Q.EXP is v3's q48_exp_approx: the Taylor series to 10 terms on |q|,
stopping below 50 ulp, 1.0 Q./ e^|q| for q < 0, e^0 = 1.0, and for
|q| >= 16.0 either 0 or Q max (v3 returned all ones, which is -ulp read
signed under D-8). x, term, sum, sign and term index live in a new
8-cell variable (QE). Checked bit for bit against v3's q48_exp_approx
(ported into the test) on 23 edge values and 3000 pseudo-random q in
(-16.0, 16.0), at both cell widths, optimised and ASan+UBSan; dropping a
term, moving the 50-ulp stop or skipping the reciprocal all fail.
Q.EXP calls Q./, which calls (UQ/), which calls D2*C, and at first it
left its caller no return entries. Lightened along the chain:
- D2*C takes cin in A and builds 2hi+m with `over + +`: no ROT, no SWAP.
- (UQ/) parks the quotient bit instead of using ROT, stores q without
SWAP, and does its full trial subtraction (with borrow) in line
instead of calling DNEGATE D+.
- Q.EXP keeps its term index in (QE) instead of a FOR count.
Headroom (data under args / return): (UQ/) 4/3 -> 4/4, Q./ 3/2 -> 3/3,
Q.EXP 3/2.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
