Refactor decimal64 fma

Author: mborland

include/boost/decimal/decimal64.hpp

@@ -2353,6 +2353,7 @@ constexpr auto copysignd64(decimal64 mag, decimal64 sgn) noexcept -> decimal64
     return mag;
 }
 
+/*
 constexpr auto fmad64(decimal64 x, decimal64 y, decimal64 z) noexcept -> decimal64
 {
     // First calculate x * y without rounding
@@ -2417,6 +2418,7 @@ constexpr auto fmad64(decimal64 x, decimal64 y, decimal64 z) noexcept -> decimal
 
     return {result.sig, result.exp, result.sign};
 }
+*/
 
 } //namespace decimal
 } //namespace boost

include/boost/decimal/detail/cmath/fma.hpp

@@ -83,6 +83,75 @@ constexpr auto fmad32(decimal32 x, decimal32 y, decimal32 z) noexcept -> decimal
     return {result.sig, result.exp, result.sign};
 }
 
+constexpr auto fmad64(decimal64 x, decimal64 y, decimal64 z) noexcept -> decimal64
+{
+    // First calculate x * y without rounding
+    constexpr decimal64 zero {0, 0};
+
+    const auto res {detail::check_non_finite(x, y)};
+    if (res != zero)
+    {
+        return res;
+    }
+
+    auto sig_lhs {x.full_significand()};
+    auto exp_lhs {x.biased_exponent()};
+    detail::normalize<decimal64>(sig_lhs, exp_lhs);
+
+    auto sig_rhs {y.full_significand()};
+    auto exp_rhs {y.biased_exponent()};
+    detail::normalize<decimal64>(sig_rhs, exp_rhs);
+
+    auto mul_result {d64_mul_impl(sig_lhs, exp_lhs, x.isneg(), sig_rhs, exp_rhs, y.isneg())};
+    const decimal64 dec_result {mul_result.sig, mul_result.exp, mul_result.sign};
+
+    const auto res_add {detail::check_non_finite(dec_result, z)};
+    if (res_add != zero)
+    {
+        return res_add;
+    }
+
+    bool lhs_bigger {dec_result > z};
+    if (dec_result.isneg() && z.isneg())
+    {
+        lhs_bigger = !lhs_bigger;
+    }
+    bool abs_lhs_bigger {abs(dec_result) > abs(z)};
+
+    // To avoid the rounding step we promote the constituent pieces to the next higher type
+    detail::decimal128_components promoted_mul_result {static_cast<detail::uint128>(mul_result.sig),
+                                                       mul_result.exp, mul_result.sign};
+
+    detail::normalize<decimal128>(promoted_mul_result.sig, promoted_mul_result.exp);
+
+    auto sig_z {static_cast<detail::uint128>(z.full_significand())};
+    auto exp_z {z.biased_exponent()};
+    detail::normalize<decimal128>(sig_z, exp_z);
+    detail::decimal128_components z_components {sig_z, exp_z, z.isneg()};
+
+    if (!lhs_bigger)
+    {
+        detail::swap(promoted_mul_result, z_components);
+        abs_lhs_bigger = !abs_lhs_bigger;
+    }
+
+    detail::decimal128_components result {};
+
+    if (!promoted_mul_result.sign && z_components.sign)
+    {
+        result = d128_sub_impl(promoted_mul_result.sig, promoted_mul_result.exp, promoted_mul_result.sign,
+                               z_components.sig, z_components.exp, z_components.sign,
+                               abs_lhs_bigger);
+    }
+    else
+    {
+        result = d128_add_impl(promoted_mul_result.sig, promoted_mul_result.exp, promoted_mul_result.sign,
+                               z_components.sig, z_components.exp, z_components.sign);
+    }
+
+    return {result.sig, result.exp, result.sign};
+}
+
 BOOST_DECIMAL_EXPORT constexpr auto fma(decimal32 x, decimal32 y, decimal32 z) noexcept -> decimal32
 {
     return fmad32(x, y, z);