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Optimize count_digits

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This commit is contained in:
Victor Zverovich 2021-06-04 06:45:20 -07:00
parent 2ac0bfe59e
commit 3eeb084e71
1 changed files with 18 additions and 5 deletions
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23
include/fmt/format.h
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@ -995,13 +995,26 @@ template <> auto count_digits<4>(detail::fallback_uintptr n) -> int;
#ifdef FMT_BUILTIN_CLZ
// Optional version of count_digits for better performance on 32-bit platforms.
FMT_CONSTEXPR20 inline auto count_digits(uint32_t n) -> int {
if (is_constant_evaluated()) {
if (is_constant_evaluated() || FMT_MSC_VER) {
return count_digits_fallback(n);
}
auto t = bsr2log10(FMT_BUILTIN_CLZ(n | 1) ^ 31);
constexpr const uint32_t zero_or_powers_of_10[] = {0, 0,
FMT_POWERS_OF_10(1U)};
return t - (n < zero_or_powers_of_10[t]);
// 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) - 1) << 32) - T)
constexpr uint64_t table[] = {
FMT_INC(0), FMT_INC(0), FMT_INC(0), // 8
FMT_INC(10), FMT_INC(10), FMT_INC(10), // 64
FMT_INC(100), FMT_INC(100), FMT_INC(100), // 512
FMT_INC(1000), FMT_INC(1000), FMT_INC(1000), // 4096
FMT_INC(10000), FMT_INC(10000), FMT_INC(10000), // 32k
FMT_INC(100000), FMT_INC(100000), FMT_INC(100000), // 256k
FMT_INC(1000000), FMT_INC(1000000), FMT_INC(1000000), // 2048k
FMT_INC(10000000), FMT_INC(10000000), FMT_INC(10000000), // 16M
FMT_INC(100000000), FMT_INC(100000000), FMT_INC(100000000), // 128M
FMT_INC(1000000000), FMT_INC(1000000000), FMT_INC(1000000000), // 1024M
FMT_INC(1000000000), FMT_INC(1000000000) // 4B
};
return static_cast<int>((n + table[FMT_BUILTIN_CLZ(n | 1) ^ 31]) >> 32);
}
#endif