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Optimize count_digits for powers of 2

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This commit is contained in:
Victor Zverovich 2021-03-10 13:12:37 -08:00
parent 85ba271639
commit 605b603735
1 changed files with 7 additions and 6 deletions
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13
include/fmt/format.h
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@ -1022,26 +1022,27 @@ FMT_CONSTEXPR inline int count_digits(uint128_t n) {
}
#endif
#ifdef FMT_BUILTIN_CLZLL
// 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 int count_digits(uint64_t n) {
if (is_constant_evaluated()) {
return count_digits_fallback(n);
}
#ifdef FMT_BUILTIN_CLZLL
// https://github.com/fmtlib/format-benchmark/blob/master/digits10
auto t = bsr2log10(FMT_BUILTIN_CLZLL(n | 1) ^ 63);
return t - (n < data::zero_or_powers_of_10_64_new[t]);
}
#else
// Fallback version of count_digits used when __builtin_clz is not available.
FMT_CONSTEXPR inline int count_digits(uint64_t n) {
return count_digits_fallback(n);
}
#endif
}
// Counts the number of digits in n. BITS = log2(radix).
template <unsigned BITS, typename UInt> FMT_CONSTEXPR int count_digits(UInt n) {
template <int BITS, typename UInt> FMT_CONSTEXPR int count_digits(UInt n) {
#ifdef FMT_BUILTIN_CLZ
if (num_bits<UInt>() == 32)
return (FMT_BUILTIN_CLZ(static_cast<uint32_t>(n) | 1) ^ 31) / BITS + 1;
#endif
int num_digits = 0;
do {
++num_digits;