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Fix handling of implicit bit

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This commit is contained in:
Victor Zverovich 2022-03-10 11:41:56 -08:00
parent 32d477e5f1
commit b4a4189d0c
1 changed files with 11 additions and 5 deletions
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16
include/fmt/format-inl.h
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@ -215,11 +215,16 @@ template <typename T> struct bits {
static_cast<int>(sizeof(T) * std::numeric_limits<unsigned char>::digits);
};
template <typename Float> constexpr bool has_implicit_bit() {
return std::numeric_limits<Float>::digits != 64;
}
// Returns the number of significand bits in Float excluding the implicit bit.
template <typename Float> constexpr int num_significand_bits() {
// Subtract 1 to account for an implicit most significant bit in the
// normalized form.
return std::numeric_limits<Float>::digits - 1;
// normalized form..
return std::numeric_limits<Float>::digits -
(has_implicit_bit<Float>() ? 1 : 0);
}
// A floating-point number f * pow(2, e).
@ -248,8 +253,9 @@ template <typename F> struct basic_fp {
// Assume float is in the format [sign][exponent][significand].
const int num_float_significand_bits =
detail::num_significand_bits<Float>();
const uint64_t implicit_bit = 1ULL << num_float_significand_bits;
using carrier_uint = typename dragonbox::float_info<Float>::carrier_uint;
const carrier_uint implicit_bit = carrier_uint(1)
<< num_float_significand_bits;
const carrier_uint significand_mask = implicit_bit - 1;
auto u = bit_cast<carrier_uint>(n);
f = static_cast<uint64_t>(u & significand_mask);
@ -260,11 +266,11 @@ template <typename F> struct basic_fp {
// than the smallest normalized number (biased_e > 1).
bool is_predecessor_closer = f == 0 && biased_e > 1;
if (biased_e != 0)
f += implicit_bit;
f += static_cast<uint64_t>(implicit_bit);
else
biased_e = 1; // Subnormals use biased exponent 1 (min exponent).
const int exponent_bias = std::numeric_limits<Float>::max_exponent - 1;
e = biased_e - exponent_bias - num_float_significand_bits;
e = biased_e - exponent_bias - std::numeric_limits<Float>::digits + 1;
return is_predecessor_closer;
}