Apply similar logic to 128-bit add/sub path

Author: mborland

include/boost/decimal/detail/add_impl.hpp

@@ -276,9 +276,53 @@ constexpr auto d128_add_impl(T lhs_sig, U lhs_exp, bool lhs_sign,
         {
             // rounding mode == fe_dec_upward
             // Unconditionally round up. Could be 5e+95 + 4e-100 -> 5.000001e+95
-            return lhs_sig != 0U && (lhs_exp > rhs_exp) ?
-                ReturnType{lhs_sig + 1U, lhs_exp, lhs_sign} :
-                ReturnType{rhs_sig + 1U, rhs_exp, rhs_sign};
+            const bool use_lhs {lhs_sig != 0U && (lhs_exp > rhs_exp)};
+
+            if (use_lhs)
+            {
+                if (rhs_sig != 0U)
+                {
+                    if (lhs_sign)
+                    {
+                        if (is_power_of_10(lhs_sig))
+                        {
+                            --lhs_sig;
+                            lhs_sig *= 10U;
+                            lhs_sig += 9U;
+                            --lhs_exp;
+                        }
+                        else
+                        {
+                            --lhs_sig;
+                        }
+                    }
+                    else
+                    {
+                        ++lhs_sig;
+                    }
+                }
+
+                return ReturnType{lhs_sig, lhs_exp, lhs_sign} ;
+            }
+            else
+            {
+                if (lhs_sig != 0U)
+                {
+                    if (rhs_sign)
+                    {
+                        --rhs_sig;
+                        rhs_sig *= 10U;
+                        rhs_sig += 9U;
+                        --rhs_exp;
+                    }
+                    else
+                    {
+                        ++rhs_sig;
+                    }
+                }
+
+                return ReturnType{rhs_sig, rhs_exp, rhs_sign};
+            }
         }
     }