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Improve digit count

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This commit is contained in:
Victor Zverovich
2021-07-18 09:05:47 -07:00
parent f20f50368f
commit 561834650a
1 changed files with 26 additions and 18 deletions
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44
include/fmt/format.h
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@ -916,23 +916,31 @@ FMT_CONSTEXPR inline auto count_digits(uint128_t n) -> int {
}
#endif
// It is a separate function rather than a part of count_digits to workaround
// the lack of static constexpr in constexpr functions.
inline auto do_count_digits(uint64_t n) -> int {
// This has comparable performance to the version by Kendall Willets
// (https://github.com/fmtlib/format-benchmark/blob/master/digits10)
// but uses smaller tables.
// Maps bsr(n) to ceil(log10(pow(2, bsr(n) + 1) - 1)).
static constexpr uint8_t bsr2log10[] = {
1, 1, 1, 2, 2, 2, 3, 3, 3, 4, 4, 4, 4, 5, 5, 5,
6, 6, 6, 7, 7, 7, 7, 8, 8, 8, 9, 9, 9, 10, 10, 10,
10, 11, 11, 11, 12, 12, 12, 13, 13, 13, 13, 14, 14, 14, 15, 15,
15, 16, 16, 16, 16, 17, 17, 17, 18, 18, 18, 19, 19, 19, 19, 20};
auto t = bsr2log10[FMT_BUILTIN_CLZLL(n | 1) ^ 63];
static constexpr const uint64_t zero_or_powers_of_10[] = {
0, 0, FMT_POWERS_OF_10(1U), FMT_POWERS_OF_10(1000000000ULL),
10000000000000000000ULL};
return t - (n < zero_or_powers_of_10[t]);
}
// Returns the number of decimal digits in n. Leading zeros are not counted
// except for n == 0 in which case count_digits returns 1.
FMT_CONSTEXPR20 inline auto count_digits(uint64_t n) -> int {
#ifdef FMT_BUILTIN_CLZLL
if (!is_constant_evaluated()) {
// https://github.com/fmtlib/format-benchmark/blob/master/digits10
// Maps bsr(n) to ceil(log10(pow(2, bsr(n) + 1) - 1)).
constexpr uint16_t bsr2log10[] = {
1, 1, 1, 2, 2, 2, 3, 3, 3, 4, 4, 4, 4, 5, 5, 5,
6, 6, 6, 7, 7, 7, 7, 8, 8, 8, 9, 9, 9, 10, 10, 10,
10, 11, 11, 11, 12, 12, 12, 13, 13, 13, 13, 14, 14, 14, 15, 15,
15, 16, 16, 16, 16, 17, 17, 17, 18, 18, 18, 19, 19, 19, 19, 20};
auto t = bsr2log10[FMT_BUILTIN_CLZLL(n | 1) ^ 63];
constexpr const uint64_t zero_or_powers_of_10[] = {
0, 0, FMT_POWERS_OF_10(1U), FMT_POWERS_OF_10(1000000000ULL),
10000000000000000000ULL};
return t - (n < zero_or_powers_of_10[t]);
return do_count_digits(n);
}
#endif
return count_digits_fallback(n);
@ -956,9 +964,9 @@ template <> auto count_digits<4>(detail::fallback_uintptr n) -> int;
// It is a separate function rather than a part of count_digits to workaround
// the lack of static constexpr in constexpr functions.
FMT_INLINE uint64_t count_digits_inc(int n) {
// An optimization by Kendall Willets from https://bit.ly/3uOIQrB.
// This increments the upper 32 bits (log10(T) - 1) when >= T is added.
FMT_INLINE auto do_count_digits(uint32_t n) -> int {
// An optimization by Kendall Willets from https://bit.ly/3uOIQrB.
// This increments the upper 32 bits (log10(T) - 1) when >= T is added.
#define FMT_INC(T) (((sizeof(#T) - 1ull) << 32) - T)
static constexpr uint64_t table[] = {
FMT_INC(0), FMT_INC(0), FMT_INC(0), // 8
@ -973,15 +981,15 @@ FMT_INLINE uint64_t count_digits_inc(int n) {
FMT_INC(1000000000), FMT_INC(1000000000), FMT_INC(1000000000), // 1024M
FMT_INC(1000000000), FMT_INC(1000000000) // 4B
};
return table[n];
auto inc = table[FMT_BUILTIN_CLZ(n | 1) ^ 31];
return static_cast<int>((n + inc) >> 32);
}
// Optional version of count_digits for better performance on 32-bit platforms.
FMT_CONSTEXPR20 inline auto count_digits(uint32_t n) -> int {
#ifdef FMT_BUILTIN_CLZ
if (!is_constant_evaluated()) {
auto inc = count_digits_inc(FMT_BUILTIN_CLZ(n | 1) ^ 31);
return static_cast<int>((n + inc) >> 32);
return do_count_digits(n);
}
#endif
return count_digits_fallback(n);