stillwater-sc / universal / 25352381793

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Commit Message

feat(microfloat): full constexpr support across all operators (#811)

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

Promotes microfloat (8-bit float family: e4m3, e5m2, e2m1) to fully
constexpr across construction, assignment, arithmetic, comparison, and
conversion.  microfloat is its own implementation (not a cfloat alias as
the issue body suggested -- 845 lines of dedicated code), so the
promotion surface is non-trivial.  Part of Epic #723.

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

include/sw/universal/number/microfloat/microfloat_impl.hpp:
  * All native-type ctors / assignment operators / conversion-out
    operators marked constexpr.
  * Increment / decrement (++ --) marked constexpr.
  * Unary minus, +=, -=, *=, /= marked constexpr (delegate to to_float
    -> arithmetic -> from_float, all of which are constexpr after the
    rewrite below).
  * Comparison operators (== != < <= > >=) for microfloat-microfloat,
    microfloat-float, and float-microfloat marked constexpr.
  * operator>= changed from `!operator<` to `operator> || operator==`
    -- pre-fix code returned true for any NaN operand because operator<
    returns false for NaN, and !false = true.  Same bug pattern as the
    e8m0 PR #810 CodeRabbit catch.

  * to_float() rewritten via std::is_constant_evaluated() dispatch:
    runtime path keeps std::ldexp; constexpr path uses a new private
    cx_ldexp helper (power-of-2 multiplication loop, bounded by the
    type's exponent range -- at most ~14 iterations for e5m2).  Both
    paths produce bit-identical results because 2.0/0.5 are exactly
    representable in IEEE 754 float.

  * from_float() rewritten via std::is_constant_evaluated() dispatch:
    - std::signbit -> bit_cast<uint32_t>(v) >> 31
    - std::isinf   -> (rawExp == 0xFF && rawFrac == 0)
    - std::frexp   -> IEEE 754 field extraction with subnormal-source
                      leading-zero scan to match frexp's mantissa
                      normalization
    - std::ld... (continued)

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3 existing lines in 1 file now uncovered.

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