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Fix normalizing the significand

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This commit is contained in:
Matt Borland
2023-07-05 15:04:40 +02:00
parent 5a3a9b3999
commit 140e8d5f44
1 changed files with 18 additions and 3 deletions
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21
include/boost/charconv/detail/compute_float80.hpp
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@@ -306,12 +306,27 @@ inline ResultType compute_float80(std::int64_t q, Unsigned_Integer w, bool negat
// Step 3: Compute the number of leading zeros of w and store as leading_zeros
// UB when w is 0 but this has already been filtered out in step 1
auto leading_zeros = clz_u128(w);
const auto leading_zeros = clz_u128(w);
// Step 4: Normalize the significand
w = (1 << leading_zeros) * w;
if (leading_zeros != 0)
{
w = (1 << leading_zeros) * w;
}
while (w < std::numeric_limits<uint128>::max() / 2)
{
w <<= 1;
}
// Step 5a: Compute the truncated 256-bit product stopping after 1 multiplication if
#ifdef BOOST_CHARCONV_DEBUG_FLOAT128
std::memset(buffer, '\0', sizeof(buffer));
to_chars128(buffer, buffer+sizeof(buffer), static_cast<boost::uint128_type>(w));
std::cerr << "\nw: " << buffer << std::endl;
BOOST_CHARCONV_ASSERT(w <= std::numeric_limits<uint128>::max());
BOOST_CHARCONV_ASSERT(w >= std::numeric_limits<uint128>::max() / 2);
#endif
// Step 5a: Compute the truncated 256-bit product stopping after 1 multiplication if
// no more are required to represent the number exactly
auto z = umul256(significand_256_high[q - smallest_power], w) / UINT64_MAX;