stillwater-sc / universal / 26909111685

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Commit Message

fix(elreal): rescue #1044 leading-pair overlap on wide hosts only (#1045)

priestRenorm could emit a non-0-overlap (DBL_k) list for a block pool with
catastrophic cancellation between nearly-equal high-precision values. A single
Priest pass (sweepUp / recurse / sweepDown) collapses the cancelling leading
limbs into a small leading term (e.g. x - pi/2 ~ 3.67e-6) while a following limb
sits closer than k, and the pass prepends the leading term without folding that
limb -- leaving an adjacent pair closer than k. Surfaced by Phase 7.6 trig
argument reduction t = x - n*(pi/2) for x near a multiple of pi/2.

The fix must be narrow. A blind "iterate priestRenorm to a 0-overlap fixpoint"
regresses narrow hosts: on bfloat16 (k=8) a deep expansion bottoms out at the
denormal floor where the two smallest limbs are closer than k and twoSumRN cannot
separate them, so extra passes never converge -- they only reshuffle and lengthen
the list, and *any* change to a narrow-host renorm result perturbs the lazy ZBCL
structure so a tail overlap main never forces lands where it IS forced, tripping
the 0-overlap assertion in ZBCL::tail() (caught by the ASan/UBSan Debug jobs on
el_math_constants and el_math_sqrt).

So the rescue is gated two ways:
  - host width: only k >= 24 (float and wider). The series/trig functions that
    need it are wide-host only anyway (a narrow mantissa cannot hold the
    expansions). Narrow hosts get exactly the single Priest pass -- byte-identical
    to the behaviour every merged test was validated against.
  - overlap shape: only a *leading-pair* overlap (the #1044 cancellation
    signature) triggers one extra pass; benign deep-tail overlaps are left as the
    single pass produces them, so well-conditioned wide-host computations stay
    identical and fast. The extra pass result is accepted only if it is fully
    0-overlap, else we fall back to the single pass.

Regression: elastic/elreal/arithmetic/renorm_cancellation.cpp pools x - n*(pi/... (continued)

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