stillwater-sc / universal / 27519576221

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Committed by web-flow

Commit Message

fix(elreal): arrest the streaming multiply at the host floor (#1068) (#1072)

mul_online kept emitting product blocks once a product reached the host's
smallest normal exponent (~2^-1022). block_two_mult places the residual `low`
a full k below `high`, so near the floor these blocks DENORMALISE -- and a
subnormal block cannot hold its k significand bits, so the streaming product
could no longer keep consecutive blocks k apart. Two subnormal blocks ~22 apart
near 2^-1074 broke the ZBCL 0-overlap invariant (assert in Debug; silently
non-canonical in Release). This is why dense online division was capped at ~8
blocks / ~118 digits: deeper Newton iterations reached the floor.

Root cause confirmed by instrumenting the assert -- the violating pair was two
SUBNORMAL blocks (E=-1052, E=-1074), i.e. the host floor, NOT a "~ -550
canonicalisation" limit as an earlier banner guessed.

Fix: singleMultHelper drops GENUINELY subnormal blocks at the source (the eager
mul()/div() do this via priestRenorm + keep_normalised every step; a streaming
producer cannot post-renormalise):
  - high subnormal -> stop the series (gs descends, so all later f*g are smaller)
  - low subnormal  -> emit `high` only (the dropped residual is < 2^-1022)
A ZERO product block is NOT a floor condition -- it is value-neutral and passes
through (drop_zeros handles it). Distinguishing zero from subnormal was the key:
keying on is_normalised() alone stopped the series on the zero blocks that
2 - b*r produces, collapsing the reciprocal to ~1 block.

With no subnormal block ever entering a stream, the 0-overlap break is gone and
the dense Newton reciprocal refines to the host floor: dense div now reaches ~17
components / ~265 digits (was ~8 / ~118), 0-overlap canonical, reconstructing
q*b == a exactly. The mul_canonical_cap=8 stopgap in div_online is removed.

verify_div_dense_deep strengthened to assert >=14 blocks, lastE <= -750, all
blocks normal, deep reconstruction. PASS on gcc 13.3 + clan... (continued)

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3 existing lines in 1 file now uncovered.

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