stillwater-sc / universal / 25293401272

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Commit Message

feat(hfloat): full constexpr support across all operators (#806)

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

Promotes hfloat (IBM System/360 hexadecimal floating-point) to fully
constexpr across construction, assignment, arithmetic, comparison, and
conversion operators -- direct sibling of dfloat PR #805 / dfixpnt PR
#803 / qd PR #800.  Part of Epic #723.

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

include/sw/universal/number/hfloat/hfloat_impl.hpp:
  * Trivial wrappers (clear, setbit, getbit, setbits, setsign, setzero)
    marked constexpr.  hfloat stores its bits in a plain bt[] array
    with no internal building-block dependencies, so these are pure
    integer/array operations with no stdlib calls.
  * Selectors (sign, iszero, isone, ispos, isneg, isinf, isnan, scale)
    marked constexpr.  isinf() and isnan() always return false in HFP.
  * Storage helpers (unpack, pack, normalize_and_pack) marked constexpr.
  * Native-type constructors and operator= for signed/unsigned int and
    float/double routed through constexpr-capable convert_signed,
    convert_unsigned, convert_ieee754.
  * Plain `char` routed through convert_signed via integral promotion
    so hfloat(char(-1)) honors the platform's char signedness (CodeRabbit
    finding from PR #805 applied here too).
  * Arithmetic (+= -= *= /=), increment/decrement (++ --), unary minus,
    SpecificValue construction, all marked constexpr.
  * operator/= divide-by-zero throw fenced under
    !std::is_constant_evaluated() so HFLOAT_THROW_ARITHMETIC_EXCEPTION=1
    callers retain throw behavior at runtime; constant-evaluation falls
    through to setzero() (HFP has no NaN/inf to return).
  * Comparison free functions (== != < <= > >=) for hfloat-hfloat,
    hfloat-double, double-hfloat, all marked constexpr.
  * Free functions abs, fabs marked constexpr.

  * convert_ieee754(double) rewritten to drop std::isnan / std::isinf /
    std::fabs / std::frexp / std::ldexp:
      - NaN detected via x != ... (continued)

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30 existing lines in 1 file now uncovered.

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