stillwater-sc / universal / 24922868498

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Commit Message

feat(posit): full constexpr arithmetic, comparison, and conversion-out (#755)

* feat(posit): full constexpr arithmetic, comparison, and conversion-out

Resolves #718. Builds on #714, #716, #717, #725, #752 and completes the
posit constexpr surface. After this PR:

  constexpr posit<32,2> a(2.0), b(3.0);
  constexpr auto sum  = a + b;     // works
  constexpr auto prod = a * b;     // works
  constexpr auto neg  = -a;        // works
  static_assert(a < b);            // works

The chain promoted to constexpr (in dependency order):

  blockbinary::operator+, operator-, to_ull (free + member)
  decode_regime, extract_fields, decode (free posit helpers)
  positRegime: ctors, reset, scale, set, setzero, setinf, regime_size,
               assign, assign_regime_pattern, iszero, bits, nrBits,
               positRegime_k, positRegime_run
               (value() left non-constexpr -- uses std::ldexp)
               (increment() left non-constexpr -- uses std::cout)
  positExponent: ctors, reset, setzero, nrBits, scale, value, bits,
                 set, setNrBits, extract_exponent_bits
  positFraction: ctors, none, bits, nrBits, value, reset, setzero, set
  convert(blocktriple, posit) free function
  posit::operator-(), operator++, operator--, operator+=, -=, *=, /=
  posit free binary operator+, -, *, /
  posit free comparison operator==, !=, <, >, <=, >= (and matching
    friend declarations)

Also converted four `if (_trace_X) std::cout` runtime branches in posit
compound operators to `if constexpr`, since the trace constants are
already `constexpr false` and the runtime form blocked constant
evaluation.

Tests: static/tapered/posit/api/api.cpp gets a third constexpr block
(after the integer and IEEE-754 ones from prior PRs) covering:
  constexpr a + b, a - b, a * b, b / a, -a
  constexpr lambda += and *=
  static_assert on comparison results (==, <, >=)
  bit-equivalent cross-check vs runtime construction
  exact arithmetic on representable values (2+3 ... (continued)

Coverage Stats

54 of 55 new or added lines in 5 files covered. (98.18%)

3 existing lines in 1 file now uncovered.

44954 of 53277 relevant lines covered (84.38%)

6445788.52 hits per line