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Reuse log10_2_significand constant

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This commit is contained in:
Victor Zverovich
2020-09-23 13:19:21 -07:00
parent 1305cbeb6f
commit 51f8d0cc21
2 changed files with 16 additions and 15 deletions
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29
include/fmt/format-inl.h
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@ -1197,11 +1197,11 @@ inline fp operator*(fp x, fp y) { return {multiply(x.f, y.f), x.e + y.e + 64}; }
// Returns a cached power of 10 `c_k = c_k.f * pow(2, c_k.e)` such that its // Returns a cached power of 10 `c_k = c_k.f * pow(2, c_k.e)` such that its
// (binary) exponent satisfies `min_exponent <= c_k.e <= min_exponent + 28`. // (binary) exponent satisfies `min_exponent <= c_k.e <= min_exponent + 28`.
inline fp get_cached_power(int min_exponent, int& pow10_exponent) { inline fp get_cached_power(int min_exponent, int& pow10_exponent) {
const int64_t one_over_log2_10 = 0x4d104d42; // round(pow(2, 32) / log2(10)) const int shift = 32;
int index = static_cast<int>( int index = static_cast<int>(((min_exponent + fp::significand_size - 1) *
((min_exponent + fp::significand_size - 1) * one_over_log2_10 + (data::log10_2_significand >> shift) +
((int64_t(1) << 32) - 1)) // ceil ((int64_t(1) << shift) - 1)) // ceil
>> 32 // arithmetic shift >> 32 // arithmetic shift
); );
// Decimal exponent of the first (smallest) cached power of 10. // Decimal exponent of the first (smallest) cached power of 10.
const int first_dec_exp = -348; const int first_dec_exp = -348;
@ -1752,15 +1752,16 @@ inline uint64_t umul96_lower64(uint32_t x, uint64_t y) FMT_NOEXCEPT {
return x * y; return x * y;
} }
// Various fast log computations. // Computes floor(log10(pow(2, e))) for e in [-1700, 1700] using the method from
// https://fmt.dev/papers/Grisu-Exact.pdf#page=5, section 3.4.
inline int floor_log10_pow2(int e) FMT_NOEXCEPT { inline int floor_log10_pow2(int e) FMT_NOEXCEPT {
FMT_ASSERT(e <= 1700 && e >= -1700, "too large exponent"); FMT_ASSERT(e <= 1700 && e >= -1700, "too large exponent");
const uint64_t log10_2_fractional_digits = 0x4d104d427de7fbcc; const int shift = 22;
const int shift_amount = 22; return (e * static_cast<int>(data::log10_2_significand >> (64 - shift))) >>
return (e * shift;
static_cast<int>(log10_2_fractional_digits >> (64 - shift_amount))) >>
shift_amount;
} }
// Various fast log computations.
inline int floor_log2_pow10(int e) FMT_NOEXCEPT { inline int floor_log2_pow10(int e) FMT_NOEXCEPT {
FMT_ASSERT(e <= 1233 && e >= -1233, "too large exponent"); FMT_ASSERT(e <= 1233 && e >= -1233, "too large exponent");
const uint64_t log2_10_integer_part = 3; const uint64_t log2_10_integer_part = 3;
@ -1773,10 +1774,9 @@ inline int floor_log2_pow10(int e) FMT_NOEXCEPT {
} }
inline int floor_log10_pow2_minus_log10_4_over_3(int e) FMT_NOEXCEPT { inline int floor_log10_pow2_minus_log10_4_over_3(int e) FMT_NOEXCEPT {
FMT_ASSERT(e <= 1700 && e >= -1700, "too large exponent"); FMT_ASSERT(e <= 1700 && e >= -1700, "too large exponent");
const uint64_t log10_2_fractional_digits = 0x4d104d427de7fbcc;
const uint64_t log10_4_over_3_fractional_digits = 0x1ffbfc2bbc780375; const uint64_t log10_4_over_3_fractional_digits = 0x1ffbfc2bbc780375;
const int shift_amount = 22; const int shift_amount = 22;
return (e * static_cast<int>(log10_2_fractional_digits >> return (e * static_cast<int>(data::log10_2_significand >>
(64 - shift_amount)) - (64 - shift_amount)) -
static_cast<int>(log10_4_over_3_fractional_digits >> static_cast<int>(log10_4_over_3_fractional_digits >>
(64 - shift_amount))) >> (64 - shift_amount))) >>
@ -2324,8 +2324,7 @@ template <class T> FMT_SAFEBUFFERS decimal_fp<T> to_decimal(T x) FMT_NOEXCEPT {
exponent += float_info<T>::exponent_bias - float_info<T>::significand_bits; exponent += float_info<T>::exponent_bias - float_info<T>::significand_bits;
// Shorter interval case; proceed like Schubfach. // Shorter interval case; proceed like Schubfach.
if (significand == 0) if (significand == 0) return shorter_interval_case<T>(exponent);
return shorter_interval_case<T>(exponent);
significand |= significand |=
(static_cast<carrier_uint>(1) << float_info<T>::significand_bits); (static_cast<carrier_uint>(1) << float_info<T>::significand_bits);

2
include/fmt/format.h
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@ -879,6 +879,8 @@ template <typename T = void> struct FMT_EXTERN_TEMPLATE_API basic_data {
static const divtest_table_entry<uint64_t> divtest_table_for_pow5_64[]; static const divtest_table_entry<uint64_t> divtest_table_for_pow5_64[];
static const uint64_t dragonbox_pow10_significands_64[]; static const uint64_t dragonbox_pow10_significands_64[];
static const uint128_wrapper dragonbox_pow10_significands_128[]; static const uint128_wrapper dragonbox_pow10_significands_128[];
// log10(2) = 0x0.4d104d427de7fbcc...
static const uint64_t log10_2_significand = 0x4d104d427de7fbcc;
#if !FMT_USE_FULL_CACHE_DRAGONBOX #if !FMT_USE_FULL_CACHE_DRAGONBOX
static const uint64_t powers_of_5_64[]; static const uint64_t powers_of_5_64[];
static const uint32_t dragonbox_pow10_recovery_errors[]; static const uint32_t dragonbox_pow10_recovery_errors[];