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Apply get_cached_power optimization by jk-jeon

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This commit is contained in:
Victor Zverovich 2019-10-30 08:07:01 -07:00
parent 8e700619b7
commit 791294d17b
1 changed files with 7 additions and 2 deletions
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9
include/fmt/format-inl.h
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@ -480,9 +480,14 @@ FMT_FUNC fp operator*(fp x, fp y) {
// 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`.
FMT_FUNC fp get_cached_power(int min_exponent, int& pow10_exponent) {
const double one_over_log2_10 = 0.30102999566398114; // 1 / log2(10)
const uint64_t one_over_log2_10 = 0x4d104d42; // round(pow(2, 32) / log2(10))
int index = static_cast<int>(
std::ceil((min_exponent + fp::significand_size - 1) * one_over_log2_10));
static_cast<int64_t>(
(min_exponent + fp::significand_size - 1) * one_over_log2_10 +
((uint64_t(1) << 32) - 1) // ceil
) >>
32 // arithmetic shift
);
// Decimal exponent of the first (smallest) cached power of 10.
const int first_dec_exp = -348;
// Difference between 2 consecutive decimal exponents in cached powers of 10.