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Optimize count_digits

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This commit is contained in:
Victor Zverovich
2020-08-02 07:39:14 -07:00
parent 8d9ab96736
commit 208291205d
2 changed files with 14 additions and 6 deletions
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13
include/fmt/format.h
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@@ -770,6 +770,8 @@ using uint32_or_64_or_128_t =
// Static data is placed in this class template for the header-only config.
template <typename T = void> struct FMT_EXTERN_TEMPLATE_API basic_data {
// Maps the result of bsr(n) to ceil(log10(n)).
static const uint16_t bsr2log10[];
static const uint64_t powers_of_10_64[];
static const uint32_t zero_or_powers_of_10_32[];
static const uint64_t zero_or_powers_of_10_64[];
@@ -799,10 +801,9 @@ struct data : basic_data<> {};
// Returns the number of decimal digits in n. Leading zeros are not counted
// except for n == 0 in which case count_digits returns 1.
inline int count_digits(uint64_t n) {
// Based on http://graphics.stanford.edu/~seander/bithacks.html#IntegerLog10
// and the benchmark https://github.com/localvoid/cxx-benchmark-count-digits.
int t = ((FMT_BUILTIN_CLZLL(n | 1) ^ 63) + 1) * 1233 >> 12;
return t - (n < data::zero_or_powers_of_10_64[t]) + 1;
// https://github.com/fmtlib/format-benchmark/blob/master/digits10
auto t = data::bsr2log10[FMT_BUILTIN_CLZLL(n | 1) ^ 63];
return t - (n < data::zero_or_powers_of_10_64[t - 1]);
}
#else
// Fallback version of count_digits used when __builtin_clz is not available.
@@ -859,8 +860,8 @@ template <> int count_digits<4>(detail::fallback_uintptr n);
#ifdef FMT_BUILTIN_CLZ
// Optional version of count_digits for better performance on 32-bit platforms.
inline int count_digits(uint32_t n) {
int t = ((FMT_BUILTIN_CLZ(n | 1) ^ 31) + 1) * 1233 >> 12;
return t - (n < data::zero_or_powers_of_10_32[t]) + 1;
auto t = data::bsr2log10[FMT_BUILTIN_CLZ(n | 1) ^ 31];
return t - (n < data::zero_or_powers_of_10_32[t - 1]);
}
#endif