Fr0zka 353c023dfd test: C9 is only half fixed — the libm half survives FPSemantics
All 9 tests green. LargeSeedSurvives proves C1 fixed by property rather than by
comparison: seed 2e9 (what FMath::Rand produces) now yields 400 distinct heights
over 400 samples where it previously gave a constant field. The three equivalence
tests stayed green through an 85-site rewrite.

The digest's NearIso warning fired at 2/115000 and its text blamed the /fp:fast vs
precise split, which is fixed. First instinct was "stale warning, soften it". Checked
instead, and the risk is real by a different mechanism:

sinf/cosf are not specified by IEEE-754. FPSemantics = Precise makes MSVC and Clang
agree on expression evaluation and says nothing about the math library; MSVC's CRT
and glibc's libm may differ by ~1 ULP. FMath::Sin/Cos are used throughout the density
path — layer lines, ribs, room placement, rotations. So C9's compiler half is closed
by construction and its library half is not, and no build flag can close it.

The measurement was also over-stating by ~100x: a single 1e-4 band is far too wide
for a libm-scale delta (~1e-6 absolute at densities of magnitude ~10). Replaced with
a three-band profile; only the tight band warns.

UNVERIFIED: the reworded test.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
2026-07-27 16:38:19 +02:00
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