moonbitlang / core / 6903

89%

Build Type

push

github

Committed by bobzhang

Commit Message

fix(math): evaluate log2 in hi/lo parts on non-JS targets

`log2(x)` was `ln(f) / LN2 + e`: a division by a rounded constant and an
addition on top of `ln`'s own error, up to 1.9 ulp off. Port the tail of
FreeBSD's e_log2.c (the algorithm of musl <= 1.1.21 and of Rust's `libm`)
instead: keep log(1+f) as a hi + lo pair, multiply by a two-part 1/ln2 and
add the exponent with the rounding error carried along.

On 72,799 positive finite inputs against an 80-digit reference: correctly
rounded 80.59% -> 97.97%, max error 1.903 -> 0.729 ulp, results off by more
than 1 ulp 2,182 -> 0. Results are bit-identical to a C build of the
reference on native, wasm and wasm-gc. It is also faster (one inlined
reduction instead of two `frexp` calls and a division): 8.3x on native and
2.7x on wasm-gc for 1,024 calls.

Part of #4336.

Reviewed by Codex CLI (xhigh): "Constants, operation order, bit helpers,
and integer ranges match the reference. Special-value result categories
are preserved."

Signed-off-by: Codex CLI <codex@openai.com>

Coverage Stats

12 of 12 new or added lines in 1 file covered. (100.0%)

1 existing line in 1 file now uncovered.

16696 of 18667 relevant lines covered (89.44%)

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