '0') : 10u;
if (d4 > 9u) {
value = (uint16_t)v;
answer.ec = std::errc();
answer.ptr = p + 4;
return answer;
}
if (len > 5) {
uint32_t d5 = uint32_t(p[5]) - uint32_t('0');
if (d5 <= 9u) {
const UC *q = p + 6;
while (q < pend && uint32_t(*q) - uint32_t('0') <= 9u)
++q;
answer.ec = std::errc::result_out_of_range;
answer.ptr = q;
return answer;
}
}
// overflow check
if (v > 6553u || (v == 6553u && d4 > 5u)) {
answer.ec = std::errc::result_out_of_range;
answer.ptr = p + 5;
return answer;
}
value = (uint16_t)(v * 10u + d4);
answer.ec = std::errc();
answer.ptr = p + 5;
return answer;
}
}
uint64_t i = 0;
if (base == 10) {
loop_parse_if_eight_digits(p, pend, i); // use SIMD if possible