stillwater-sc / universal / 28203072856

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Commit Message

feat(efloat): implement lossless bit-level ieee-754 conversions (#1105)

* feat(efloat): implement lossless bit-level ieee-754 conversions (#1093)

Rewrite convert_to_ieee754() inside efloat_impl.hpp to perform correct,
lossless, bit-level conversions to float and double. It directly packs sign,
exponent, and rounded significand bits into C++20 sw::bit_cast layouts,
handling normal/subnormal ranges, directed rounding, and overflow thresholds.

Replace sw::universal::sign() with std::signbit() inside convert_ieee754()
to ensure that the sign of negative zero (-0.0) is preserved correctly upon
construction rather than being silently hardcoded to positive.

Create lossless.cpp inside elastic/efloat/conversion/ to rigorously verify the
conversions, testing identity double/float preservation, special IEEE values,
smallest subnormals, overflow and underflow boundaries, and directed
rounding behaviors under ties-to-even and infinity rounding.

Resolves #1093
Relates to Epic #1101, PR #1104

* fix(efloat): resolve CodeRabbit review comments on lossless conversions

1. Fix shift-by-64 UB: change subnormal underflow cutoff from sub_shift > K
   to sub_shift >= K in both float and double branches, preventing eff_shift from
   reaching 64 inside bitwise operations.

2. Fix full-underflow rounding: specifically handle eff_shift >= 64 (full underflow)
   rounding-up decisions, allowing directed rounding to correctly produce the
   smallest subnormal (1ULL << 63) instead of forcing +0.0f.

3. Fix halfway-tie test pattern: use limb configuration { 0x80000080, 0 }
   inside lossless.cpp to test true guard-bit ties (guard=1, sticky=0) at the 40th
   bit, successfully exercising RoundToNearest ties-to-even.

4. Include <bit> directly in lossless.cpp to ensure std::bit_cast portability.

Resolves #1093, PR #1105

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