stillwater-sc / universal / 25291271294

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Commit Message

feat(dfloat): full constexpr support across all operators (#805)

* feat(dfloat): full constexpr support across all operators

Promotes dfloat (IEEE 754-2008 decimal floating-point) to fully constexpr
across construction, assignment, arithmetic, comparison, and conversion
operators -- direct sibling of dfixpnt PR #803.

What changed
------------

include/sw/universal/number/dfloat/dfloat_impl.hpp:
  * Trivial wrappers (clear, setbit, getbit, setbits, setsign, setinf,
    setnan, setzero) marked constexpr -- each delegates to a constexpr
    blockbinary primitive.
  * Selectors (sign, iszero, isinf, isnan, isone, ispos, isneg, scale)
    marked constexpr.
  * Storage helpers (pow10_s, count_digits_s, unpack, pack,
    normalize_and_pack, dpd_decode_trailing_wide,
    dpd_encode_trailing_wide) marked constexpr.
  * Native-type constructors and operator= for signed/unsigned int and
    float/double routed through constexpr-capable convert_signed,
    convert_unsigned, convert_ieee754.
  * Arithmetic (+= -= *= /=), increment/decrement (++ --), unary minus,
    SpecificValue construction, all marked constexpr.
  * Comparison free functions (== != < <= > >=) for dfloat-dfloat,
    dfloat-double, double-dfloat, all marked constexpr.  >= now uses
    the NaN-safe (a > b || a == b) pattern (PR #797 convention).
  * Free functions abs, fabs marked constexpr.
  * convert_ieee754(double) rewritten to drop std::isnan/isinf/signbit/
    fabs/floor/log10/pow/round.  NaN detected via x != x; infinity via
    bracket against numeric_limits::max(); -0.0 detected via constexpr
    sw::bit_cast<uint64_t>; floor(log10) replaced by a constexpr loop
    bracketing v into [1, 10); pow(10, e) replaced by table lookup
    (pow10_64) plus reciprocal saturation helper pow10_64_safe for
    n >= 20.
  * convert_to_double() rewritten without std::pow.  For decimal128
    (sig_bits > 64) the std::strtod path is fenced under
    !std::is_constant_evaluated(); a constant-evaluated wi... (continued)

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33 existing lines in 1 file now uncovered.

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