Parse the decimal128 max significand at compile time

[ibmibmibm]

Summary

For decimal128, max_significand_v() returns the _u128 literal of 34 nines. When it rounds up, fenv_round_impl calls this function at run time, thus each call parses 34 digits. This PR keeps the literal in a local constexpr variable, thus the compiler parses it. Decimal128 addition is 1.36 times faster and tan is 1.29 times faster.

Cause

• fenv_rounding.hpp lines 602 and 606 call max_significand_v<TargetType>() in a normal expression, not in a constant expression. Thus the compiler can evaluate operator""_u128 at run time.
• With GCC 16.2.1 at -O2, from_chars_integer_impl takes 7% of the time of a loop of decimal128 tan. Half of it comes from tan, and half from the += in the loop.
• The other _u128 literals are in constexpr tables, thus the compiler parses them.

Speed

Google Benchmark, median of 9 runs, 1024 random operands with 34 or 16 digits, GCC 16.2.1 at -O3:

Operation	develop	this PR	Speed-up
decimal128_t +	46.4 ns	34.2 ns	1.36x
decimal128_t *	29.2 ns	30.4 ns	0.96x
decimal128_t /	50.2 ns	49.8 ns	1.01x
decimal128_t sin	249.8 ns	214.2 ns	1.17x
decimal128_t tan	565.8 ns	437.7 ns	1.29x
decimal64_t +	13.3 ns	13.5 ns	0.99x
decimal64_t *	14.8 ns	14.8 ns	1.00x

Multiplication, division and decimal64 do not change, as expected.

Tests

• GCC 16.2.1, C++17, the warning flags of the Jamfile: 162 of the 163 run tests pass. test_format_fmtlib does not build on develop either (the char32_t sign conversion in fmt_format.hpp).
• Clang 22.1.8, C++14 and C++20: test_fenv, test_cmath, test_trig_rounding, test_sqrt_rounding, test_decimal_quantum and test_big_uints pass.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.6%. Comparing base (5b03306) to head (bcd3a62).

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #1480     +/-   ##
=========================================
+ Coverage     98.6%   98.6%   +0.1%     
=========================================
  Files          312     312             
  Lines        26265   26266      +1     
  Branches      2264    2264             
=========================================
+ Hits         25892   25893      +1     
  Misses         373     373             

Files with missing lines	Coverage Δ
include/boost/decimal/detail/attributes.hpp	100.0% <100.0%> (ø)

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[mborland]

Good catch. Thanks!