nx_exp.nx

nx_exp.nx -- exp(x) primitive in Q10 fixed point. Bits-up math primitive. Replaces / generalises the previously private _attn_exp_q10 helper inside nx_attention.nx. Public so nx_silu, nx_softmax (future generalisation), nx_gelu, attention, and any future kernel that needs exp() can compose against ONE canonical implementation. ===== Domain ===================================================== Q10-only. Input x_q10 in i64. We split by sign for stability: x >= 0: exp grows; in Q10 i64 we have headroom only up to exp(40) ~= 2.35e17 which fits i64. Return clamped at i64-max-headroom for x > 40 (rare in our pipelines). x < 0: exp decays toward 0. Use the 16-entry lookup table for x in [-7.5, 0) at 0.5 Q10 bin width; for x <= -7.5 return 0 (negligible under Q10 precision). We expose three entrypoints: nx_exp_q10_neg(x_q10) -- caller-asserts x <= 0; lookup path nx_exp_q10(x_q10) -- general; dispatches by sign nx_exp_q10_clamped(x_q10) -- same as nx_exp_q10 but saturates large positives to a documented ceiling for softmax-like usage ===== Lookup table ============================================== Values of exp(-k*0.5) for k = 0..15 in Q10: k=0 exp(0) = 1024 k=1 exp(-0.5) = 621 k=2 exp(-1.0) = 376 k=3 exp(-1.5) = 228 k=4 exp(-2.0) = 138 k=5 exp(-2.5) = 84 k=6 exp(-3.0) = 51 k=7 exp(-3.5) = 31