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Use the optimized multiply path for decimal32_t #1465

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Merged
mborland merged 1 commit into from
Sep 25, 2026
Merged

Use the optimized multiply path for decimal32_t #1465

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13 changes: 5 additions & 8 deletions include/boost/decimal/detail/mul_impl.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -204,12 +204,9 @@ BOOST_DECIMAL_FORCE_INLINE BOOST_DECIMAL_CUDA_CONSTEXPR auto mul_impl_dispatch(
using mul_type = std::conditional_t<TVal < 64, std::uint_fast64_t, int128::uint128_t>;
const bool sign {lhs_c.sign != rhs_c.sign};

// d32 IEEE: revert to the simple multiply + constructor handoff. The
// hardware uint64 divide the constructor's coefficient_rounding does
// is already cheap enough that the expand + pre-shrink + direct_pack
// path's overhead doesn't pay off here. d32_fast still benefits and
// continues to use the optimized path below.
BOOST_DECIMAL_IF_CONSTEXPR (std::is_same<ReturnType, decimal32_t>::value)
// d32 IEEE: a product below the subnormal exponent goes to the constructor
// unrounded, because the optimized path below would round it twice.
if (std::is_same<ReturnType, decimal32_t>::value && BOOST_DECIMAL_UNLIKELY(lhs_c.exp + rhs_c.exp < etiny_v<decimal32_t>))
{
const auto res_sig {static_cast<mul_type>(lhs_c.sig) * static_cast<mul_type>(rhs_c.sig)};
const auto res_exp {lhs_c.exp + rhs_c.exp};
Expand Down Expand Up @@ -325,8 +322,8 @@ BOOST_DECIMAL_FORCE_INLINE BOOST_DECIMAL_CUDA_CONSTEXPR auto mul_impl(T lhs_sig,

const bool sign {lhs_sign != rhs_sign};

// d32 IEEE: revert to the simple multiply + constructor handoff (see
// generic mul_impl above for rationale).
// d32 IEEE: keep the simple multiply + constructor handoff, because the
// path below gives a zero product a different exponent.
BOOST_DECIMAL_IF_CONSTEXPR (std::is_same<ReturnType, decimal32_t>::value)
{
const auto res_sig {static_cast<mul_type>(lhs_sig) * static_cast<mul_type>(rhs_sig)};
Expand Down
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